Artificial intelligence-assisted quantitative CT analysis of airway changes following SABR for central lung tumors.

INTRODUCTION: Use of stereotactic ablative radiotherapy (SABR) for central lung tumors can result in up to a 35% incidence of late pulmonary toxicity. We evaluated an automated scoring method to quantify post-SABR bronchial changes by using artificial intelligence (AI)-based airway segmentation. MATERIALS AND METHODS: Central lung SABR patients treated at Amsterdam UMC (AUMC, internal reference dataset) and Peter MacCallum Cancer Centre (PMCC, external validation dataset) were identified. Patients were eligible if they had pre- and post-SABR CT scans with ≤ 1 mm resolution. The first step of the automated scoring method involved AI-based airway auto-segmentation using MEDPSeg, an end-to-end deep learning-based model. The Vascular Modeling Toolkit in 3D Slicer was then used to extract a centerline curve through the auto-segmented airway lumen, and cross-sectional measurements were computed along each bronchus for all CT scans. For AUMC patients, airway stenosis/occlusion was evaluated by both visual assessment and automated scoring. Only the automated method was applied to the PMCC dataset. RESULTS: Study patients comprised 26 from AUMC, and 33 from PMCC. Visual scoring identified stenosis/occlusion in 8 AUMC patients (31 %), most frequently in the segmental bronchi. After airway auto-segmentation, minor manual edits were needed in 9 % of patients. Segmentation for a single scan averaged 83sec (range 73-136). Automated scoring nearly doubled detected airway stenosis/occlusion (n = 15, 58 %), and allowed for earlier detection in 5/8 patients who had also visually scored changes. Estimated rates were 48 % and 66 % at 1- and 2-years, respectively, for the internal dataset. The automated detection rate was 52 % in the external dataset, with 1- and 2-year risks of 56 % and 61 %, respectively. CONCLUSION: An AI-based automated scoring method allows for detection of more bronchial stenosis/occlusion after lung SABR, and at an earlier time-point. This tool can facilitate studies to determine early airway changes and establish more reliable airway tolerance doses.

An ESTRO-ACROP guideline on quality assurance and medical physics commissioning of online MRI guided radiotherapy systems based on a consensus expert opinion.

OBJECTIVE: The goal of this consensus expert opinion was to define quality assurance (QA) tests for online magnetic resonance image (MRI) guided radiotherapy (oMRgRT) systems and to define the important medical physics aspects for installation and commissioning of an oMRgRT system. MATERIALS AND METHODS: Ten medical physicists and two radiation oncologists experienced in oMRgRT participated in the survey. In the first round of the consensus expert opinion, ideas on QA and commissioning were collected. Only tests and aspects different from commissioning of a CT guided radiotherapy (RT) system were considered. In the following two rounds all twelve participants voted on the importance of the QA tests, their recommended frequency and their suitability for the two oMRgRT systems approved for clinical use as well as on the importance of the aspects to consider during medical physics commissioning. RESULTS: Twenty-four QA tests were identified which are potentially important during commissioning and routine QA on oMRgRT systems compared to online CT guided RT systems. An additional eleven tasks and aspects related to construction, workflow development and training were collected. Consensus was found for most tests on their importance, their recommended frequency and their suitability for the two approved systems. In addition, eight aspects mostly related to the definition of workflows were also found to be important during commissioning. CONCLUSIONS: A program for QA and commissioning of oMRgRT systems was developed to support medical physicists to prepare for safe handling of such systems.

Same-day consultation, simulation and lung Stereotactic Ablative Radiotherapy delivery on a Magnetic Resonance-linac.

Background and Purpose: Magnetic resonance-guided radiotherapy (MRgRT) with real-time intra-fraction tumor motion monitoring allows for high precision Stereotactic Ablative Radiotherapy (SABR). This study aimed to investigate the clinical feasibility, patient satisfaction and delivery accuracy of single-fraction MR-guided SABR in a single day (one-stop-shop, OSS). Methods and Materials: Ten patients with small lung tumors eligible for single fraction treatments were included. The OSS procedure consisted of consultation, treatment simulation, treatment planning and delivery. Following SABR delivery, patients completed a reported experience measure (PREM) questionnaire. Prescribed doses ranged 28-34 Gy. Median GTV was 2.2 cm3 (range 1.3-22.9 cm3). A gating boundary of 3 mm, and PTV margin of 5 mm around the GTV, were used with auto-beam delivery control. Accuracy of SABR delivery was studied by analyzing delivered MR-cines reconstructed from machine log files. Results: All 10 patients completed the OSS procedure in a single day, and all reported satisfaction with the process. Median time for the treatment planning step and the whole procedure were 2.8 h and 6.6 h, respectively. With optimization of the procedure, treatment could be completed in half a day. During beam-on, the 3 mm tracking boundary encompassed between 78.0 and 100 % of the GTV across all patients, with corresponding PTV values being 94.4-100 % (5th-95th percentiles). On average, system-latency for triggering a beam-off event comprised 5.3 % of the delivery time. Latency reduced GTV coverage by an average of -0.3 %. Duty-cycles during treatment delivery ranged from 26.1 to 64.7 %. Conclusions: An OSS procedure with MR-guided SABR for lung cancer led to good patient satisfaction. Gated treatment delivery was highly accurate with little impact of system-latency.

Magnetic resonance imaging-guided radiotherapy for intermediate- and high-risk prostate cancer: Trade-off between planning target volume margin and online plan adaption.

Magnetic resonance-guided radiotherapy with daily plan adaptation for intermediate- and high-risk prostate cancer is time and labor intensive. Fifty adapted plans with 3 mm planning target volume (PTV)-margin were compared with non-adapted plans using 3 or 5 mm margins. Adequate (V95% ≥ 95%) prostate coverage was achieved in 49 fractions with 5 mm PTV without plan adaptation, however, coverage of the seminal vesicles (SV) was insufficient in 15 of 50 fractions. There was no insufficient coverage for prostate and SV using plan adaptation with 3 mm. Hence, daily adaptation is recommended to obtain adequate SV-coverage when using 3 mm PTV.

Dose accumulation for personalized stereotactic MR-guided adaptive radiation therapy in prostate cancer.

BACKGROUND AND PURPOSE: Adaptive MR-guided radiotherapy (MRgRT) is an innovative approach for delivering stereotactic body radiotherapy (SBRT) in prostate cancer (PC). Despite the increased clinical use of SBRT for PC, there is limited data on the relation between the actual delivered dose and toxicity. We aimed to identify dose parameters based on the total accumulated delivered bladder dose (DOSEACCTX). Furthermore, for future personalization, we studied whether prospective accumulation of the first 3 of 5 fractions (DOSEACC3FR) could be used as a representative of DOSEACCTX. MATERIALS AND METHODS: We deployed a recently validated deformable image registration-based dose accumulation strategy to reconstruct DOSEACCTX and DOSEACC3FR in 101 PC patients treated with stereotactic MRgRT. IPSS scores at baseline, end of MRgRT, at 6 and 12 weeks after treatment were analyzed to identify a clinically relevant increase of acute urinary symptoms. A receiver operator characteristic curve analysis was used to investigate the correlation of an increase in IPSS and bladder DOSEACCTX (range V5-V36.25 Gy, D1cc, D5cc) and DOSEACC3FR (range V6-V21.8 Gy, D1cc, D5cc) parameters. RESULTS: A clinically relevant increase in IPSS in the three months following MRgRT was observed in 25 patients. The V20Gy-32Gy from DOSEACCTX and V15Gy-18Gy from DOSEACC3FR showed good correlation with IPSS increase with area under the curve (AUC) values ranging from 0.71 to 0.75. In contrast, baseline dosimetry showed a poor correlation with AUC values between 0.53 and 0.62. CONCLUSION: DOSEACCTX was superior to baseline dosimetry in predicting acute urinary symptoms. Because DOSEACC3FR also showed good correlation, this can potentially be used to optimize MRgRT for the remaining fractions.

Impact of daily plan adaptation on organ-at-risk normal tissue complication probability for adrenal lesions undergoing stereotactic ablative radiation therapy.

INTRODUCTION: Stereotactic ablative radiotherapy (SABR) can achieve good local control for metastatic adrenal lesions. Magnetic resonance (MR)-guidance with daily on-table plan adaptation can augment the delivery of SABR with greater dose certainty. The goal of this study was to quantify the potential clinical benefit MR-guided daily-adaptive adrenal SABR using the normal tissue complication probability (NTCP) framework. METHODS: Patients treated with adrenal MR-guided SABR at a single institution were retrospectively reviewed. Lyman-Kutcher-Burman NTCP models were used to calculate the NTCP of upper abdominal organs-at-risk (OARs) at simulation and both before and after daily on-table plan adaptation. Differences in OAR NTCPs were assessed using signed-rank tests. Potential predictors of the benefits of adaptation were assessed by linear regression. RESULTS: Fifty-two adrenal MR-guided SABR courses were analyzed. The baseline simulation plan underestimated the absolute stomach NTCP by 10.0% on average (95% confidence interval: 4.7-15.2%, p < 0.001). Daily on-table adaptation lowered absolute NTCP by 8.7% (4.2-13.2%, p < 0.001). The most significant predictor of the benefits of adaptation was lesion laterality (p = 0.018), with left-sided lesions benefitting more (13.3% [6.3-20.4%], p < 0.001) than right-sided lesions (2.1% [-1.6-5.7%], p = 0.25). Sensitivity analyses did not change the statistical significance of the findings. CONCLUSION: NTCP analysis revealed that patients with left adrenal tumors were more likely to benefit from MR-guided daily on-table adaptive SABR using current dose/fractionation regimens due to reductions in predicted gastric toxicity. Right-sided adrenal lesions may be considered for dose escalation due to low predicted NTCP.

ESTRO-ACROP recommendations on the clinical implementation of hybrid MR-linac systems in radiation oncology.

Online magnetic resonance-guided radiotherapy (oMRgRT) represents one of the most innovative applications of current image-guided radiation therapy (IGRT). The revolutionary concept of oMRgRT systems is the ability to acquire MR images for adaptive treatment planning and also online imaging during treatment delivery. The daily adaptive planning strategies allow to improve targeting accuracy while avoiding critical structures. This ESTRO-ACROP recommendation aims to provide an overview of available systems and guidance for best practice in the implementation phase of hybrid MR-linac systems. Unlike the implementation of other radiotherapy techniques, oMRgRT adds the MR environment to the daily practice of radiotherapy, which might be a new experience for many centers. New issues and challenges that need to be thoroughly explored before starting clinical treatments will be highlighted.

An end-to-end test for MR-guided online adaptive radiotherapy.

In the evolving field of adaptive MR guided radiotherapy, the need for dedicated procedures for acceptance and quality assurance is increasing. Research has been devoted to MR compatible dosimeters and phantoms, but to date no end-to-end test has been presented that covers an MRgRT workflow. Such an end-to-end test should comprise each step of the workflow and include all associated uncertainties. The purpose of this study was to investigate the usability of an anthropomorphic deformable and multimodal pelvis (ADAM-pelvis) phantom in combination with film dosimetry for end-to-end testing of an MRgRT adaptive workflow. The ADAM-pelvis phantom included surrogates for muscle tissue, adipose and bone, as well as deformable silicone organs mimicking a prostate patient. At the interfaces of the critical structures (bladder and rectum), small pieces of GafChromic EBT3 films were placed to measure delivered dose. Pre-treatment MR imaging of the phantom was used to delineate the prostate, rectum and bladder and to generate a treatment plan to deliver 2 Gy to the prostate. Electron density (ED) map from CT imaging was used for dose calculation after deformable image registration (DIR) to the pre-treatment MR scan. At each fraction, bladder- and rectum filling was varied and a new adapted plan was generated. Dose calculation was performed using both a DIR-based ED map and a CT-based ED map after acquisition of a new CT scan of the phantom at each fraction. All dose calculations were performed taking into account the magnetic field. A good agreement between measured and calculated dose was found using both, the CT-derived and the DIR-based ED map (2.0% and 2.8% dose difference, respectively). The gamma index pass-rate (3%/2 mm) varied from 96.4% to 100%.The ADAM-pelvis phantom was suitable for end-to-end testing in MR-guided radiotherapy and a very good agreement with the calculated dose was achieved.

The Role of Daily Adaptive Stereotactic MR-Guided Radiotherapy for Renal Cell Cancer.

Novel magnetic-resonance-guided radiotherapy (MRgRT) permits real-time soft-tissue visualization, respiratory-gated delivery with minimal safety margins, and time-consuming daily plan re-optimisation. We report on early clinical outcomes of MRgRT and routine plan re-optimization for large primary renal cell cancer (RCC). Thirty-six patients were treated with MRgRT in 40 Gy/5 fractions. Prior to each fraction, re-contouring of tumor and normal organs on a pretreatment MR-scan allowed daily plan re-optimization. Treatment-induced toxicity and radiological responses were scored, which was followed by an offline analysis to evaluate the need for such daily re-optimization in 180 fractions. Mean age and tumor diameter were 78.1 years and 5.6 cm, respectively. All patients completed MRgRT with an average fraction duration of 45 min. Local control (LC) and overall survival rates at one year were 95.2% and 91.2%. No grade ≥3 toxicity was reported. Plans without re-optimization met institutional radiotherapy constraints in 83.9% of 180 fractions. Thus, daily plan re-optimization was required for only a minority of patients, who can be identified upfront by a higher volume of normal organs receiving 25 Gy in baseline plans. In conclusion, stereotactic MRgRT for large primary RCC showed low toxicity and high LC, while daily plan re-optimization was required only in a minority of patients.

Clinical implementation of magnetic resonance imaging guided adaptive radiotherapy for localized prostate cancer.

BACKGROUND AND PURPOSE: Magnetic resonance-guided radiation therapy (MRgRT) has recently become available in clinical practice and is expected to expand significantly in coming years. MRgRT offers marker-less continuous imaging during treatment delivery, use of small clinical target volume (CTV) to planning target volume (PTV) margins, and finally the option to perform daily plan re-optimization. MATERIALS AND METHODS: A total of 140 patients (700 fractions) have been treated with MRgRT and online plan adaptation for localized prostate cancer since early 2016. Clinical workflow for MRgRT of prostate cancer consisted of patient selection, simulation on both MR- and computed tomography (CT) scan, inverse intensity-modulated radiotherapy (IMRT) treatment planning and daily plan re-optimization prior to treatment delivery with partial organs at risk (OAR) recontouring within the first 2 cm outside the PTV. For each adapted plan online patient-specific quality assurance (QA) was performed by means of a secondary Monte Carlo 3D dose calculation and gamma analysis comparison. Patient experiences with MRgRT were assessed using a patient-reported outcome questionnaire (PRO-Q) after the last fraction. RESULTS: In 97% of fractions, MRgRT was delivered using the online adapted plan. Intrafractional prostate drifts necessitated 2D-corrections during treatment in approximately 20% of fractions. The average duration of an uneventful fraction of MRgRT was 45 min. PRO-Q's (N = 89) showed that MRgRT was generally well tolerated, with disturbing noise sensations being most commonly reported. CONCLUSIONS: MRgRT with daily online plan adaptation constitutes an innovative approach for delivering SBRT for prostate cancer and appears to be feasible, although necessitating extended timeslots and logistical challenges.

Identification of patients with locally advanced pancreatic cancer benefitting from plan adaptation in MR-guided radiation therapy.

BACKGROUND AND PURPOSE: MR-guided radiation therapy (MRgRT) with daily plan adaptation is a novel but time- and resource-intensive treatment for locally advanced pancreatic cancer (LAPC). We analyzed the benefit in target coverage and organ-at-risk (OAR) sparing of daily plan adaptation in 36 consecutive LAPC patients treated with MRgRT to 40 Gy in 5 fractions. MATERIALS AND METHODS: Adaptive planning was assessed for 180 fractions by comparing non-adapted plans with re-optimized plans using (a) GTV coverage and OAR high-doses, and (b) compliance with institutional objectives for GTV coverage and high-dose OAR constraints. Using these criteria, plan adaptation for each fraction was characterized as "not needed", "beneficial", or "no benefit". Decision-tree analysis was performed to identify subgroups most likely or not to benefit from routine plan adaptation. RESULTS: The percentage of plans fulfilling institutional constraints increased from 43.9% (non-adapted plans) to 83.3% after online plan adaptation, with significant improvements in GTV coverage and lower V33Gy OAR doses. Adaptive re-optimization was found to be "not needed" in 80 fractions (44.4%), "beneficial" in 95 fractions (52.8%) and of "no benefit" in 5 fractions (2.8%). Decision-tree analysis identified a grouping based on distance from tumor to OAR of ≤3 mm and GTV size, respectively, to be the major determinants for the benefit of daily plan adaptation. CONCLUSION: MRgRT with daily plan adaptation for LAPC was of benefit in approximately half of fractions, improving target coverage and OAR sparing. Plan adaptation appeared to be relevant mainly in cases where the GTV to adjacent OAR distance was ≤3 mm.

End-to-end empirical validation of dose accumulation in MRI-guided adaptive radiotherapy for prostate cancer using an anthropomorphic deformable pelvis phantom.

BACKGROUND AND PURPOSE: This work evaluates the accuracy of deformable dose accumulation for organs at risk (OAR) in MR-guided prostate SBRT using an anthropomorphic deformable phantom. MATERIALS AND METHODS: Six MR-guided prostate SBRT treatment courses were simulated using volumetric OAR (bladder and rectum) information derived from actual patient data. Deformed OAR contours, geometrical landmarks and GafChromic EBT3 film strips (1.25 × 2.0 cm2) placed at the surface of the OARs were used to validate DIR-based dose accumulation in MRgRT. Two DIR methods were applied: an intensity-based deformation (IB-D) applied to the whole image, and a contour-based deformation (CB-D), resulting in a separate deformation and dose accumulation for each OAR. Dosimetric accuracy was evaluated by quantifying the dose differences, and performing a gamma-index analysis between measured and DIR-derived accumulated dose for both OARs. Geometrical accuracy was assessed by measuring the Dice similarity coefficient (DSC), Hausdorff distance (HDD) and residual distance error (RDE) for all markers at each fraction. RESULTS: CB-D resulted in an average dose deviation from film measurements for rectum and bladder surfaces of 0.6% and 0.3%, respectively. IB-D led to worse results resulting in an overall average dose accumulation inaccuracy of 7.2% and 2.5% for rectum and bladder. CB-D also showed a higher geometrical accuracy than IB-D with significantly higher DSC values and lower RDE and HDD deviations. CONCLUSION: Empirical validation of dose accumulation in MR-guided SBRT for prostate cancer obtained a good agreement with reference film measurements when using a contour-based DIR approach.

Pitfalls of Ovarian Ablative Magnetic Resonance-guided Radiation Therapy for Refractory Endometriosis.

In this case presentation, we describe the challenges of performing magnetic resonance-guided radiation therapy (MRgRT) with plan adaptation in a patient with advanced endometriosis, in whom several prior therapeutic attempts were unsuccessful and extensive pelvic irradiation was regarded as being too toxic. Treatment was delivered in two sessions, first for the seemingly only active right ovary, and at a later stage for the left ovary. Some logistical problems were encountered during the preparation of the first treatment, which were subsequently optimized for the second treatment by using transvaginal ultrasound to determine the optimum time point for simulation and delivery. Using breath-hold gated delivery and plan adaptation, radiation dose to the bowel could be minimized, resulting in good tolerance of treatment. Because of the need to simulate and deliver in a brief optimal time span for visibility of the follicles in the ovaries, a single fraction dose of 8 Gy was used in our patient. Hormonal outcome after her second treatment is still pending. In conclusion, MRgRT with plan adaptation is feasible for the occasional patient with refractory endometriosis. Simulation and delivery needs to be synchronized with the menstrual cycle, ensuring that the Graafian follicles allow the ovaries to be visible on magnetic resonance imaging (MRI). Because the ovaries are only visible on T2-weighted MRI for a very brief period of time, we suggest that it is preferable to use single fraction radiotherapy with a brief interval between simulation imaging and delivery.

Combined Inter- and Intrafractional Plan Adaptation Using Fraction Partitioning in Magnetic Resonance-guided Radiotherapy Delivery.

Magnetic resonance-guided radiation therapy (MRgRT) not only allows for superior soft-tissue setup and online MR-guidance during delivery but also for inter-fractional plan re-optimization or adaptation. This plan adaptation involves repeat MR imaging, organs at risk (OARs) re-contouring, plan prediction (i.e., recalculating the baseline plan on the anatomy of that moment), plan re-optimization, and plan quality assurance. In contrast, intrafractional plan adaptation cannot be simply performed by pausing delivery at any given moment, adjusting contours, and re-optimization because of the complex and composite nature of deformable dose accumulation. To overcome this limitation, we applied a practical workaround by partitioning treatment fractions, each with half the original fraction dose. In between successive deliveries, the patient remained in the treatment position and all steps of the initial plan adaptation were repeated. Thus, this second re-optimization served as an intrafractional plan adaptation at 50% of the total delivery. The practical feasibility of this partitioning approach was evaluated in a patient treated with MRgRT for locally advanced pancreatic cancer (LAPC). MRgRT was delivered in 40Gy in 10 fractions, with two fractions scheduled successively on each treatment day. The contoured gross tumor volume (GTV) was expanded by 3 mm, excluding parts of the OARs within this expansion to derive the planning target volume for daily re-optimization (PTVOPT). The baseline GTVV95% achieved in this patient was 80.0% to adhere to the high-dose constraints for the duodenum, stomach, and bowel (V33 Gy <1 cc and V36 Gy <0.1 cc). Treatment was performed on the MRIdian (ViewRay Inc, Mountain View, USA) using video-assisted breath-hold in shallow inspiration. The dual plan adaptation resulted, for each partitioned fraction, in the generation of PlanPREDICTED1, PlanRE-OPTIMIZED1 (inter-fractional adaptation), PlanPREDICTED2, and PlanRE-OPTIMIZED2 (intrafractional adaptation). An offline analysis was performed to evaluate the benefit of inter-fractional versus intrafractional plan adaptation with respect to GTV coverage and high-dose OARs sparing for all five partitioned fractions. Interfractional changes in adjacent OARs were substantially larger than intrafractional changes. Mean GTV V95% was 76.8 ± 1.8% (PlanPREDICTED1), 83.4 ± 5.7% (PlanRE-OPTIMIZED1), 82.5 ± 4.3% (PlanPREDICTED2),and 84.4 ± 4.4% (PlanRE-OPTIMIZED2). Both plan re-optimizations appeared important for correcting the inappropriately high duodenal V33 Gy values of 3.6 cc (PlanPREDICTED1) and 3.9 cc (PlanPREDICTED2) to 0.2 cc for both re-optimizations. To a smaller extent, this improvement was also observed for V25 Gy values. For the stomach, bowel, and all other OARs, high and intermediate doses were well below preset constraints, even without re-optimization. The mean delivery time of each daily treatment was 90 minutes. This study presents the clinical application of combined inter-fractional and intrafractional plan adaptation during MRgRT for LAPC using fraction partitioning with successive re-optimization. Whereas, in this study, interfractional plan adaptation appeared to benefit both GTV coverage and OARs sparing, intrafractional adaptation was particularly useful for high-dose OARs sparing. Although all necessary steps lead to a prolonged treatment duration, this may be applied in selected cases where high doses to adjacent OARs are regarded as critical.

Role of Daily Plan Adaptation in MR-Guided Stereotactic Ablative Radiation Therapy for Adrenal Metastases.

PURPOSE: To study interfractional organ changes during magnetic resonance (MR)-guided stereotactic ablative radiation therapy for adrenal metastases and to evaluate the dosimetric advantages of online plan adaptation. METHODS AND MATERIALS: Seventeen patients underwent a total of 84 fractions of video-assisted, respiration-gated, MR-guided adaptive radiation therapy to deliver either 50 Gy (5 fractions), 60 Gy (8 fractions), or 24 Gy (3 fractions). An MR scan was repeated before each fraction, followed by rigid coregistration to the gross tumor volume (GTV) on the pretreatment MR scan. Contour deformation, planning target volume (PTV) (GTV + 3 mm) expansion, and online plan reoptimization were then performed. Reoptimized plans were compared with baseline treatment plans recalculated on the anatomy-of-the-day ("predicted plans"). Interfractional changes in organs at risk (OARs) were quantified according to OAR volume changes within a 3 cm distance from the PTV surface, center of mass displacements, and the Dice similarity coefficient. Plan quality evaluation was based on target coverage (GTV and PTV) and high dose sparing of all OARs (V36Gy, V33Gy, and V25Gy). RESULTS: Substantial center of mass displacements were observed for stomach, bowel, and duodenum, 17, 27 and 36 mm, respectively. Maximum volume changes for the stomach, bowel, and duodenum within 3 cm of PTV were 23.8, 20.5, and 20.9 cm3, respectively. Dice similarity coefficient values for OARs ranged from 0.0 to 0.9 for all fractions. Baseline plans recalculated on anatomy-of-the-day revealed underdosage of target volumes and variable OAR sparing, leading to a failure to meet institutional constraints in a third of fractions. Online reoptimization improved target coverage in 63% of fractions and reduced the number of fractions not meeting the V95% objective for GTV and PTV. Reoptimized plans exhibited significantly better sparing of OARs. CONCLUSIONS: Significant interfractional changes in OAR positions were observed despite breath-hold stereotactic ablative radiation therapy delivery under MR-guidance. Online reoptimization of treatment plans led to significant improvements in target coverage and OAR sparing.

Isotoxic radiosurgery planning for brain metastases.

PURPOSE/OBJECTIVE(S): Radionecrosis (RN) has previously been correlated with radiosurgery (RS) dose, lesion volume, and the volume of the brain receiving specific doses, i.e. V10-14Gy. A knowledge-based individualized estimation of the optimum RS dose has been derived based on lesional volume and brain toxicity parameters. METHODS AND MATERIALS: A prediction model for brain toxicity parameters and estimation of the optimum RS dose was derived using 30 historical linac-based dynamic conformal arc RS plans for single brain metastases (BM) (0.2-20.3cc) with risk-adapted dose prescription ranging from 15 to 24Gy. Derivation of the model followed a three-step process: (1) Derivation of formulas for the prediction of brain toxicity parameters V10-18Gy; (2) Establishing the relationship of the coefficients used for the prediction of V12Gy with prescription dose; (3) Derivation of the optimum prescription dose for a given maximum V12Gy as a function of a given lesion volume. Model validation was performed on 65 new patients with 138 lesions (44 with multiple BM) treated with non-coplanar volumetric modulated stereotactic arc treatment (VMAT). RESULTS: A linear dependence with the PTV size was found for all investigated brain toxicity parameters (V10-18Gy). Individualized RS prescription doses can be calculated for any given PTV size based on a linear relationship between V12Gy and PTV size, according to the formula PD=[V12Gy+0.96+(1.44×PTV)]/[0.12+(0.12×PTV)]. A very good correlation (R(2)=0.991) was found between the predicted V12Gy and the resulting V12Gy in 65 new patients with 138 lesions treated with non-coplanar VMAT technique in our clinic. CONCLUSIONS: A simple formula is proposed for estimation of the optimal individual RS dose for any given lesion volume for patients with (multiple) BM. This formula is based on calculation of the brain toxicity parameter, V12Gy, for the normal brain minus PTV.

The effect of induction chemotherapy on tumor volume and organ-at-risk doses in patients with locally advanced oropharyngeal cancer.

BACKGROUND AND PURPOSE: To retrospectively report changes in gross tumor volume (GTV) and organ-at-risk (OAR) doses after induction chemotherapy (IC) in oropharyngeal cancer using different contouring strategies. MATERIALS AND METHODS: GTV and OARs were delineated on pre- and post-IC planning CT. Two post-IC GTV contours were made: (1) a 'consensus set' using published guidelines (GTVconsensus), and (2) 'visible set', delineating only visible post-IC GTV (GTVvisible). Pre-IC interactively optimized volumetric modulated arc therapy plans were generated. The pre-IC planning constraints served as the starting point for both post-IC plans. Results reflect pooled data from all 10 patients. RESULTS: Mean reduction in volume post-IC was 24% and 47% for consensus and visible primary tumor and 57% and 60% for consensus and visible nodes. Compared to pre-IC plans, average mean OAR dose for post-IC GTVconsensus plans was significantly lower for CL parotid. For GTVvisible plans both parotids, upper/lower larynx, inferior pharyngeal constrictor and cricopharyngeal muscles were significantly lower. However reductions compared with post-IC GTVconsensus plans were modest (1.6/1.5/1.2/3.7/5.9/2.6Gy, respectively). CONCLUSION: IC in patients with oropharyngeal carcinoma results in substantial reductions in GTVs. If post-IC GTVs are used, which is contrary to current consensus, statistically significant but relatively small OAR dose reductions are observed.