Dosimetric evaluation of a spinal cord dose-limiting 3D-CRT technique for radiotherapy of spinal metastases.

BACKGROUND: To evaluate the possible advantages of a simple spinal cord (SC) dose-limiting three-dimensional conformal radiotherapy (3D-CRT) technique in comparison to conventional two-dimensional (2D) techniques and other 3D-CRT techniques for spinal bone irradiation. METHODS: For 41 spinal target volumes, seven different techniques were evaluated, using a standard schedule of 30 Gy in 10 fractions. The SC dose-limiting 3D-CRT technique 1F2S-18MV using a single posterior field (F) supplemented by two anterior segment fields (S) and 18-MV photon beams was compared to two conventional 2D techniques (a single posterior field, PA, and two opposed anterior-posterior fields, APPA), three other 3D-CRT techniques (a single posterior field supplemented by four segment fields, 1F4S; two wedged fields, WD, and the SC dose-limiting variant using 6 MV, 1F2S-6MV) along with the original clinically applied plans. RESULTS: 1F2S-18MV demonstrated notably better results for all target volume parameters compared to the conventional 2D techniques (p < 0.001). Limitation of the SC dose was significantly superior with 1F2S-18MV in comparison to PA and APPA (SC Dmean: 28.9 ± 0.4  vs. 30.1 ± 0.6 Gy and 30.1 ± 0.4 Gy; SC Dmax: 30.9 ± 0.7  vs. 32.5 ± 1.0 Gy and 31.8 ± 0.7 Gy; SC D1cm3 : 30.1 ± 0.6  vs. 31.7 ± 0.9 Gy and 31.1 ± 0.6 Gy; p < 0.001). Likewise, lower mean SC doses with 1F2S-18MV were observed in comparison to the more treatment time-consuming 3D-CRT techniques (1F4S, WD) and the original plans without relevant compromises on the dose homogeneity in the target volume and the dose exposure to the other OARs. CONCLUSION: In treatment planning of spinal metastases, simple variants of 3D-CRT-techniques like 1F2S-18MV can offer a significant dose limitation to the SC while providing a sufficient dose coverage of the target volume. Especially in patients with favorable life expectancy and potential need for re-irradiation, such SC dose-limiting 3D-CRT techniques may be a reasonable approach.

Comparing Iridium-192 with Cobalt-60 sources in high-dose-rate brachytherapy boost for localized prostate cancer.

BACKGROUND: Dosimetric and clinical comparison of two cohorts of Iridium-192 (Ir-192) and Cobalt-60 (Co-60) high-dose-rate brachytherapy (DR-BT) boost for localized prostate cancer. MATERIAL AND METHODS: Patients with localized prostate cancer receiving either Ir-192 or Co-60 high-dose-rate brachytherapy (HDR-BT) boost in combination with external beam radiotherapy (EBRT) in the period of 2002-2019 were evaluated for dosimetric differences, side effects, biochemical relapse-free survival (bRFS), metastasis-free survival (MFS), and overall survival (OS). EBRT, delivered in 46 Gy (DMean) in conventional fractionation, was followed by two fractions HDR-BT boost with 9 Gy (D90%) 2 and 4 weeks after EBRT. Genitourinary (GU)/gastrointestinal (GI) toxicity were evaluated utilizing the Common Toxicity Criteria for Adverse Events version 5.0 and biochemical failure was defined according to the Phoenix definition. RESULTS: A total of 338 patients with a median follow-up of 101.8 (IQR 65.7-143.0) months were evaluated. At 10 years the estimated bRFS, MFS, and OS in our patient sample were 81.1%/71.2% (p=.073), 87.0%/85.7% (p=.862), and 70.1%/69.7% (p=.998) for Ir-192/Co-60, respectively. Cumulative 5-year late grade ≥2 GU toxicity was 20% for Ir-192 and 18.3% for Co-60 (p=.771). Cumulative 5-year late grade ≥2 GI toxicity was 5.8% for Ir-192 and 4.6% for Co-60 (p=.610). Grade 3 late GU side effects were pronounced in the Ir-192 cohort with 8.1% versus 1.4% in the Co-60 cohort (p=.01), which was associated with significantly lower dose to the organs at risk in the Co-60 cohort. PTV D90% was 9.3 ± 0.8 Gy versus 9.0 ± 1.1 Gy (p=.027) for Ir-192 versus Co-60. PTV V100% and PTV V150% were not significantly different between both cohorts. CONCLUSION: Co-60 brachytherapy sources are an effective alternative to Ir-192 in combined prostate HDR-BT boost + EBRT.

The role of beam density and arrangement in non-coplanar IMRT exemplified by the irradiation of brain tumors - Parallels to computed tomographic imaging.

PURPOSE: Parallels between the fields of non-coplanar IMRT and non-coplanar computed tomographic reconstruction are highlighted exemplified by the identification of qualified beam configurations for the irradiation of brain tumors. METHODS AND MATERIALS: Four types of beam configurations, i.e. a pure coplanar, a quasi-isotropic and two transitional arrangements, served to systematically examine the impact of parameters such as the sampling rate and the degree of accessibility on plan quality. The resulting set of treatment techniques was compared by means of a Pinnacle3 based retrospective planning study on 18 brain tumor cases. RESULTS AND DISCUSSION: A consistent ranking of IMRT beam constellations according to plan quality was established, which directly reflects the necessities of high-quality CT imaging. Once a sufficient dense beam sampling is secured (given by compliance to Nyquist's theorem), the quasi-isotropic (QIso) irradiation produced best treatment plans, followed by a coplanar irradiation complemented by a single orthogonal non-coplanar beam (CoPl+1). Beams evenly distributed in two orthogonal planes (2-Pl), although using larger portions of the 4π space, proved to be less favorable as the beam sequence becomes less dense. The most unfavorable technique is the pure coplanar technique (CoPl). Generally, techniques with large interbeam distance, i.e. the 2-Pl technique and, to a lesser extent, QIso, are particularly sensitive to a beam number reduction. CONCLUSIONS: Rules established for high quality non-coplanar tomographic imaging are also relevant for non-coplanar IMRT. In this regard, the degree of coverage of 4π space is less important than a sufficient dense sampling.

Comparison of sliding window and field-in-field techniques for tangential whole breast irradiation using the Halcyon and Synergy Agility systems.

BACKGROUND: To implement a tangential treatment technique for whole breast irradiation using the Varian Halcyon and to compare it with Elekta Synergy Agility plans. METHODS: For 20 patients two comparable treatment plans with respect to dose coverage and normal tissue sparing were generated. Tangential field-in-field treatment plans (Pinnacle/Synergy) were replanned using the sliding window technique (Eclipse/Halcyon). Plan specific QA was performed using the portal Dosimetry and the ArcCHECK phantom. Imaging and treatment dose were evaluated for treatment delivery on both systems using a modified CIRS Phantom. RESULTS: The mean number of monitor units for a fraction dose of 2.67 Gy was 515 MUs and 260 MUs for Halcyon and Synergy Agility plans, respectively. The homogeneity index and dose coverage were similar for both treatment units. The plan specific QA showed good agreement between measured and calculated plans. All Halcyon plans passed portal dosimetry QA (3%/2 mm) with 100% points passing and ArcCheck QA (3%/2 mm) with 99.5%. Measurement of the cumulated treatment and imaging dose with the CIRS phantom resulted in lower dose to the contralateral breast for the Halcyon plans. CONCLUSIONS: For the Varian Halcyon a plan quality similar to the Elekta Synergy device was achieved. For the Halcyon plans the dose contribution from the treatment fields to the contralateral breast was even lower due to less interleaf transmission of the Halcyon MLC and a lower contribution of scattered dose from the collimator system.

Moderately hypofractionated radiotherapy for localized prostate cancer: updated long-term outcome and toxicity analysis.

PURPOSE: Evaluation of long-term outcome and toxicity of moderately hypofractionated radiotherapy using intensity-modulated radiotherapy (IMRT) with simultaneous integrated boost treatment planning and cone beam CT-based image guidance for localized prostate cancer. METHODS: Between 2005 and 2015, 346 consecutive patients with localized prostate cancer received primary radiotherapy using cone beam CT-based image-guided intensity-modulated radiotherapy (IG-IMRT) and volumetric modulated arc therapy (IG-VMAT) with a simultaneous integrated boost (SIB). Total doses of 73.9 Gy (n = 44) and 76.2 Gy (n = 302) to the high-dose PTV were delivered in 32 and 33 fractions, respectively. The low-dose PTV received a dose (D95) of 60.06 Gy in single doses of 1.82 Gy. The pelvic lymph nodes were treated in 91 high-risk patients to 45.5 Gy (D95). RESULTS: Median follow-up was 61.8 months. The 5­year biochemical relapse-free survival (bRFS) was 85.4% for all patients and 93.3, 87.4, and 79.4% for low-, intermediate-, and high-risk disease, respectively. The 5­year prostate cancer-specific survival (PSS) was 94.8% for all patients and 98.7, 98.9, 89.3% for low-, intermediate-, and high-risk disease, respectively. The 5­year and 10-year overall survival rates were 83.8 and 66.3% and the 5­year and 10-year freedom from distant metastasis rates were 92.2 and 88.0%, respectively. Cumulative 5­year late GU toxicity and late GI toxicity grade ≥2 was observed in 26.3 and 12.1% of the patients, respectively. Cumulative 5­year late grade 3 GU/GI toxicity occurred in 4.0/1.2%. CONCLUSION: Moderately hypofractionated radiotherapy using SIB treatment planning and cone beam CT image guidance resulted in high biochemical control and survival with low rates of late toxicity.

Impact of beam configuration on VMAT plan quality for Pinnacle3Auto-Planning for head and neck cases.

BACKGROUND: The purpose of this study was to compare automatically generated VMAT plans to find the superior beam configurations for Pinnacle3 Auto-Planning and share "best practices". METHODS: VMAT plans for 20 patients with head and neck cancer were generated using Pinnacle3 Auto-Planning Module (Pinnacle3 Version 9.10) with different beam setup parameters. VMAT plans for single (V1) or double arc (V2) and partial or full gantry rotation were optimized. Beam configurations with different collimator positions were defined. Target coverage and sparing of organs at risk were evaluated based on scoring of an evaluation parameter set. Furthermore, dosimetric evaluation was performed based on the composite objective value (COV) and a new cross comparison method was applied using the COVs. RESULTS: The evaluation showed a superior plan quality for double arcs compared to one single arc or two single arcs for all cases. Plan quality was superior if a full gantry rotation was allowed during optimization for unilateral target volumes. A double arc technique with collimator setting of 15° was superior to a double arc with collimator 60° and a two single arcs with collimator setting of 15° and 345°. CONCLUSION: The evaluation showed that double and full arcs are superior to single and partial arcs in terms of organs at risk sparing even for unilateral target volumes. The collimator position was found as an additional setup parameter, which can further improve the target coverage and sparing of organs at risk.

Properties of the anisotropy of dose contributions: A planning study on prostate cases.

PURPOSE: To characterize the static properties of the anisotropy of dose contributions for different treatment techniques on real patient data (prostate cases). From this, we aim to define a class of treatment techniques with invariant anisotropy distribution carrying information of target coverage and organ-at-risk (OAR) sparing. The anisotropy presumably is a helpful quantity for plan adaptation problems. METHODS: The anisotropy field is analyzed for different intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) techniques for a total of ten planning CTs of prostate cases. Primary irradiation directions ranged from 5 to 15. The uniqueness of anisotropy was explored: In particular, the anisotropy distribution inside the planning treatment volume (PTV) and in its vicinity was investigated. Furthermore, deviations of the anisotropy under beam rotations were explored by direct plan comparison as an indicating the susceptibility of each planned technique to changes in the geometric plan configuration. In addition, plan comparisons enabled the categorization of treatment techniques in terms of their anisotropy distribution. RESULTS: The anisotropy profile inside the PTV and in the transition between OAR and PTV is independent of the treatment technique as long as a sufficient number of beams contribute to the dose distribution. Techniques with multiple beams constitute a class of almost identical and technique-independent anisotropy distribution. For this class of techniques, substructures of the anisotropy are particularly pronounced in the PTV, thus offering good options for applying adaptation rules. Additionally, the techniques forming the mentioned class fortunately allow a better OAR sparing at constant PTV coverage. Besides the characterization of the distribution, a pairwise plan comparison reveals each technique's susceptibility to deviations which decreases for an increasing number of primary irradiation directions. CONCLUSIONS: Techniques using many irradiation directions form a class of almost identical anisotropy distributions which are assumed to provide a basis for improved adaptation procedures. Encouragingly, these techniques deliver quite invariant anisotropy distributions with respect to rotations correlated with good plan qualities than techniques using few gantry angles. The following will be the next steps toward anisotropy-based adaptation: first, the quantification of anisotropy regarding organ deformations; and second, establishing the interrelation between the anisotropy and beam shaping.

Steeper dose gradients resulting from reduced source to target distance-a planning system independent study.

PURPOSE: To quantify the contribution of penumbra in the improvement of healthy tissue sparing at reduced source-to-axis distance (SAD) for simple spherical target and different prescription isodoses (PI). METHOD: A TPS-independent method was used to estimate three-dimensional (3D) dose distribution for stereotactic treatment of spherical targets of 0.5 cm radius based on single beam two-dimensional (2D) film dosimetry measurements. 1 cm target constitutes the worst case for the conformation with standard Multi-Leaf Collimator (MLC) with 0.5 cm leaf width. The measured 2D transverse dose cross-sections and the profiles in leaf and jaw directions were used to calculate radial dose distribution from isotropic beam arrangement, for both quadratic and circular beam openings, respectively. The results were compared for standard (100 cm) and reduced SAD 70 and 55 cm for different PI. RESULTS: For practical reduction of SAD using quadratic openings, the improvement of healthy tissue sparing (HTS) at distances up to 3 times the PTV radius was at least 6%-12%; gradient indices (GI) were reduced by 3-39% for PI between 40% and 90%. Except for PI of 80% and 90%, quadratic apertures at SAD 70 cm improved the HTS by up to 20% compared to circular openings at 100 cm or were at least equivalent; GI were 3%-33% lower for reduced SAD in the PI range 40%-70%. For PI = 80% and 90% the results depend on the circular collimator model. CONCLUSION: Stereotactic treatments of spherical targets delivered at reduced SAD of 70 or 55 cm using MLC spare healthy tissue around the target at least as good as treatments at SAD 100 cm using circular collimators. The steeper beam penumbra at reduced SAD seems to be as important as perfect target conformity. The authors argue therefore that the beam penumbra width should be addressed in the stereotactic studies.

Finer leaf resolution and steeper beam edges using a virtual isocentre in concurrence to PTV-shaped collimators in standard distance - a planning study.

PURPOSE: Investigation of a reduced source to target distance to improve organ at risk sparing during stereotactic irradiation (STX). METHODS: The authors present a planning study with perfectly target-volume adapted collimator compared with multi-leaf collimator (MLC) at reduced source to virtual isocentre distance (SVID) in contrast to normal source to isocentre distance (SID) for stereotactic applications. The role of MLC leaf width and 20-80% penumbra was examined concerning the healthy tissue sparing. Several prescription schemes and target diameters are considered. RESULTS: Paddick's gradient index (GI) as well as comparison of the mean doses to spherical shells at several distances to the target is evaluated. Both emphasize the same results: the healthy tissue sparing in the high dose area around the planning target volume (PTV) is improved at reduced SVID ≤ 70 cm. The effect can be attributed more to steeper penumbra than to finer leaf resolution. Comparing circular collimators at different SVID just as MLC-shaped collimators, always the GI was reduced. Even MLC-shaped collimator at SVID 70 cm had better healthy tissue sparing than an optimal shaped circular collimator at SID 100 cm. Regarding penumbra changes due to varying SVID, the results of the planning study are underlined by film dosimetry measurements with Agility™ MLC. CONCLUSION: Penumbra requires more attention in comparing studies, especially studies using different planning systems. Reduced SVID probably allows usage of conventional MLC for STX-like irradiations.

Anisotropy of dose contributions-An instrument to upgrade real time IMRT and VMAT adaptation?

PURPOSE: To suggest a definition of dose deposition anisotropy for the purpose of ad hoc adaptation of intensity modulated arc therapy (IMRT) and volumetric arc therapy (VMAT), particularly in the vicinity of important organs at risk (OAR), also for large deformations. METHODS: Beam's-eye-view (BEV) based fluence warping is a standard adaptation method with disadvantages for strongly varying OAR shapes. 2-Step-adaptation overcomes these difficulties by a deeper analysis of the 3D properties of adaptation processes, but requires separate arcs for every OAR to spare, which makes it impractical for cases with multiple OARs. The authors aim to extend the 2-Step method to arbitrary intensity modulated plan by analyzing the anisotropy of dose contributions. Anisotropy was defined as a second term of Fourier transformation of gantry angle dependent dose contributions. For a cylindrical planning target volume (PTV) surrounding an OAR of varying diameter, the anisotropy and the dose-normalized anisotropy were analyzed for several scenarios of optimized fluence distributions. 2-Step adaptation to decreasing and increasing OAR diameter was performed, and compared to a usual fluence based adaptation method. For two clinical cases, prostate and neck, the VMAT was generated and the behavior of anisotropy was qualitatively explored for deformed organs at risk. RESULTS: Dose contribution anisotropy in the PTV peaks around nearby OARs. The thickness of the "anisotropy wall" around OAR increases for increasing OAR radius, as also does the width of 2-Step dose saturating fluence peak adjacent to the OAR [K. Bratengeier et al., "A comparison between 2-Step IMRT and conventional IMRT planning," Radiother. Oncol. 84, 298-306 (2007)]. Different optimized beam fluence profiles resulted in comparable radial dependence of normalized anisotropy. As predicted, even for patient cases, anisotropy was inflated even more than increasing diameters of OAR. CONCLUSIONS: For cylindrically symmetric cases, the dose distribution anisotropy defined in the present work implicitly contains adaptation-relevant information about 3D relationships between PTV and OAR and degree of OAR sparing. For more complex realistic cases, it shows the predicted behavior qualitatively. The authors claim to have found a first component for advancing a 2-Step adaptation to a universal adaptation algorithm based on the BEV projection of the dose anisotropy. Further planning studies to explore the potential of anisotropy for adaptation algorithms using phantoms and clinical cases of differing complexity will follow.

Characteristics of non-coplanar IMRT in the presence of target-embedded organs at risk.

BACKGROUND: The aim is to analyze characteristics and to study the potentials of non-coplanar intensity modulated radiation therapy (IMRT) techniques. The planning study applies to generalized organ at risk (OAR) - planning target volume (PTV) geometries. METHODS: The authors focus on OARs embedded in the PTV. The OAR shapes are spherically symmetric (A), cylindrical (B), and bended (C). Several IMRT techniques are used for the planning study: a) non-coplanar quasi-isotropic; b) two sets of equidistant coplanar beams, half of beams incident in a plane perpendicular to the principal plane; c) coplanar equidistant (reference); d) coplanar plus one orthogonal beam. The number of beam directions varies from 9 to 16. The orientation of the beam sets is systematically changed; dose distributions resulting from optimal fluence are explored. A selection of plans is optimized with direct machine parameter optimization (DMPO) allowing 120 and 64 segments. The overall plan quality, PTV coverage, and OAR sparing are evaluated. RESULTS: For all fluence based techniques in cases A and C, plan quality increased considerably if more irradiation directions were used. For the cylindrically symmetric case B, however, only a weak beam number dependence was observed for the best beam set orientation, for which non-coplanar directions could be found where OAR- and PTV-projections did not overlap. IMRT plans using quasi-isotropical distributed non-coplanar beams showed stable results for all topologies A, B, C, as long as 16 beams were chosen; also the most unfavorable beam arrangement created results of similar quality as the optimally oriented coplanar configuration. For smaller number of beams or application in the trunk, a coplanar technique with additional orthogonal beam could be recommended. Techniques using 120 segments created by DMPO could qualitatively reproduce the fluence based results. However, for a reduced number of segments the beam number dependence declined or even reversed for the used planning system and the plan quality degraded substantially. CONCLUSIONS: Topologies with targets encompassing sensitive OAR require sufficient number of beams of 15 or more. For the subgroup of topologies where beam incidences are possible which cover the whole PTV without direct OAR irradiation, the quality dependence on the number of beams is much less pronounced above 9 beams. However, these special non-coplanar beam directions have to be found. On the basis of this work the non-coplanar IMRT techniques can be chosen for further clinical planning studies.

Towards automated on-line adaptation of 2-Step IMRT plans: QUASIMODO phantom and prostate cancer cases.

BACKGROUND: The standard clinical protocol of image-guided IMRT for prostate carcinoma introduces isocenter relocation to restore the conformity of the multi-leaf collimator (MLC) segments to the target as seen in the cone-beam CT on the day of treatment. The large interfractional deformations of the clinical target volume (CTV) still require introduction of safety margins which leads to undesirably high rectum toxicity. Here we present further results from the 2-Step IMRT method which generates adaptable prostate IMRT plans using Beam Eye View (BEV) and 3D information. METHODS: Intermediate/high-risk prostate carcinoma cases are treated using Simultaneous Integrated Boost at the Universitätsklinkum Würzburg (UKW). Based on the planning CT a CTV is defined as the prostate and the base of seminal vesicles. The CTV is expanded by 10 mm resulting in the PTV; the posterior margin is limited to 7 mm. The Boost is obtained by expanding the CTV by 5 mm, overlap with rectum is not allowed. Prescription doses to PTV and Boost are 60.1 and 74 Gy respectively given in 33 fractions.We analyse the geometry of the structures of interest (SOIs): PTV, Boost, and rectum, and generate 2-Step IMRT plans to deliver three fluence steps: conformal to the target SOIs (S0), sparing the rectum (S1), and narrow segments compensating the underdosage in the target SOIs due to the rectum sparing (S2). The width of S2 segments is calculated for every MLC leaf pair based on the target and rectum geometry in the corresponding CT layer to have best target coverage. The resulting segments are then fed into the DMPO optimizer of the Pinnacle treatment planning system for weight optimization and fine-tuning of the form, prior to final dose calculation using the collapsed cone algorithm.We adapt 2-Step IMRT plans to changed geometry whilst simultaneously preserving the number of initially planned Monitor Units (MU). The adaptation adds three further steps to the previous isocenter relocation: 1) 2-Step generation for the geometry of the day using the relocated isocenter, MU transfer from the planning geometry; 2) Adaptation of the widths of S2 segments to the geometry of the day; 3) Imitation of DMPO fine-tuning for the geometry of the day. RESULTS AND CONCLUSION: We have performed automated 2-Step IMRT adaptation for ten prostate adaptation cases. The adapted plans show statistically significant improvement of the target coverage and of the rectum sparing compared to those plans in which only the isocenter is relocated. The 2-Step IMRT method may become a core of the automated adaptive radiation therapy system at our department.

Considerations on IMRT for quasi-isotropic non-coplanar irradiation.

The purpose of this study was the mathematical analysis of IMRT with many non-coplanar fields for planning target volumes (PTV) surrounding nearly spherical organs at risk (OAR). Our approach is partially analogous to the well known inverse planning for a cylindrically symmetric (CS) case (Brahme et al 1982 Phys. Med. Biol. 27 1221-9) and leads to a spherically symmetric (SS) solution. For the planning study we approximated isotropic 4 Pi irradiation by a quasi-isotropic non-coplanar IMRT technique with 16 fields which we compared to a coplanar IMRT technique with 15 equidistant fields. A virtual spherical phantom contained a spherical central organ at risk which was surrounded by a PTV shaped like a spherical shell with a gap towards the spherical OAR. We compared three types of plans: (1) non-segmented inversely planned fluence distributions prior to sequencing, (2) plans obtained by direct machine parameter optimization (DMPO) with up to 120 segments (good approximation of non-segmented fluence) and (3) more practical DMPO plans with up to 64 segments. In this study we sought an analytical SS solution for the non-segmented fluence distribution in 4 Pi-geometry. For the CS case Brahme et al found that a special narrow fluence peak ('Brahme peak') has to be applied to improve dose uniformity in PTV areas adjacent to the OAR. We showed that in the SS case the peak was steeper but the area under the peak was smaller. The relevance of the peak decreased for increasing gap between the OAR and the PTV. The plan quality of the non-segmented SS plans was higher albeit the fluence distributions were less uniform. The plan quality of the segmented plans degraded if the allowed number of segments was reduced; the degradation was quicker for the SS beam arrangement than for the CS beam arrangement. For 64 segments, the SS plans delivered less uniform and more conformal dose distributions than the CS plans, ensuring better sparing of the healthy tissue. Also, the SS plans always needed less monitor units than the CS plans. In conclusion, due to substructures or steeper fluence gradients, the improved potential of quasi-isotropic SS-plan quality can only be exploited, if many segments are allowed. SS plans seem to spare normal tissue better. Further analysis of non-coplanar beam arrangements with less degree of symmetry is planned, followed by a study on non-coplanar intensity modulated arc techniques.

Methods for monitor-unit-preserving adaptation of intensity modulated arc therapy techniques to the daily target-A simple comparison.

PURPOSE: For fast adaptation of step and shoot intensity modulated radiotherapy (IMRT) plans, monitor units (MU)-preserving methods which modify only the segment shapes have been proposed in the literature. In this work, two such adaptation methods are applied to intensity modulated arc therapy (IMAT) and their results are compared to that of a newly optimized IMAT plan. METHODS: In a simplified cylindrically symmetric model, the organ at risk (OAR) is surrounded by the planning target volume (PTV). For the initial plan, a steep dose gradient is produced by variants of double arc (IMAT) plans. To simulate situations which require adaptation, the OAR radius and the inner PTV radius have been varied. One adaptation method (Warp) is based on a mesh spanned over structures identified within the beam's eye view (BEV). Changes to the structure projections warp the mesh. For the adaptation, the segment shapes are fixed to the mesh. The other method (2-Step) uses geometrical 3D information from the computed tomography (CT). For comparison, the objective function representing the dose to the PTV as well as the mean and the maximum dose to the OAR is used. RESULTS: For the narrow segments that compensate the underdosage in the PTV areas proximate to the OAR, the Warp method suggests contrary adaptation rules compared to the 2-Step method. In contrast to Warp, the 2-Step method approximates the behavior of a newly optimized plan and leads to better dose homogeneity in the clinical target volume (CTV) and the PTV, whilst simultaneously sparing the OAR. CONCLUSIONS: For minor changes associated with less steep dose gradients, both Warp and 2-Step methods are suitable. However, the 2-Step method should be preferred for more challenging cases, where steep dose gradients between the OAR and the concave PTV are needed. For considerable interfractional reductions of the gap between the OAR and the PTV, where especially steep dose gradients have to be generated, MU-preserving adaptation techniques are not adequate. In this case, narrower segments in the initial plan can be used to facilitate the adaptation. Otherwise, non-MU-preserving adaptation methods have to be applied. Further work is needed to include clinical cases with more complex geometries and expand the methods to IMRT techniques.

Fast IMRT by increasing the beam number and reducing the number of segments.

PURPOSE: The purpose of this work is to develop fast deliverable step and shoot IMRT technique. A reduction in the number of segments should theoretically be possible, whilst simultaneously maintaining plan quality, provided that the reduction is accompanied by an increased number of gantry angles. A benefit of this method is that the segment shaping could be performed during gantry motion, thereby reducing the delivery time. The aim was to find classes of such solutions whose plan quality can compete with conventional IMRT. MATERIALS/METHODS: A planning study was performed. Step and shoot IMRT plans were created using direct machine parameter optimization (DMPO) as a reference. DMPO plans were compared to an IMRT variant having only one segment per angle ("2-Step Fast"). 2-Step Fast is based on a geometrical analysis of the topology of the planning target volume (PTV) and the organs at risk (OAR). A prostate/rectum case, spine metastasis/spinal cord, breast/lung and an artificial PTV/OAR combination of the ESTRO-Quasimodo phantom were used for the study. The composite objective value (COV), a quality score, and plan delivery time were compared. The delivery time for the DMPO reference plan and the 2-Step Fast IMRT technique was measured and calculated for two different linacs, a twelve year old Siemens Primus™ ("old" linac) and two Elekta Synergy™ "S" linacs ("new" linacs). RESULTS: 2-Step Fast had comparable or better quality than the reference DMPO plan. The number of segments was smaller than for the reference plan, the number of gantry angles was between 23 and 34. For the modern linac the delivery time was always smaller than that for the reference plan. The calculated (measured) values showed a mean delivery time reduction of 21% (21%) for the new linac, and of 7% (3%) for the old linac compared to the respective DMPO reference plans. For the old linac, the data handling time per beam was the limiting factor for the treatment time reduction. CONCLUSIONS: 2-Step Fast plans are suited to reduce the delivery time, especially if the data handling time per beam is short. The plan quality can be retained or even increased for fewer segments provided more gantry angles are used.

Fast intensity-modulated arc therapy based on 2-step beam segmentationa.

PURPOSE: Single or few are intensity-modulated arc therapy (IMAT) is intended to be a time saving irradiation method, potentially replacing classical intensity-modulated radiotherapy (IMRT). The aim of this work was to evaluate the quality of different IMAT methods with the potential of fast delivery, which also has the possibility of adapting to the daily shape of the target volume. METHODS: A planning study was performed. Novel double and triple IMAT techniques based on the geometrical analysis of the target organ at risk geometry (2-step IMAT) were evaluated. They were compared to step and shoot IMRT reference plans generated using direct machine parameter optimization (DMPO). Volumetric arc (VMAT) plans from commercial preclinical software (SMARTARC) were used as an additional benchmark to classify the quality of the novel techniques. Four cases with concave planning target volumes (PTV) with one dominating organ at risk (OAR), viz., the PTV/OAR combination of the ESTRO Quasimodo phantom, breast/lung, spine metastasis/ spinal cord, and prostate/rectum, were used for the study. The composite objective value (COV) and other parameters representing the plan quality were studied. RESULTS: The novel 2-step IMAT techniques with geometry based segment definition were as good as or better than DMPO and were superior to the SMARTARC VMAT techniques. For the spine metastasis, the quality measured by the COV differed only by 3%, whereas the COV of the 2-step IMAT for the other three cases decreased by a factor of 1.4-2.4 with respect to the reference plans. CONCLUSIONS: Rotational techniques based on geometrical analysis of the optimization problem (2-step IMAT) provide similar or better plan quality than DMPO or the research version of SMARTARC VMAT variants. The results justify pursuing the goal of fast IMAT adaptation based on 2-step IMAT techniques.

Is ad-hoc plan adaptation based on 2-Step IMRT feasible?

BACKGROUND: The ability of a geometry-based method to expeditiously adapt a "2-Step" step and shoot IMRT plan was explored. Both changes of the geometry of target and organ at risk have to be balanced. A retrospective prostate planning study was performed to investigate the relative benefits of beam segment adaptation to the changes in target and organ at risk coverage. METHODS: Four patients with six planning cases with extraordinarily large deformations of rectum and prostate were chosen for the study. A 9-field IMRT plan (A) using 2-Step IMRT segments was planned on an initial CT study. The plan had to fulfil all the requirements of a conventional high-quality step and shoot IMRT plan. To adapt to changes of the anatomy in a further CT data set, three approaches were considered: the original plan with optimized isocentre position (B), a newly optimized plan (C) and the original plan, adapted using the 2-Step IMRT optimization rules (D). DVH parameters were utilized for quantification of plan quality: D(99) for the CTV and the central planning target volume (PTV), D(95) for an outer PTV, V(95), V(80) and V(50) for rectum and bladder. RESULTS: The adapted plan (D) achieved almost the same target coverage as the newly optimized plan (C). Target coverage for plan B was poor and for the organs at risk, the rectum V(80) was slightly increased. The volume with more than 95% of the target dose (V(95)) was 1.5+/-1.5 cm(3) for the newly optimized plan (C), compared to 2.2+/-1.3 cm(3) for the original plan (A) and 7.2+/-4.8 cm(3) (B) on the first and the second CT, respectively. The adapted plan resulted in 4.3+/-2.1 cm(3) (D), an intermediate dose load to the rectum. All other parameters were comparable for the newly optimized and the adapted plan. CONCLUSIONS: The first results for adaptation of interfractional changes using the 2-Step IMRT algorithm are encouraging. The plans were superior to plans with optimized isocentre position and only marginally inferior to a newly optimized plan.

Remarks on reporting and recording consistent with the ICRU reference dose.

BACKGROUND: ICRU 50/62 provides a framework to facilitate the reporting of external beam radiotherapy treatments from different institutions. A predominant role is played by points that represent "the PTV dose". However, for new techniques like Intensity Modulated Radiotherapy (IMRT) - especially step and shoot IMRT - it is difficult to define a point whose dose can be called "characteristic" of the PTV dose distribution. Therefore different volume based methods of reporting of the prescribed dose are in use worldwide. Several of them were compared regarding their usability for IMRT and compatibility with the ICRU Reference Point dose for conformal radiotherapy (CRT) in this study. METHODS: The dose distributions of 45 arbitrarily chosen volumes treated by CRT plans and 57 volumes treated by IMRT plans were used for comparison. Some of the IMRT methods distinguish the planning target volume (PTV) and its central part PTVx (PTV minus a margin region of x mm). The reporting of dose prescriptions based on mean and median doses together with the dose to 95% of the considered volume (D95) were compared with each other and in respect of a prescription report with the aid of one or several possible ICRU Reference Points. The correlation between all methods was determined using the standard deviation of the ratio of all possible pairs of prescription reports. In addition the effects of boluses and the characteristics of simultaneous integrated boosts (SIB) were examined. RESULTS: Two types of methods result in a high degree of consistency with the hitherto valid ICRU dose reporting concept: the median dose of the PTV and the mean dose to the central part of the PTV (PTVx). The latter is similar to the CTV, if no nested PTVs are used and no patient model surfaces are involved. A reporting of dose prescription using the CTV mean dose tends to overestimate the plateau doses of the lower dose plateaus of SIB plans. PTVx provides the possibility to approach biological effects using the standard deviation of the dose within this volume. CONCLUSION: The authors advocate reporting the PTV median dose or preferably the mean dose of the central dose plateau PTVx as a potential replacement or successor of the ICRU Reference Dose - both usable for CRT and IMRT.

Steep dose gradients for simultaneous integrated boost IMRT.

Steep dose gradients between two planning target volumes (PTVs) as may be required for simultaneous integrated boosts (SIB) should be an option provided by IMRT algorithms. The aim was to analyse the geometry of the SIB problem and to implement the results in an algorithm for IMRT segment generation denoted two-step intensity modulated radiotherapy (2-Step IMRT). It was hypothesized that a gap between segments directed to the inner and the outer PTV would steepen the dose gradient. The mathematical relationships were derived from the individual dose levels and the geometry (diameters) of the PTVs. The results generated by means of 2-Step IMRT segments were equivalent or better than the segment generation using a commercial IMRT planning system. The dose to both the inner and the outer PTV was clearly more homogeneous and the composite objective value was the lowest. The segment numbers were lower or equal--with better sparing of the surrounding tissue. In summary, it was demonstrated that 2-Step IMRT was able to achieve steep dose gradients for SIB constellations.