Experience With Normal Breathhold Planning Scans for Radiosurgery of Moving Targets With Live Tracking.

PURPOSE: Utilization of breathhold scans with live tracking has a long track record of good published outcomes for stereotactic body radiation therapy (SBRT) and is recommended by the manufacturer of the Synchrony tracking system. However, the popularity of four-dimensional computed tomography (4DCT) scans challenges the validity of the breathhold scan with live tracking technique. Although this study is not intended to prove the superiority of either method, we demonstrate the feasibility of using the breathhold scans with a phantom test and clinical examples. METHODS: A 4DCT of a perfect sphere was scanned at 20 breaths per minute and compared to a 4DCT of a small lung tumor in one patient and a 4DCT of a larger renal tumor in another patient, as well as to fiducial matching in a patient with pancreatic cancer. Normal exhale and normal inhale breathhold CT scans were performed for the pancreatic cancer patient, combined with Synchrony tracking on CyberKnife (Sunnyvale, CA: Accuray) for treatment. RESULTS: The 4DCT scan of the phantom exhibited considerable apparent deformation, which must be entirely due to imaging artifact since the perfect sphere in the phantom is known to be completely rigid. The 4DCT of the lung and renal tumors in patients had similar apparent deformation. Usually in patients, from 4DCT alone, it is difficult to determine how much was due to deformation and how much was due to artifact. Fiducial positions in the final normal exhale and normal inhale breathhold scans for Synchrony matched each other within 1mm for the pancreatic cancer patient. CONCLUSION: We demonstrated the feasibility of breathhold scans with Synchrony live tracking, as recommended by the manufacturer. More studies will be needed to determine whether this method is better than using a 4DCT.

Results from the AAPM Task Group 324 respiratory motion management in radiation oncology survey.

PURPOSE: To quantify the clinical practice of respiratory motion management in radiation oncology. METHODS: A respiratory motion management survey was designed and conducted based on clinician survey guidelines. The survey was administered to American Association of Physicists in Medicine (AAPM) members on 17 August 2020 and closed on 13 September 2020. RESULTS: A total of 527 respondents completed the entire survey and 651 respondents completed part of the survey, with the partially completed surveys included in the analysis. Overall, 84% of survey respondents used deep inspiration breath hold for left-sided breast cancer. Overall, 83% of respondents perceived respiratory motion management for thoracic and abdominal cancer radiotherapy patients to be either very important or required. Overall, 95% of respondents used respiratory motion management for thoracic and abdominal sites, with 36% of respondents using respiratory motion management for at least 90% of thoracic and abdominal patients. The majority (60%) of respondents used the internal target volume method to treat thoracic and abdominal cancer patients, with 25% using breath hold or abdominal compression and 13% using gating or tracking. CONCLUSIONS: A respiratory motion management survey has been completed by AAPM members. Respiratory motion management is generally considered very important or required and is widely used for breast, thoracic, and abdominal cancer treatments.

Resolution of Radiation-Induced Necrosis in Arteriovenous Malformation with Bevacizumab: A Case Report and Review of Current Literature.

Stereotactic radiosurgery (SRS) is a proven treatment modality for inoperable arteriovenous malformations (AVMs). However, the rate of radiation-induced necrosis (RIN) is as high as 10%. A 6-year-old female patient presented with severe headache, emesis, and syncope, and workup revealed a Spetzler-Martin grade 4 AVM with intraventricular hemorrhage and hydrocephalus. The patient underwent a right frontal ventriculostomy followed by a linear accelerator-based SRS of 16.9 Gy. At 19 years, she developed progressive neurological symptoms. Diagnostic magnetic resonance imaging (MRI) revealed a recurrent parietal AVM nidus. We delivered the linear accelerator-based SRS of 18.5 Gy to the AVM nidus. Within 9 months, she experienced episodic headaches and left-sided weakness and spasticity; symptoms were initially managed with dexamethasone. Follow-up MRI was notable for edema and nondetectable blood flow, consistent with RIN and AVM obliteration. The second course of steroids did not provide the symptom control. Persistent RIN was noted on MRI, and she had stigmata of steroid toxicity (centripetal obesity, depression, and sleep disorder). Two infusions of bevacizumab (5 mg/kg) were administered concurrently with a tapering dose of dexamethasone. The patient noted a near immediate improvement in her headaches, and 2 months following the second bevacizumab infusion, she reported a near-complete resolution of her symptoms and displayed improved ambulation. The development of RIN remains a noteworthy concern post-SRS of AVMs. While steroids aid with initial management of RIN, for persistent and recurrent symptoms, bevacizumab infusions serve as a viable treatment course, with the added benefit of reducing the likelihood of adverse effects resulting from prolonged steroid therapy.

Contrast-Enhanced Ultrasound to Detect Early Microvascular Changes in Skeletal Muscle after High-Dose Radiation Treatment.

The biological response of normal tissue to high-dose radiation treatment remains poorly understood. Alterations to the microenvironment, specifically the microvasculature, have been implicated as a significant contributor to tumoral cytotoxicity. We used contrast-enhanced ultrasound (CEU) perfusion imaging, which is uniquely suited to assess functional status of the microcirculation, to measure microvascular blood flow after high-dose irradiation to normal skeletal muscle tissue in a murine model. Proximal hindlimbs of wild-type C57Bl/6 mice were irradiated with a single fraction using 6 MV photons, 1 cm bolus and a dynamic wedge. Quantitative perfusion CEU imaging of the skeletal muscle was performed at days 1 and 8 postirradiation in three different regions of interest (ROIs): 1. 15 Gy external-beam irradiated leg; 2. 12 Gy irradiated 5 mm proximal area; 3. single ROI in the nonirradiated contralateral (CL) hindlimb. Perfusion imaging was also performed in the hindlimb of nonirradiated mice. CEU time-intensity data were analyzed to measure microvascular blood flow (MBF, also referred to as perfusion), and its parametric components of microvascular flux rate and functional microvascular blood volume (MBV). Plasma measurements of two potent vasoconstrictors, endothelin-1 and angiotensin II, were also performed to assess systemic response. CEU perfusion imaging values for the 12 and 15 Gy irradiated limb regions were pooled. At day 1, MBF in the irradiated limb was significantly lower than in the CL limb (P = 0.016) but quite similar to the nonirradiated mice. At day 8, both limbs of irradiated mice exhibited a trend towards lower MBF than the limbs of nonirradiated mice (28% decrease in mean MBF, P = 0.149 for CL; 39% decrease, P = 0.065 for irradiated limb). Compared to nonirradiated animals, the reduction in perfusion in irradiated limbs at day 8 may have been more influenced by the microvascular flux rate (25% decrease in the mean, P = 0.079) than the MBV (12% decrease in the mean, P = 0.328). Examination of vasoactive compounds revealed that the average plasma concentration for endothelin-1 at day 8 postirradiation was significantly higher in 14 irradiated animals than in 4 nonirradiated animals (3.07 pg/ ml vs. 2.51 pg/ml; P = 0.011). Up to day 8 after high-dose irradiation, flow deficits in irradiated muscle appear to be a consequence of increased vascular resistance more so than loss or functional de-recruitment of microvascular units.

Safety and Efficacy of Accelerated Hypofractionation and Stereotactic Body Radiation Therapy for Hepatocellular Carcinoma Patients With Varying Degrees of Hepatic Impairment.

PURPOSE: To report the toxicities and outcomes for stereotactic body radiation therapy (SBRT) and accelerated hypofractionated radiation therapy (AHRT) in patients with Child-Pugh (CP) class A, B, or C and albumin-bilirubin (ALBI) score 1, 2, or 3 hepatocellular carcinoma. METHODS AND MATERIALS: We retrospectively reviewed the data from 146 patients with hepatocellular carcinoma who had undergone SBRT (50 Gy in 5 fractions) or AHRT (45 Gy in 18 fractions). The primary endpoint was liver toxicity, defined as an increase in the CP score of ≥2 within 6 months of radiation therapy. The secondary endpoints of ALBI change, overall survival, and local control were also calculated. RESULTS: The median follow-up was 23 months (range 1-59). Most received SBRT (72%), and 28% received AHRT. Of all 146 patients, 45 (31%) had a CP score elevation of ≥2 within 6 months of radiation therapy (RT) (27 patients [28%] with baseline CP-A/B7 and 18 [35%] with baseline CP-B8/B9/C cirrhosis; P = .45). On multivariate analysis, neither baseline CP nor ALBI score was predictive of toxicity. No patient with a decline in liver functionality of CP ≥2 within 6 months of RT returned to baseline at later time points. Eleven grade 4 toxicities were observed. The mean change in the raw ALBI score at ∼6 months was similar for all baseline ALBI groups. Twenty-two patients underwent orthotopic liver transplantation after RT, 13 of whom had baseline CP-B8/B9/C liver functionality. For all patients, the 1- and 2-year treated-lesion local control was greater for SBRT than for AHRT (2-year 94% vs 65%, P < .0001). CONCLUSIONS: The tolerability of SBRT or AHRT as measured by a CP score decline of ≥2 within 6 months of RT was similar across baseline liver functionality groups. Compared with AHRT, SBRT was associated with superior local control. Because the true tolerability of limited-volume RT for patients with CP-B or CP-C cirrhosis is unknown, prospective trials validating its safety and efficacy are warranted.

The impact of anatomic tumor location on inter-fraction tumor motion during lung stereotactic body radiation therapy (SBRT).

PURPOSE: Narrow PTV margins and steep dose gradients underscore the importance of evaluating breathing-associated tumor motion for lung SBRT. The specific aim of this study was to determine the impact of anatomic tumor location on inter-fraction tumor motion. METHODS AND MATERIALS: Forty-one patients underwent standard free-breathing 4DCT simulation and daily image-guidance 4DCTs during lung SBRT. Absolute tumor motion amplitude in the mediolateral (ML), anterior-posterior (AP), and superior-inferior (SI) directions was analyzed from 159 total 4DCT scans (simulation and daily pre-treatment). RESULTS: Overall, the inter-fraction tumor motion amplitude in the ML, AP, and SI directions was small (mean ≤2.5 mm). Similarly, while both upper lobe (UL) and lower lobe (LL) tumors exhibited limited inter-fraction motion in both the ML and AP directions (mean ≤2.2 mm), tumors in the LL had increased inter-fraction motion in the SI direction compared to UL tumors (mean 4.3±4.0 mm vs. 1.7±1.7 mm, p=0.008). Moreover, 28.6% (n=4) of LL tumors exhibited mean inter-fraction motion along the SI direction >5 mm (all of which resided in the supra-diaphragmatic basal segments of the LL). CONCLUSIONS: Mean inter-fraction tumor motion amplitude along the SI direction exceeded our PTV margins (an isotropic 5 mm expansion of the ITV) in 28.6% of LL tumors (all of which resided in the basal segments). These results suggest that typical ITV-to-PTV margins may be insufficient for a subset of LL lesions and that increased PTV margins, daily breathing motion re-assessment and/or adaptive re-planning may benefit patients with supra-diaphragmatic tumors in the LL.

Volumetric-modulated arc radiotherapy for pancreatic malignancies: dosimetric comparison with sliding-window intensity-modulated radiotherapy and 3-dimensional conformal radiotherapy.

Volumetric-modulated arc radiotherapy (VMAT) is an iteration of intensity-modulated radiotherapy (IMRT), both of which deliver highly conformal dose distributions. Studies have shown the superiority of VMAT and IMRT in comparison with 3-dimensional conformal radiotherapy (3D-CRT) in planning target volume (PTV) coverage and organs-at-risk (OARs) sparing. This is the first study examining the benefits of VMAT in pancreatic cancer for doses more than 55.8 Gy. A planning study comparing 3D-CRT, IMRT, and VMAT was performed in 20 patients with pancreatic cancer. Treatments were planned for a 25-fraction delivery of 45 Gy to a large field followed by a reduced-volume 8-fraction external beam boost to 59.4 Gy in total. OARs and PTV doses, conformality index (CI) deviations from 1.0, monitor units (MUs) delivered, and isodose volumes were compared. IMRT and VMAT CI deviations from 1.0 for the large-field and the boost plans were equivalent (large field: 0.032 and 0.046, respectively; boost: 0.042 and 0.037, respectively; p > 0.05 for all comparisons). Both IMRT and VMAT CI deviations from 1.0 were statistically superior to 3D-CRT (large field: 0.217, boost: 0.177; p < 0.05 for all comparisons). VMAT showed reduction of the mean dose to the boost PTV (VMAT: 61.4 Gy, IMRT: 62.4 Gy, and 3D-CRT: 62.3 Gy; p < 0.05). The mean number of MUs per fraction was significantly lower for VMAT for both the large-field and the boost plans. VMAT delivery time was less than 3 minutes compared with 8 minutes for IMRT. Although no statistically significant dose reduction to the OARs was identified when comparing VMAT with IMRT, VMAT showed a reduction in the volumes of the 100% isodose line for the large-field plans. Dose escalation to 59.4 Gy in pancreatic cancer is dosimetrically feasible with shorter treatment times, fewer MUs delivered, and comparable CIs for VMAT when compared with IMRT.

Electromagnetic navigational bronchoscopy-guided fiducial markers for lung stereotactic body radiation therapy: analysis of safety, feasibility, and interfraction stability.

BACKGROUND: Embolization coils as fiducial markers for pulmonary stereotactic body radiation therapy (SBRT) are perceived to be the optimal marker type, given their ability to conform and anchor within the small airways. The aim of our study was to assess retention, placement, migration, feasibility, and safety of electromagnetic navigational bronchoscopy (ENB)-guided embolization coil markers throughout courses of SBRT. METHODS: Thirty-one patients with 34 nodules underwent ENB-guided fiducial placement of several 4 mm fibered platinum embolization coils before SBRT. Patient and nodule positioning was confirmed with daily pretreatment cone-beam computed tomography (CBCT). Fiducial positional characteristics were analyzed utilizing radiation treatment-planning software comparing the simulation CT with daily CBCTs. RESULTS: Of 105 fiducials placed, 103 were identifiable on simulation CT (retention rate: 98.1%). Incidence of asymptomatic pneumothoraces was 6%. One patient experienced hemoptysis requiring hospitalization. Eighty-six percent of fiducials were placed within 1 cm of the nodule, with 52% of fiducials placed directly on the nodule surface. Throughout a 5-fraction SBRT course, fiducial displacement was <7, 5, and 2 mm in 98%, 96%, and 67% of pretreatment CBCTs. CONCLUSIONS: ENB placement of embolization coils as fiducials for lung SBRT image guidance is associated with a low rate of iatrogenic pneumothoraces, and resulted in reliable placement of the fiducials in close proximity to the lung nodule. Embolization coils retained their relative position to the nodule throughout the course of SBRT, and provide an excellent alternative to linear gold seeds.

Real-time prostate motion assessment: image-guidance and the temporal dependence of intra-fraction motion.

BACKGROUND: The rapid adoption of image-guidance in prostate intensity-modulated radiotherapy (IMRT) results in longer treatment times, which may result in larger intrafraction motion, thereby negating the advantage of image-guidance. This study aims to qualify and quantify the contribution of image-guidance to the temporal dependence of intrafraction motion during prostate IMRT. METHODS: One-hundred and forty-three patients who underwent conventional IMRT (n=67) or intensity-modulated arc therapy (IMAT/RapidArc, n=76) for localized prostate cancer were evaluated. Intrafraction motion assessment was based on continuous RL (lateral), SI (longitudinal), and AP (vertical) positional detection of electromagnetic transponders at 10 Hz. Daily motion amplitudes were reported as session mean, median, and root-mean-square (RMS) displacements. Temporal effect was evaluated by categorizing treatment sessions into 4 different classes: IMRTc (transponder only localization), IMRTcc (transponder + CBCT localization), IMATc (transponder only localization), or IMATcc (transponder + CBCT localization). RESULTS: Mean/median session times were 4.15/3.99 min (IMATc), 12.74/12.19 min (IMATcc), 5.99/5.77 min (IMRTc), and 12.98/12.39 min (IMRTcc), with significant pair-wise difference (p<0.0001) between all category combinations except for IMRTcc vs. IMATcc (p>0.05). Median intrafraction motion difference between CBCT and non-CBCT categories strongly correlated with time for RMS (t-value=17.29; p<0.0001), SI (t-value=-4.25; p<0.0001), and AP (t-value=2.76; p<0.0066), with a weak correlation for RL (t-value=1.67; p=0.0971). Treatment time reduction with non-CBCT treatment categories showed reductions in the observed intrafraction motion: systematic error (Σ)<0.6 mm and random error (σ)<1.2 mm compared with ≤0.8 mm and <1.6 mm, respectively, for CBCT-involved treatment categories. CONCLUSIONS: For treatment durations >4-6 minutes, and without any intrafraction motion mitigation protocol in place, patient repositioning is recommended, with at least the acquisition of the lateral component of an orthogonal image pair in the absence of volumetric imaging.

Circumferential or sectored beam arrangements for stereotactic body radiation therapy (SBRT) of primary lung tumors: effect on target and normal-structure dose-volume metrics.

To compare 2 beam arrangements, sectored (beam entry over ipsilateral hemithorax) vs circumferential (beam entry over both ipsilateral and contralateral lungs), for static-gantry intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT) delivery techniques with respect to target and organs-at-risk (OAR) dose-volume metrics, as well as treatment delivery efficiency. Data from 60 consecutive patients treated using stereotactic body radiation therapy (SBRT) for primary non-small-cell lung cancer (NSCLC) formed the basis of this study. Four treatment plans were generated per data set: IMRT/VMAT plans using sectored (-s) and circumferential (-c) configurations. The prescribed dose (PD) was 60Gy in 5 fractions to 95% of the planning target volume (PTV) (maximum PTV dose ~ 150% PD) for a 6-MV photon beam. Plan conformality, R50 (ratio of volume circumscribed by the 50% isodose line and the PTV), and D2cm (Dmax at a distance ≥2cm beyond the PTV) were evaluated. For lungs, mean doses (mean lung dose [MLD]) and percent V30/V20/V10/V5Gy were assessed. Spinal cord and esophagus Dmax and D5/D50 were computed. Chest wall (CW) Dmax and absolute V30/V20/V10/V5Gy were reported. Sectored SBRT planning resulted in significant decrease in contralateral MLD and V10/V5Gy, as well as contralateral CW Dmax and V10/V5Gy (all p < 0.001). Nominal reductions of Dmax and D5/D50 for the spinal cord with sectored planning did not reach statistical significance for static-gantry IMRT, although VMAT metrics did show a statistically significant decrease (all p < 0.001). The respective measures for esophageal doses were significantly lower with sectored planning (p < 0.001). Despite comparable dose conformality, irrespective of planning configuration, R50 significantly improved with IMRT-s/VMAT-c (p < 0.001/p = 0.008), whereas D2cm significantly improved with VMAT-c (p < 0.001). Plan delivery efficiency improved with sectored technique (p < 0.001); mean monitor unit (MU)/cGy of PD decreased from 5.8 ± 1.9 vs 5.3 ± 1.7 (IMRT) and 2.7 ± 0.4 vs 2.4 ± 0.3 (VMAT). The sectored configuration achieves unambiguous dosimetric advantages over circumferential arrangement in terms of esophageal, contralateral CW, and contralateral lung sparing, in addition to being more efficient at delivery.

Characterization of a gated fiber-optic-coupled detector for application in clinical electron beam dosimetry.

PURPOSE: Assessment of the fundamental dosimetric characteristics of a novel gated fiber-optic-coupled dosimetry system for clinical electron beam irradiation. METHODS: The response of fiber-optic-coupled dosimetry system to clinical electron beam, with nominal energy range of 6-20 MeV, was evaluated for reproducibility, linearity, and output dependence on dose rate, dose per pulse, energy, and field size. The validity of the detector system's response was assessed in correspondence with a reference ionization chamber. RESULTS: The fiber-optic-coupled dosimetry system showed little dependence to dose rate variations (coefficient of variation +/- 0.37%) and dose per pulse changes (with 0.54% of reference chamber measurements). The reproducibility of the system was +/- 0.55% for dose fractions of approximately 100 cGy. Energy dependence was within +/- 1.67% relative to the reference ionization chamber for the 6-20 MeV nominal electron beam energy range. The system exhibited excellent linear response (R2 = 1.000) compared to reference ionization chamber in the dose range of 1-1000 cGy. The output factors were within +/- 0.54% of the corresponding reference ionization chamber measurements. CONCLUSIONS: The dosimetric properties of the gated fiber-optic-coupled dosimetry system compare favorably to the corresponding reference ionization chamber measurements and show considerable potential for applications in clinical electron beam radiotherapy.

Assessment of planning target volume margins for intensity-modulated radiotherapy of the prostate gland: role of daily inter- and intrafraction motion.

PURPOSE: To determine planning target volume margins for prostate intensity-modulated radiotherapy based on inter- and intrafraction motion using four daily localization techniques: three-point skin mark alignment, volumetric imaging with bony landmark registration, volumetric imaging with implanted fiducial marker registration, and implanted electromagnetic transponders (beacons) detection. METHODS AND MATERIALS: Fourteen patients who underwent definitive intensity-modulated radiotherapy for prostate cancer formed the basis of this study. Each patient was implanted with three electromagnetic transponders and underwent a course of 39 treatment fractions. Daily localization was based on three-point skin mark alignment followed by transponder detection and patient repositioning. Transponder positioning was verified by volumetric imaging with cone-beam computed tomography of the pelvis. Relative motion between the prostate gland and bony anatomy was quantified by offline analyses of daily cone-beam computed tomography. Intratreatment organ motion was monitored continuously by the Calypso® System for quantification of intrafraction setup error. RESULTS: As expected, setup error (that is, inter- plus intrafraction motion, unless otherwise stated) was largest with skin mark alignment, requiring margins of 7.5 mm, 11.4 mm, and 16.3 mm, in the lateral (LR), longitudinal (SI), and vertical (AP) directions, respectively. Margin requirements accounting for intrafraction motion were smallest for transponder detection localization techniques, requiring margins of 1.4 mm (LR), 2.6 mm (SI), and 2.3 mm (AP). Bony anatomy alignment required 2.1 mm (LR), 9.4 mm (SI), and 10.5 mm (AP), whereas image-guided marker alignment required 2.8 mm (LR), 3.7 mm (SI), and 3.2 mm (AP). No marker migration was observed in the cohort. CONCLUSION: Clinically feasible, rapid, and reliable tools such as the electromagnetic transponder detection system for pretreatment target localization and, subsequently, intratreatment target location monitoring allow clinicians to reduce irradiated volumes and facilitate safe dose escalation, where appropriate.

Performance characteristics of a gated fiber-optic-coupled dosimeter in high-energy pulsed photon radiation dosimetry.

Fiber-optic-coupled dosimeters (FOCDs) are a new class of in vivo dosimetry systems that are finding increased clinical applications. Utility of FOCDs has been limited in dosimetric applications due Cerenkov-ray signal contamination. The current study reports on the characterization of a novel FOCD, with a gated detection system for the discrimination and effective elimination of the direct contribution of Cerenkov radiation, for use in the radiotherapeutic realm. System reproducibility, linearity and output dependence on dose rate, energy, field size, and temperature response were characterized for 6, 10, and 15MV photon energies. The system exhibited a linear response to absorbed dose ranging from 1 to 2400cGy and showed little dependence to dose rate variations. Overall system reproducibility was 0.52% with no field-geometry and temperature dependence.

Implications of a high-definition multileaf collimator (HD-MLC) on treatment planning techniques for stereotactic body radiation therapy (SBRT): a planning study.

PURPOSE: To assess the impact of two multileaf collimator (MLC) systems (2.5 and 5 mm leaf widths) on three-dimensional conformal radiotherapy, intensity-modulated radiotherapy, and dynamic conformal arc techniques for stereotactic body radiation therapy (SBRT) of liver and lung lesions. METHODS: Twenty-nine SBRT plans of primary liver (n=11) and lung (n=18) tumors were the basis of this study. Five-millimeter leaf width 120-leaf Varian Millennium (M120) MLC-based plans served as reference, and were designed using static conformal beams (3DCRT), sliding-window intensity-modulated beams (IMRT), or dynamic conformal arcs (DCA). Reference plans were either re-optimized or recomputed, with identical planning parameters, for a 2.5-mm width 120-leaf BrainLAB/Varian high-definition (HD120) MLC system. Dose computation was based on the anisotropic analytical algorithm (AAA, Varian Medical Systems) with tissue heterogeneity taken into account. Each plan was normalized such that 100% of the prescription dose covered 95% of the planning target volume (PTV). Isodose distributions and dose-volume histograms (DVHs) were computed and plans were evaluated with respect to target coverage criteria, normal tissue sparing criteria, as well as treatment efficiency. RESULTS: Dosimetric differences achieved using M120 and the HD120 MLC planning were generally small. Dose conformality improved in 51.7%, 62.1% and 55.2% of the IMRT, 3DCRT and DCA cases, respectively, with use of the HD120 MLC system. Dose heterogeneity increased in 75.9%, 51.7%, and 55.2% of the IMRT, 3DCRT and DCA cases, respectively, with use of the HD120 MLC system. DVH curves demonstrated a decreased volume of normal tissue irradiated to the lower (90%, 50% and 25%) isodose levels with the HD120 MLC. CONCLUSION: Data derived from the present comparative assessment suggest dosimetric merit of the high definition MLC system over the millennium MLC system. However, the clinical significance of these results warrants further investigation in order to determine whether the observed dosimetric advantages translate into outcome improvements.

Stereotactic radiotherapy for unilateral orbital lymphoma and orbital pseudo-tumors: a planning study.

Orbital lymphoma and Grave's ophthalmopathy (GO) are successfully treated with radiation therapy. The lenses are blocked to prevent cataract formation. However, blocking of the lens by traditional methods can be difficult for tumors located anteriorly and extending into the retrobulbar space. We present a series of 3 patients with orbital lymphoma and 1 with GO treated with stereotactic intensity modulated radiation therapy (IMRT) to spare normal structures, including the lens. Three patients with orbital lymphomas and 1 with unilateral GO were treated with IMRT using a linac with stereotactic capabilities. Avoidance structures included the ipsilateral lens and globe, the contralateral lens and globe, the chiasm, and the brainstem. Two patients with orbital lymphoma were treated with 24 Gy in 12 fractions, and 1 patient was treated with 30.6 Gy in 17 fractions. The patient with GO was treated with 20 Gy in 10 fractions. The dosimetry was analyzed to determine the dose to normal tissues. Patient follow-up varies between 8 and 24 months. The mean minimal dose to the ipsilateral lens was 13.6% of the total dose, the mean maximal dose was 33.5%, and the mean median dose was 19.3%. The mean median dose to the contralateral eye was 1.1% of the total dose. The mean median dose to the chiasm was 14.9% of the total dose. The mean median dose to the brainstem was 1.9% of the total dose. No cataracts or other complications were noted in the 4 patients treated with this technique so far. IMRT gives a more conformal treatment to the orbital contents while sparing normal tissues such as the ipsilateral lens and adjacent critical structures. This should result in fewer complications such as cataracts.

Volumetric image-guidance: does routine usage prompt adaptive re-planning? An institutional review.

PURPOSE: To investigate how the use of volumetric image-guidance using an on-board cone-beam computed tomography (CBCT) system impacts on the frequency of adaptive re-planning. MATERIAL AND METHODS: Treatment courses of 146 patients who have undergone a course of external beam radiation therapy (EBRT) using volumetric CBCT image-guidance were analyzed. Target locations included the brain, head and neck, chest, abdomen, as well as prostate and non-prostate pelvis. The majority of patients (57.5%) were treated with hypo-fractionated treatment regimens (three to 15 fraction courses). The frequency of image-guidance ranged from daily (87.7%) to weekly or twice weekly. The underlying medical necessity for adaptive re-planning as well as frequency and consequences of plan adaptation to dose-volume parameters was assessed. RESULTS: Radiation plans of 34 patients (23.3%) were adapted at least once (up to six time) during their course of EBRT as a result of image-guidance CBCT review. Most common causes for adaptive planning were: tumor change (mostly shrinkage: 10 patients; four patients more than one re-plan), change in abdominal girth (systematic change in hollow organ filling; n=7, two patients more than one re-plan), weight loss (n=5), and systematic target setup deviation from simulation (n=5). Adaptive re-plan was required mostly for conventionally fractionated courses; only 5 patient plans undergoing hypo-fractionated treatment were adjusted. In over 91% of adapted plans, the dose-volume parameters did deviate from the prescribed plan parameters by more than 5% for at least 10% of the target volume, or organs-at-risk in close proximity to the target volume. DISCUSSION: Routine use of volumetric image-guidance has in our practice increased the demand for adaptive re-planning. Volumetric CBCT image-guidance provides sufficient imaging information to reliably predict the need for dose adjustment. In the vast majority of cases evaluated, the initial and adapted dose-volume parameters differed to a degree that was considered clinically significant.

MOSFET sensitivity dependence on integrated dose from high-energy photon beams.

The ability of a commercially available dual bias, dual MOSFET dosimetry system to measure therapeutic doses reproducibly throughout its vendor-defined dose-based lifetime has been evaluated by characterizing its sensitivity variation to integrated/cumulative doses from,high-energy (6 and 15 MV) photon radiotherapy beams. The variation of sensitivity as a function of total integrated dose was studied for three different dose-per-fraction levels; namely, 50, 200, and 1200 cGy/fraction. In standard sensitivity mode (i.e., measurements involving dose-per-fraction levels > or =100 cGy), the response of the MOSFET system to identical irradiations increased with integrated dose for both energies investigated. Dose measurement reproducibility for the low (i.e., 50 cGy) dose fractions was within 2.1% (if the system was calibrated before each in-phantom measurement) and 3.1% [if the system was calibrated prior to first use, with no intermediate calibration(s)]. Similarly, dose measurement reproducibility was between 2.2% and 6.6% for the conventional (i.e., 200 cGy) dose fractions and between 1.8% and 7.9% for escalated (i.e., 1200 cGy) dose fractions. The results of this study suggest that, due to the progressively increasing sensitivity resulting from the dual-MOSFET design, frequent calibrations are required to achieve measurement accuracy of < or =3% (within one standard deviation).

Phantom investigation of 3D motion-dependent volume aliasing during CT simulation for radiation therapy planning.

PURPOSE: To quantify volumetric and positional aliasing during non-gated fast- and slow-scan acquisition CT in the presence of 3D target motion. METHODS: Single-slice fast, single-slice slow, and multi-slice fast scan helical CTs were acquired of dynamic spherical targets (1 and 3.15 cm in diameter), embedded in an anthropomorphic phantom. 3D target motions typical of clinically observed tumor motion parameters were investigated. Motion excursions included +/- 5, +/- 10, and +/- 15 mm displacements in the S-I direction synchronized with constant displacements of +/- 5 and +/- 2 mm in the A-P and lateral directions, respectively. For each target, scan technique, and motion excursion, eight different initial motion-to-scan phase relationships were investigated. RESULTS: An anticipated general trend of target volume overestimation was observed. The mean percentage overestimation of the true physical target volume typically increased with target motion amplitude and decreasing target diameter. Slow-scan percentage overestimations were larger, and better approximated the time-averaged motion envelope, as opposed to fast-scans. Motion induced centroid misrepresentation was greater in the S-I direction for fast-scan techniques, and transaxial direction for the slow-scan technique. Overestimation is fairly uniform for slice widths < 5 mm, beyond which there is gross overestimation. CONCLUSION: Non-gated CT imaging of targets describing clinically relevant, 3D motion results in aliased overestimation of the target volume and misrepresentation of centroid location, with little or no correlation between the physical target geometry and the CT-generated target geometry. Slow-scan techniques are a practical method for characterizing time-averaged target position. Fast-scan techniques provide a more reliable, albeit still distorted, target margin.