Mapping Local Failure Following Bladder Radiotherapy According to Dose.

AIMS: To determine the relationship between local relapse following radical radiotherapy for muscle-invasive bladder cancer (MIBC) and radiation dose. MATERIALS AND METHODS: Patients with T2-4N0-3M0 MIBC were recruited to a phase II study assessing the feasibility of intensity-modulated radiotherapy to the bladder and pelvic lymph nodes. Patients were planned to receive 64 Gy/32 fractions to the bladder tumour, 60 Gy/32 fractions to the involved pelvic nodes and 52 Gy/32 fractions to the uninvolved bladder and pelvic nodes. Pre-treatment set-up was informed by cone-beam CT. For patients who experienced local relapse, cystoscopy and imaging (CT/MRI) was used to reconstruct the relapse gross tumour volume (GTVrelapse) on the original planning CT . GTVrelapse D98% and D95% was determined by co-registering the relapse image to the planning CT utilising deformable image registration (DIR) and rigid image registration (RIR). Failure was classified into five types based on spatial and dosimetric criteria as follows: A (central high-dose failure), B (peripheral high-dose failure), C (central elective dose failure), D (peripheral elective dose failure) and E (extraneous dose failure). RESULTS: Between June 2009 and November 2012, 38 patients were recruited. Following treatment, 18/38 (47%) patients experienced local relapse within the bladder. The median time to local relapse was 9.0 months (95% confidence interval 6.3-11.7). Seventeen of 18 patients were evaluable based on the availability of cross-sectional relapse imaging. A significant difference between DIR and RIR methods was seen. With the DIR approach, the median GTVrelapse D98% and D95% was 97% and 98% of prescribed dose, respectively. Eleven of 17 (65%) patients experienced type A failure and 6/17 (35%) patients type B failure. No patients had type C, D or E failure. MIBC failure occurred in 10/17 (59%) relapsed patients; of those, 7/11 (64%) had type A failure and 3/6 (50%) had type B failure. Non-MIBC failure occurred in 7/17 (41%) patients; 4/11 (36%) with type A failure and 3/6 (50%) with type B failure. CONCLUSION: Relapse following radiotherapy occurred within close proximity to the original bladder tumour volume and within the planned high-dose region, suggesting possible biological causes for failure. We advise caution when considering margin reduction for future reduced high-dose radiation volume or partial bladder radiotherapy protocols.

A Randomised Phase II Clinical Trial Comparing the Deliverability and Acute Toxicity of Wide Tangent versus Volumetric Modulated Arc Therapy to the Breast and Internal Mammary Chain.

AIMS: Inclusion of the internal mammary chain in the radiotherapy target volume (IMC-RT) improves disease-free and overall survival in higher risk breast cancer patients, but increases radiation doses to heart and lungs. Dosimetric data show that either modified wide-tangential fields (WT) or volumetric modulated arc therapy (VMAT) together with [AQ1]voluntary deep inspiration breath hold (vDIBH) keep mean heart doses below 4 Gy in most patients. However, the impact on departmental resources has not yet been documented. This phase II clinical trial compared the time taken to deliver IMC-RT using either WT and vDIBH or VMAT and vDIBH, together with planning time, dosimetry, set-up reproducibility and toxicity. MATERIALS AND METHODS: Left-sided breast cancer patients requiring IMC-RT were randomised to receive either WT(vDIBH) or VMAT radiotherapy. The primary outcome was treatment time, powered to detect a minimum difference of 75 min (5 min/fraction) between techniques. The population mean displacement, systematic error and random error for cone beam computed tomography chest wall matches in three directions of movement were calculated. Target volume and organ at risk doses were compared between groups. Side-effects, including skin (Radiation Therapy Oncology Group), lung and oesophageal toxicity (Common Terminology Criteria for Adverse Events v 4.03) rates, were compared between the groups over 3 months. Patient-reported outcome measures, including shoulder toxicity at baseline, 6 months and 1 year, were compared. RESULTS: Twenty-one patients were recruited from a single UK centre between February 2017 and January 2018. The mean (standard deviation) total treatment time per fraction for VMAT treatments was 13.2 min (1.7 min) compared with 28.1 min (3.3 min) for WT(vDIBH). There were no statistically significant differences in patient set-up errors in between groups. The average mean heart dose for WT(vDIBH) was 2.6 Gy compared with 3.4 Gy for VMAT(vDIBH) (P = 0.13). The mean ipsilateral lung V17Gy was 32.8% in the WT(vDIBH) group versus 34.4% in the VMAT group (P = 0.2). The humeral head (mean dose 16.8 Gy versus 2.8 Gy), oesophagus (maximum dose 37.3 Gy versus 20.1 Gy) and thyroid (mean dose 22.0 Gy versus 11.2 Gy) all received a statistically significantly higher dose in the VMAT group. There were no statistically significant differences in skin, lung or oesophageal toxicity within 3 months of treatment. Patient-reported outcomes of shoulder toxicity, pain, fatigue, breathlessness and breast symptoms were similar between groups at 1 year. CONCLUSION: VMAT(vDIBH) and WT(vDIBH) are feasible options for locoregional breast radiotherapy including the IMC. VMAT improves nodal coverage and delivers treatment more quickly, resulting in less breath holds for the patient. This is at the cost of increased dose to some non-target tissues. The latter does not appear to translate into increased toxicity in this small study.

Image-guided Adaptive Radiotherapy for Bladder Cancer.

Technological advancement has facilitated patient-specific radiotherapy in bladder cancer. This has been made possible by developments in image-guided radiotherapy (IGRT). Particularly transformative has been the integration of volumetric imaging into the workflow. The ability to visualise the bladder target using cone beam computed tomography and magnetic resonance imaging initially assisted with determining the magnitude of inter- and intra-fraction target change. It has led to greater confidence in ascertaining true anatomy at each fraction. The increased certainty of dose delivered to the bladder has permitted the safe reduction of planning target volume margins. IGRT has therefore improved target coverage with a reduction in integral dose to the surrounding tissue. Use of IGRT to feed back into plan and dose delivery optimisation according to the anatomy of the day has enabled adaptive radiotherapy bladder solutions. Here we undertake a review of the stepwise developments underpinning IGRT and adaptive radiotherapy strategies for external beam bladder cancer radiotherapy. We present the evidence in accordance with the framework for systematic clinical evaluation of technical innovations in radiation oncology (R-IDEAL).

The Development of Therapeutic Radiographers in Imaging and Adaptive Radiotherapy Through Clinical Trial Quality Assurance.

AIMS: Adaptive radiotherapy (ART) is an emerging advanced treatment option for bladder cancer patients. Therapeutic radiographers (RTTs) are central to the successful delivery of this treatment. The purpose of this work was to evaluate the image-guided radiotherapy (IGRT) and ART experience of RTTs before participating in the RAIDER trial. A plan of the day (PoD) quality assurance programme was then implemented. Finally, the post-trial experience of RTTs was evaluated, together with the impact of trial quality assurance participation on their routine practice. MATERIALS AND METHODS: A pre-trial questionnaire to assess the experience of the RTT staff group in IGRT and ART in bladder cancer was sent to each centre. Responses were grouped according to experience. The PoD quality assurance programme was implemented, and the RAIDER trial commenced. During stage 1 of the trial, RTTs reported difficulties in delivering PoD and the quality assurance programme was updated accordingly. A follow-up questionnaire was sent assessing experience in IGRT and ART post-trial. Any changes in routine practice were also recorded. RESULTS: The experience of RTTs in IGRT and ART pre-trial varied. For centres deemed to have RTTs with more experience, the initial PoD quality assurance programme was streamlined. For RTTs without ART experience, the full quality assurance programme was implemented, of which 508 RTTs completed. The quality assurance programme was updated (as the trial recruited) and it was mandated that at least one representative RTT (regardless of pre-trial experience) participated in the update in real-time. The purpose of the updated quality assurance programme was to provide further support to RTTs in delivering a complex treatment. Engagement with the updated quality assurance programme was high, with RTTs in 24/33 centres participating in the real-time online workshop. All 33 UK centres reported all RTTs reviewed the updated training offline. Post-trial, the RTTs' experience in IGRT and ART was increased. CONCLUSION: Overall, 508 RTTs undertook the PoD quality assurance programme. There was a high engagement of RTTs in the PoD quality assurance programme and trial. RTTs increased their experience in IGRT and ART and subsequently updated their practice for bladder cancer and other treatment sites.

Adaptive Radiotherapy Enabled by MRI Guidance.

Adaptive radiotherapy (ART) strategies systematically monitor variations in target and neighbouring structures to inform treatment-plan modification during radiotherapy. This is necessary because a single plan designed before treatment is insufficient to capture the actual dose delivered to the target and adjacent critical structures during the course of radiotherapy. Magnetic resonance imaging (MRI) provides superior soft-tissue image contrast over current standard X-ray-based technologies without additional radiation exposure. With integrated MRI and radiotherapy platforms permitting motion monitoring during treatment delivery, it is possible that adaption can be informed by real-time anatomical imaging. This allows greater treatment accuracy in terms of dose delivered to target with smaller, individualised treatment margins. The use of functional MRI sequences would permit ART to be informed by imaging biomarkers, so allowing both personalised geometric and biological adaption. In this review, we discuss ART solutions enabled by MRI guidance and its potential gains for our patients across tumour types.

Proposed genitalia contouring guidelines in anal cancer intensity-modulated radiotherapy.

OBJECTIVE: Intensity-modulated radiotherapy (IMRT) for anal canal carcinoma (ACC) is associated with favourable toxicity outcomes. Side effects include sexual dysfunction, skin desquamation, pain and fibrosis to perineum and genitalia region. The genitalia are situated anterior to the primary ACC between two inguinal regions providing a challenging structure to avoid. Techniques improving outcomes require robust, consistent genitalia contouring to ensure standardization and production of fully optimized IMRT plans. Official recommendations for genitalia contouring are lacking. We describe a potential genitalia contouring atlas for ACC radiotherapy. METHODS: Following a review of genitalia CT anatomy, a contouring atlas was generated for male and female patients positioned prone and supine. Particular attention was paid to the reproducibility of the genitalia contour in all planes. RESULTS: Male and female genitalia positioned prone and supine are described and represented visually through a contouring atlas. Contoured areas in males include penis and scrotum, and in females include clitoris, labia majora and minora. The muscles, bone, prostate, vagina, cervix and uterus should be excluded. The genitalia contour extends laterally to inguinal creases and includes areas of fat and skin anterior to the symphysis pubis for both genders. CONCLUSION: This atlas provides descriptive and visual guidance enabling more consistent genitalia delineation for both genders when prone and supine. The atlas can be used for other sites requiring radiotherapy planning. ADVANCES IN KNOWLEDGE: This atlas presents visual contouring guidance for genitalia in ACC radiotherapy for the first time. Contouring methods provide reproducible genitalia contours that allow the provision of accurate dose toxicity data in future studies.

Radiographer-led plan selection for bladder cancer radiotherapy: initiating a training programme and maintaining competency.

OBJECTIVE: The implementation of plan of the day selection for patients receiving radiotherapy (RT) for bladder cancer requires efficient and confident decision-making. This article describes the development of a training programme and maintenance of competency. METHODS: Cone beam CT (CBCT) images acquired on patients receiving RT for bladder cancer were assessed to establish baseline competency and training needs. A training programme was implemented, and observers were asked to select planning target volumes (PTVs) on two groups of 20 patients' images. After clinical implementation, the PTVs chosen were reviewed offline, and an audit performed after 3 years. RESULTS: A mean of 73% (range, 53-93%) concordance rate was achieved prior to training. Subsequent to training, the mean score decreased to 66% (Round 1), then increased to 76% (Round 2). Six radiographers and two clinicians successfully completed the training programme. An independent observer reviewed the images offline after clinical implementation, and a 91% (126/139) concordance rate was achieved. During the audit, 125 CBCT images from 13 patients were reviewed by a single observer and concordance was 92%. CONCLUSION: Radiographer-led selection of plan of the day was implemented successfully with the use of a training programme and continual assessment. Quality has been maintained over a period of 3 years. ADVANCES IN KNOWLEDGE: The training programme was successful in achieving and maintaining competency for a plan of the day technique.

Clinical implementation of adaptive hypofractionated bladder radiotherapy for improvement in normal tissue irradiation.

AIMS: Adaptive bladder radiotherapy, with plan of the day selection and plan library development based on individual filling patterns, has been previously modelled in patients receiving weekly hypofractionated treatment and improved geometric accuracy has been shown. The aim of this study was to assess the clinical implementation of the technique. MATERIALS AND METHODS: Conformal plans (with small, intermediate and large planning target volumes) were developed for 25 patients. After pre-treatment cone-beam computed tomography, the optimal plan of the day was selected and delivered by two trained observers. Independent off-line plan selection was also carried out. Concordance between the on-line and off-line selections, frequency of plan usage, target coverage and normal tissue sparing were assessed. RESULTS: Plan selection concordance was 91%. Fifty-five per cent of fractions were delivered using small or large plans. The mean coverage of the clinical target volume by the 95% isodose was 99%. The mean reduction in the volume of normal tissue treated to 95% of the prescription dose was 219 cm(3) compared with the previous institutional standard approach. CONCLUSIONS: Good concordance in plan selection is shown with clinical implementation of the adaptive strategy. Adequate target coverage was achieved with reduction in the volume of normal tissue irradiated to a high dose compared with the previous standard approach.

Organ-sparing Intensity-modulated radiotherapy for anal cancer using the ACTII schedule: a comparison of conventional and intensity-modulated radiotherapy plans.

AIMS: Conventional external beam radiotherapy for anal cancer is associated with a high rate of treatment-related morbidity. The purpose of this retrospective study was to compare the dosimetric advantages of three intensity-modulated radiotherapy (IMRT) plans with the conventional plan with regards to organs at risk avoidance delivering the ACTII schedule of 50.4 Gy in 1.8 Gy/fraction: 17 fractions for phase 1 and 11 fractions for phase 2. MATERIALS AND METHODS: Ten anal cancer patients (T1-3 N0-3) treated with the conventional plan using four fields and conformal boost were identified. The phase 1 planning target volume (PTV) included tumour, anal canal and inguinal, peri-rectal and internal/external iliac nodes. Phase 2 included identifiable disease only. Three step-and-shoot IMRT plans were generated: IMRT1: phase 1 inverse-planned IMRT with two- to four-field conformal phase 2; IMRT2: both phase 1 and phase 2 inverse-planned IMRT; IMRT3: phase 1 IMRT and phase 2 forward-planned IMRT. All IMRT plans were then compared against the conventional plan on PTV coverage, small bowel, genitalia, femoral heads, bladder and healthy tissue dose volume information. RESULTS: While achieving similar PTV coverage compared with the conventional plan, significant dose reductions were observed for IMRT plans in external genitalia, small bowel and healthy tissue. Reductions were also observed in the femoral heads and bladder. CONCLUSIONS: IMRT significantly reduces the dose to organs at risk while maintaining excellent PTV coverage in anal cancer radiotherapy.

Kilovoltage energy imaging with a radiotherapy linac with a continuously variable energy range.

PURPOSE: In this paper, the effect on image quality of significantly reducing the primary electron energy of a radiotherapy accelerator is investigated using a novel waveguide test piece. The waveguide contains a novel variable coupling device (rotovane), allowing for a wide continuously variable energy range of between 1.4 and 9 MeV suitable for both imaging and therapy. METHOD: Imaging at linac accelerating potentials close to 1 MV was investigated experimentally and via Monte Carlo simulations. An imaging beam line was designed, and planar and cone beam computed tomography images were obtained to enable qualitative and quantitative comparisons with kilovoltage and megavoltage imaging systems. The imaging beam had an electron energy of 1.4 MeV, which was incident on a water cooled electron window consisting of stainless steel, a 5 mm carbon electron absorber and 2.5 mm aluminium filtration. Images were acquired with an amorphous silicon detector sensitive to diagnostic x-ray energies. RESULTS: The x-ray beam had an average energy of 220 keV and half value layer of 5.9 mm of copper. Cone beam CT images with the same contrast to noise ratio as a gantry mounted kilovoltage imaging system were obtained with doses as low as 2 cGy. This dose is equivalent to a single 6 MV portal image. While 12 times higher than a 100 kVp CBCT system (Elekta XVI), this dose is 140 times lower than a 6 MV cone beam imaging system and 6 times lower than previously published LowZ imaging beams operating at higher (4-5 MeV) energies. CONCLUSIONS: The novel coupling device provides for a wide range of electron energies that are suitable for kilovoltage quality imaging and therapy. The imaging system provides high contrast images from the therapy portal at low dose, approaching that of gantry mounted kilovoltage x-ray systems. Additionally, the system provides low dose imaging directly from the therapy portal, potentially allowing for target tracking during radiotherapy treatment. There is the scope with such a tuneable system for further energy reduction and subsequent improvement in image quality.

Paraspinal volumetric modulated arc therapy.

OBJECTIVES: The processes involved in the treatment of paraspinal tumours by volumetric modulated arc therapy (VMAT) are described here by means of an illustrative case. METHODS: Az single anticlockwise arc from gantry angle 179° to 181° was constructed using SmartArc (Philips Radiation Oncology Systems, Fitchburg, WI) with control points spaced at 2°. The dose prescription was 60 Gy in 30 fractions to cover the planning target volume (PTV) as uniformly as possible while sparing the 0.3-cm planning risk volume (PRV) around the spinal cord. The plan was verified before treatment using a diode array phantom and radiochromic film. Treatment delivery was on a Synergy linear accelerator with a beam modulator head (Elekta Ltd, Crawley, UK). RESULTS: Homogeneous dose coverage of the PTV was achieved with a D(2%) of 62.0 Gy and D(98%) of 55.6 Gy. Maximum spinal cord dose was 49.9 Gy to 0.1 cm(3) and maximum dose to the spinal cord PRV was 55.4 Gy to 0.1 cm(3). At pre-treatment verification, the percentage of the high-dose region receiving a dose within 3% and 3 mm of the planned dose was 98.8% with the diode array and 93.4% with film. Delivery time was 2 min 15 s and the course of treatment was successfully completed. CONCLUSIONS: VMAT was successfully planned, verified and delivered for this challenging tumour site. VMAT provides a very suitable method of treating complex paraspinal tumours, offering a high-quality conformal dose distribution with a short delivery time.

Comparative study of a low-Z cone-beam computed tomography system.

Computed tomography images have been acquired using an experimental (low atomic number (Z) insert) megavoltage cone-beam imaging system. These images have been compared with standard megavoltage and kilovoltage imaging systems. The experimental system requires a simple modification to the 4 MeV electron beam from an Elekta Precise linac. Low-energy photons are produced in the standard medium-Z electron window and a low-Z carbon electron absorber located after the window. The carbon electron absorber produces photons as well as ensuring that all remaining electrons from the source are removed. A detector sensitive to diagnostic x-ray energies is also employed. Quantitative assessment of cone-beam computed tomography (CBCT) contrast shows that the low-Z imaging system is an order of magnitude or more superior to a standard 6 MV imaging system. CBCT data with the same contrast-to-noise ratio as a kilovoltage imaging system (0.15 cGy) can be obtained in doses of 11 and 244 cGy for the experimental and standard 6 MV systems, respectively. Whilst these doses are high for everyday imaging, qualitative images indicate that kilovoltage like images suitable for patient positioning can be acquired in radiation doses of 1-8 cGy with the experimental low-Z system.

An efficient Monte Carlo-based algorithm for scatter correction in keV cone-beam CT.

A new method is proposed for scatter-correction of cone-beam CT images. A coarse reconstruction is used in initial iteration steps. Modelling of the x-ray tube spectra and detector response are included in the algorithm. Photon diffusion inside the imaging subject is calculated using the Monte Carlo method. Photon scoring at the detector is calculated using forced detection to a fixed set of node points. The scatter profiles are then obtained by linear interpolation. The algorithm is referred to as the coarse reconstruction and fixed detection (CRFD) technique. Scatter predictions are quantitatively validated against a widely used general-purpose Monte Carlo code: BEAMnrc/EGSnrc (NRCC, Canada). Agreement is excellent. The CRFD algorithm was applied to projection data acquired with a Synergy XVI CBCT unit (Elekta Limited, Crawley, UK), using RANDO and Catphan phantoms (The Phantom Laboratory, Salem NY, USA). The algorithm was shown to be effective in removing scatter-induced artefacts from CBCT images, and took as little as 2 min on a desktop PC. Image uniformity was greatly improved as was CT-number accuracy in reconstructions. This latter improvement was less marked where the expected CT-number of a material was very different to the background material in which it was embedded.

A low Z linac and flat panel imager: comparison with the conventional imaging approach.

Experimental and Monte Carlo simulations were conducted for an Elekta Ltd Precise Treatment System linac fitted with a low Z insert of sufficient thickness to remove all primary electrons. A variety of amorphous silicon based panels employing different scintillators were modelled to determine their response to a variety of x-ray spectra and produce an optimized portal imaging system. This study has shown that in a low Z configuration the vast majority of x-rays are produced in the nickel electron window, and with a combination of a carbon insert and caesium iodide based XVI-panel, significant improvement in the object contrast was achieved. For thin, head and neck-type geometries, contrast is 4.62 times greater for 1.6 cm bone in 5.8 cm water than the standard 6 MV/iViewGT system. For thicker, pelvis-type geometries contrast increases by a factor of 1.3 for 1.6 cm of bone in 25.8 cm water. To obtain images with the same signal-to-noise ratio as the 6 MV/iViewGT system, dose reductions of a factor of 15 and 4.2 are possible for 5.8 cm and 25.8 cm phantoms respectively. This design has the advantage of being easily implemented on a standard linac and provides a portal image directly from the therapy beam aperture.

The use of gel dosimetry for verification of electron and photon treatment plans in carcinoma of the scalp.

In recent years there has been a large amount of research into the potential use of radiation sensitive gels for three-dimensional verification of clinical radiotherapy doses. In this paper we report the use of a MAGIC gel dosimeter (Fong et al 2001 Phys. Med. Biol. 46 3105) for the verification of a specific patient's radiation therapy dose distribution. A 69-year-old male patient presented with a squamous cell carcinoma extending approximately 180 degrees across the top of the scalp (anterior to posterior) and from just over midline to 90 degrees left of the skull. The patient's treatment was commenced using two electron fields. For gel dosimetry, phantoms were produced in which the outer surface spatially corresponded to the outer contours of the patient's anatomy in the region of irradiation. The phantoms were treated with either electrons or intensity modulated radiation therapy (IMRT) with photons. The results identified a hot spot between the matched electron fields and confirmed the more homogeneous dose distribution produced by the IMRT planning system. The IMRT plan was then clinically implemented. The application of a clinical dose to a phantom shaped to a specific patient as well as the ability to select a slice at will during phantom imaging means that gel dosimetry can no longer be considered to simply have potential alone, but is now in fact a useful dosimetric tool.

An evaluation of Beam cath in the verification process for prostate cancer radiotherapy.

AIMS: As the trend towards more conformal treatment continues, the accuracy of treatment delivery becomes more important. Conventionally, treatment set-up for prostate cancer patients is verified in relation to the bony anatomy. However, there can be prostate movement independent of bony anatomy. This study tested the feasibility of using Beam cath to enable online correction of treatment set-up in relation to the prostate position, and to assess inter-fraction and intra-fraction prostate movement. MATERIALS AND METHODS: Beam cath is a urethral catheter containing radio-opaque markers, which can be seen on electronic portal imaging, enabling verification of prostate rather than bony anatomy position. The Beam cath was used for planning and treatment of a boost phase of 10 Gy in 5 fractions, delivered before the conventional conformal plan of 60 Gy in 30 fractions. Patients were scanned by computed tomgography (CT), with and without the catheter, and a radio-opaque marker in the catheter was used as the isocentre of the boost phase to enable accurate and rapid pre-treatment isocentre adjustment. The set-up errors between the Beam Cath and bony images were compared to identify the magnitude of prostate movement, independent of bony anatomy. Post-treatment portal images were taken to assess intra-fraction prostate movement. RESULTS: Of 29 patients approached to take part in the study, 18 patients gave informed consent, but only five completed the intended 5 fractions of the boost phase using Beam cath. Pre- and post-treatment portal images were obtained for a total of 29 fractions in six patients. Inter-fraction prostate movement, independent of bony anatomy, was identified. The mean movements were 0.2 mm (standard deviation [SD] 1.2 mm), 2.9 mm (SD 3.1 mm) and 0.7 mm (SD 2.3 mm) in the right left (RL), cranio-caudal (CC) and anterior posterior (AP) direction, respectively. The mean intra-fraction movement was 0.2 mm (SD 1.2 mm), 2.9 mm (SD 3.1 mm) and 0.7 mm (SD 2.3 mm) in the RL, CC and AP direction, respectively. CONCLUSION: Although independent prostate movement was identified, the use of Beam cath was poorly tolerated. Alternative methods of identifying and correcting for prostate movement should be investigated.

The delivery of intensity modulated radiotherapy to the breast using multiple static fields.

BACKGROUND AND PURPOSE: To develop a method of using a multileaf collimator (MLC) to deliver intensity modulated radiotherapy (IMRT) for tangential breast fields, using an MLC to deliver a set of multiple static fields (MSFs). MATERIALS AND METHODS: An electronic portal imaging device (EPID) is used to obtain thickness maps of medial and lateral tangential breast fields. From these IMRT deliveries are designed to minimize the volume of breast above 105% of prescribed dose. The deliveries are universally-wedged beams augmented with a set of low dose shaped irradiations. Dosimetric and planning QA of this method has been compared with the standard, wedged treatment and the corresponding treatment using physical compensators. Several options for delivering the MSF treatment are presented. RESULTS: The MSF technique was found to be superior to the standard technique (P value=0.002) and comparable with the compensated technique. Both IMRT methods reduced the volume of breast above 105% dose from a mean value of 12.0% of the total breast volume to approximately 2.8% of the total breast volume. CONCLUSIONS: This MSF method may be used to reduce the high dose volume in tangential breast irradiation significantly. This may have consequences for long-term side effects, particularly cosmesis.

Direct measurement and analytical modeling of scatter in portal imaging.

In this study a direct measurement of scatter in portal imaging for various air gaps and scatterer thicknesses at a beam energy of 6 MV is presented. The experimental data are compared with results from a Monte Carlo (MC) scatter model. In the regime where the air gap is larger than 9.3 cm the MC and the experiment agree. Based on this MC model an analytical model is developed, which takes all important interaction processes into account. It comprises a rigorous treatment of first order scattering and an estimation of photons scattered more than once within the phantom. This estimation is based on the assumption that higher order scattering can be considered as isotropically distributed around a certain scatter origin located in the midplane of the phantom. It is found that relative deviations between the MC model and the analytical model are of 2% to 3% in regions where scattering is very large.

The derivation of tissue-maximum ratio from percentage depth dose requires peak scatter factor to be considered a function of source-to-surface distance.

A formula for the calculation of tissue-maximum ratio (TMR) from percentage depth dose (PDD) and peak scatter factor (PSF) is derived from first principles using a simple geometric model for the case when the field size for PDD and PSF is defined at the surface. The derivation is carried out in two ways: (a) taking field size for PDD and PSF as defined at the depth of maximum dose and then applying a conversion factor, and (b) by a direct derivation. The first of these methods yields a formula which agrees with BJR Supplement 25, but the latter yields a result which differs from it. Numerically, this difference is insignificant, but it has implications for the theoretical basis of the conversion formulae. The difference arises due to the translation of field size from one depth to another when calculating PSF: two different values of source-to-surface distance (SSD) yield two apparently different PSFs for the same size of field at the depth of maximum dose. Disagreements of this type are prevalent throughout the standard conversion formulae given in BJR Supplement 25 when field size for PDD and PSF is defined at the surface rather than at the depth of dose maximum. These disagreements are illustrated here using the conversion of PDD from one SSD to another as an example. The difficulty is overcome by considering PSF to be a function of SSD as well as field size.

Quality assurance of the dose delivered by small radiation segments.

The use of intensity modulation with multiple static fields has been suggested by many authors as a way to achieve highly conformal fields in radiotherapy. However, quality assurance of linear accelerators is generally done only for beam segments of 100 MU or higher, and by measuring beam profiles once the beam has stabilized. We propose a set of measurements to check the stability of dose delivery in small segments, and present measured data from three radiotherapy centres. The dose delivered per monitor unit, MU, was measured for various numbers of MU segments. The field flatness and symmetry were measured using either photographic films that are subsequently scanned by a densitometer, or by using a diode array. We performed the set of measurements at the three radiotherapy centres on a set of five different Philips SL accelerators with energies of 6 MV, 8 MV, 10 MV and 18 MV. The dose per monitor unit over the range of 1 to 100 MU was found to be accurate to within +/-5% of the nominal dose per monitor unit as defined for the delivery of 100 MU for all the energies. For four out of the five accelerators the dose per monitor unit over the same range was even found to be accurate to within +/-2%. The flatness and symmetry were in some cases found to be larger for small segments by a maximum of 9% of the flatness/symmetry for large segments. The result of this study provides the dosimetric evidence that the delivery of small segment doses as top-up fields for beam intensity modulation is feasible. However, it should be stressed that linear accelerators have different characteristics for the delivery of small segments, hence this type of measurement should be performed for each machine before the delivery of small dose segments is approved. In some cases it may be advisable to use a low pulse repetition frequency (PRF) to obtain more accurate dose delivery of small segments.