Impact of proton PBS machine operating parameters on the effectiveness of layer rescanning for interplay effect mitigation in lung SBRT treatment.

BACKGROUND: Rescanning is a common technique used in proton pencil beam scanning to mitigate the interplay effect. Advances in machine operating parameters across different generations of particle therapy systems have led to improvements in beam delivery time (BDT). However, the potential impact of these improvements on the effectiveness of rescanning remains an underexplored area in the existing research. METHODS: We systematically investigated the impact of proton machine operating parameters on the effectiveness of layer rescanning in mitigating interplay effect during lung SBRT treatment, using the CIRS phantom. Focused on the Hitachi synchrotron particle therapy system, we explored machine operating parameters from our institution's current (2015) and upcoming systems (2025A and 2025B). Accumulated dynamic 4D dose were reconstructed to assess the interplay effect and layer rescanning effectiveness. RESULTS: Achieving target coverage and dose homogeneity within 2% deviation required 6, 6, and 20 times layer rescanning for the 2015, 2025A, and 2025B machine parameters, respectively. Beyond this point, further increasing the number of layer rescanning did not further improve the dose distribution. BDTs without rescanning were 50.4, 24.4, and 11.4 s for 2015, 2025A, and 2025B, respectively. However, after incorporating proper number of layer rescanning (six for 2015 and 2025A, 20 for 2025B), BDTs increased to 67.0, 39.6, and 42.3 s for 2015, 2025A, and 2025B machine parameters. Our data also demonstrated the potential problem of false negative and false positive if the randomness of the respiratory phase at which the beam is initiated is not considered in the evaluation of interplay effect. CONCLUSION: The effectiveness of layer rescanning for mitigating interplay effect is affected by machine operating parameters. Therefore, past clinical experiences may not be applicable to modern machines.

A review of the clinical introduction of 4D particle therapy research concepts.

Background and purpose: Many 4D particle therapy research concepts have been recently translated into clinics, however, remaining substantial differences depend on the indication and institute-related aspects. This work aims to summarise current state-of-the-art 4D particle therapy technology and outline a roadmap for future research and developments. Material and methods: This review focused on the clinical implementation of 4D approaches for imaging, treatment planning, delivery and evaluation based on the 2021 and 2022 4D Treatment Workshops for Particle Therapy as well as a review of the most recent surveys, guidelines and scientific papers dedicated to this topic. Results: Available technological capabilities for motion surveillance and compensation determined the course of each 4D particle treatment. 4D motion management, delivery techniques and strategies including imaging were diverse and depended on many factors. These included aspects of motion amplitude, tumour location, as well as accelerator technology driving the necessity of centre-specific dosimetric validation. Novel methodologies for X-ray based image processing and MRI for real-time tumour tracking and motion management were shown to have a large potential for online and offline adaptation schemes compensating for potential anatomical changes over the treatment course. The latest research developments were dominated by particle imaging, artificial intelligence methods and FLASH adding another level of complexity but also opportunities in the context of 4D treatments. Conclusion: This review showed that the rapid technological advances in radiation oncology together with the available intrafractional motion management and adaptive strategies paved the way towards clinical implementation.

Risks of Spinal Abnormalities and Growth Impairment After Radiation to the Spine in Childhood Cancer Survivors: A PENTEC Comprehensive Review.

PURPOSE: A PENTEC (Pediatric Normal Tissue Effects in the Clinic) review was performed to estimate the dose-volume effects of radiation therapy on spine deformities and growth impairment for patients who underwent radiation therapy as children. METHODS AND MATERIALS: A systematic literature search was performed to identify published data for spine deformities and growth stunting. Data were extracted from 12 reports of children irradiated to the spine (N = 603 patients). The extracted data were analyzed to find associations between complication risks and the radiation dose (conventional fractionation throughout) as impacted by exposed volumes and age using the mixed-effects logistic regression model. When appropriate, corrections were made for radiation modality, namely orthovoltage beams. RESULTS: In the regression analysis, the association between vertebral dose and scoliosis rate was highly significant (P < .001). Additionally, young age at time of radiation was highly predictive of adverse outcomes. Clinically significant scoliosis can occur with doses ≥15 Gy to vertebrae during infancy (<2 years of age). For children irradiated at 2 to 6 years of age, overall scoliosis rates of any grade were >30% with doses >20 Gy; grade 2 or higher scoliosis was correlated with doses ≥30 Gy. Children >6 years of age remain at risk for scoliosis with doses >30 Gy; however, most cases will be mild. There are limited data regarding the effect of dose gradients across the spine on degree of scoliosis. The risk of clinically meaningful height loss was minimal when irradiating small volumes of the spine up to 20 Gy (eg, flank irradiation), except in infants who are more vulnerable to lower doses. Growth stunting was more frequent when larger segments of the spine (eg, the entire spine or craniospinal irradiation) were irradiated before puberty to doses >20 Gy. The effect was modest when patients were irradiated after puberty to doses >20 Gy. CONCLUSIONS: To reduce the risk of kyphoscoliosis and growth impairment, the dose to the spine should be kept to <20 Gy for children <6 years of age and to <10 to 15 Gy in infants. The number of vertebral bodies irradiated and dose gradients across the spine should also be limited when possible.

Effect of the initial energy layer and spot placement parameters on IMPT delivery efficiency and plan quality.

PURPOSE: Improving efficiency of intensity modulated proton therapy (IMPT) treatment can be achieved by shortening the beam delivery time. The purpose of this study is to reduce the delivery time of IMPT, while maintaining the plan quality, by finding the optimal initial proton spot placement parameters. METHODS: Seven patients previously treated in the thorax and abdomen with gated IMPT and voluntary breath-hold were included. In the clinical plans, the energy layer spacing (ELS) and spot spacing (SS) were set to 0.6-0.8 (as a scale factor of the default values). For each clinical plan, we created four plans with ELS increased to 1.0, 1.2, 1.4, and SS to 1.0 while keeping all other parameters unchanged. All 35 plans (130 fields) were delivered on a clinical proton machine and the beam delivery time was recorded for each field. RESULTS: Increasing ELS and SS did not cause target coverage reduction. Increasing ELS had no effect on critical organ-at-risk (OAR) doses or the integral dose, while increasing SS resulted in slightly higher integral and selected OAR doses. Beam-on times were 48.4 ± 9.2 (range: 34.1-66.7) seconds for the clinical plans. Time reductions were 9.2 ± 3.3 s (18.7 ± 5.8%), 11.6 ± 3.5 s (23.1 ± 5.9%), and 14.7 ± 3.9 s (28.9 ± 6.1%) when ELS was changed to 1.0, 1.2, and 1.4, respectively, corresponding to 0.76-0.80 s/layer. SS change had a minimal effect (1.1 ± 1.6 s, or 1.9 ± 2.9%) on the beam-on time. CONCLUSION: Increasing the energy layers spacing can reduce the beam delivery time effectively without compromising IMPT plan quality; increasing the SS had no meaningful impact on beam delivery time and resulted in plan-quality degradation in some cases.

Systematic review for deep inspiration breath hold in proton therapy for mediastinal lymphoma: A PTCOG Lymphoma Subcommittee report and recommendations.

PURPOSE: To systematically review all dosimetric studies investigating the impact of deep inspiration breath hold (DIBH) compared with free breathing (FB) in mediastinal lymphoma patients treated with proton therapy as compared to IMRT (intensity-modulated radiation therapy)-DIBH. MATERIALS AND METHODS: We conducted a systematic review in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guideline using the PubMed database to identify studies of mediastinal lymphoma patients with dosimetric comparisons of proton-FB and/or proton-DIBH with IMRT-DIBH. Parameters included mean heart (MHD), lung (MLD), and breast (MBD) doses, among other parameters. Case reports were excluded. Absolute differences in mean doses > 1 Gy between comparators were considered to be clinically meaningful. RESULTS: As of April 2021, eight studies fit these criteria (n = 8), with the following comparisons: proton-FB vs IMRT-DIBH (n = 5), proton-DIBH vs proton-FB (n = 5), and proton-DIBH vs IMRT-DIBH (n = 8). When comparing proton-FB with IMRT-DIBH in 5 studies, MHD was reduced with proton-FB in 2 studies, was similar (<1 Gy difference) in 2 studies, and increased in 1 study. On the other hand, MLD and MBD were reduced with proton-FB in 3 and 4 studies, respectively. When comparing proton-DIBH with proton-FB, MHD and MLD were reduced with proton DIBH in 4 and 3 studies, respectively, while MBD remained similar. Compared with IMRT-DIBH in 8 studies, proton-DIBH reduced the MHD in 7 studies and was similar in 1 study. Furthermore, MLD and MBD were reduced with proton-DIBH in 8 and 6 studies respectively. Integral dose was similar between proton-FB and proton-DIBH, and both were substantially lower than IMRT-DIBH. CONCLUSION: Accounting for heart, lung, breast, and integral dose, proton therapy (FB or DIBH) was superior to IMRT-DIBH. Proton-DIBH can lower dose to the lungs and heart even further compared with proton-FB, depending on disease location in the mediastinum, and organ-sparing and target coverage priorities.

Chemoradiation with Hypofractionated Proton Therapy in Stage II-III Non-Small Cell Lung Cancer: A Proton Collaborative Group Phase 2 Trial.

PURPOSE: Hypofractionated radiation therapy has been safely implemented in the treatment of early-stage non-small cell lung cancer (NSCLC) but not locally advanced NSCLC owing to prohibitive toxicities with photon therapy. Proton therapy, however, may allow for safe delivery of hypofractionated radiation therapy. We sought to determine whether hypofractionated proton therapy with concurrent chemotherapy improves overall survival. METHODS AND MATERIALS: The Proton Collaborative Group conducted a phase 1/2 single-arm nonrandomized prospective multicenter trial from 2013 through 2018. We received consent from 32 patients, of whom 28 were eligible for on-study treatment. Patients had stage II or III unresectable NSCLC (based on the 7th edition of the American Joint Committee on Cancer's staging manual) and received hypofractionated proton therapy at 2.5 to 4 Gy per fraction to a total 60 Gy with concurrent platin-based doublet chemotherapy. The primary outcome was 1-year overall survival comparable to the 62% reported for the Radiation Therapy Oncology Group (RTOG) 9410 trial. RESULTS: The trial closed early owing to slow accrual, in part, from a competing trial, RTOG 1308. Median patient age was 70 years (range, 50-86 years). Patients were predominantly male (n = 20), White (n = 23), and prior smokers (n = 27). Most had stage III NSCLC (n = 22), 50% of whom had adenocarcinoma. After a median follow-up of 31 months, the 1- and 3-year overall survival rates were 89% and 49%, respectively, and progression-free survival rates were 58% and 32%, respectively. No acute grade ≥3 esophagitis occurred. Only 14% developed a grade ≥3 radiation-related pulmonary toxic effect. CONCLUSIONS: Hypofractionated proton therapy delivered at 2.5 to 3.53 Gy per fraction to a total 60 Gy with concurrent chemotherapy provides promising survival, and additional examination through larger studies may be warranted.

AAPM Task Group Report 290: Respiratory motion management for particle therapy.

Dose uncertainty induced by respiratory motion remains a major concern for treating thoracic and abdominal lesions using particle beams. This Task Group report reviews the impact of tumor motion and dosimetric considerations in particle radiotherapy, current motion-management techniques, and limitations for different particle-beam delivery modes (i.e., passive scattering, uniform scanning, and pencil-beam scanning). Furthermore, the report provides guidance and risk analysis for quality assurance of the motion-management procedures to ensure consistency and accuracy, and discusses future development and emerging motion-management strategies. This report supplements previously published AAPM report TG76, and considers aspects of motion management that are crucial to the accurate and safe delivery of particle-beam therapy. To that end, this report produces general recommendations for commissioning and facility-specific dosimetric characterization, motion assessment, treatment planning, active and passive motion-management techniques, image guidance and related decision-making, monitoring throughout therapy, and recommendations for vendors. Key among these recommendations are that: (1) facilities should perform thorough planning studies (using retrospective data) and develop standard operating procedures that address all aspects of therapy for any treatment site involving respiratory motion; (2) a risk-based methodology should be adopted for quality management and ongoing process improvement.

Intensity Modulated Proton Therapy Treatment Planning for Postmastectomy Patients with Metallic Port Tissue Expanders.

PURPOSE: Proton beam therapy can significantly reduce cardiopulmonary radiation exposure compared with photon-based techniques in the postmastectomy setting for locally advanced breast cancer. For patients with metallic port tissue expanders, which are commonly placed in patients undergoing a staged breast reconstruction, dose uncertainties introduced by the high-density material pose challenges for proton therapy. In this report, we describe an intensity modulated proton therapy planning technique for port avoidance through a hybrid single-field optimization/multifield optimization approach. METHODS AND MATERIALS: In this planning technique, 3 beams are utilized. For each beam, no proton spot is placed within or distal to the metal port plus a 5 mm margin. Therefore, precise modeling of the metal port is not required, and various tissue expander manufacturers/models are eligible. The blocked area of 1 beam is dosimetrically covered by 1 or 2 of the remaining beams. Multifield optimization is used in the chest wall target region with blockage of any beam, while single-field optimization is used for remainder of chest wall superior/inferior to the port. RESULTS: Using this technique, clinical plans were created for 6 patients. Satisfactory plans were achieved in the 5 patients with port-to-posterior chest wall separations of 1.5 cm or greater, but not in the sixth patient with a 0.7 cm separation. CONCLUSIONS: We described a planning technique and the results suggest that the metallic port-to-chest wall distance may be a key parameter for optimal plan design.

The role of motion management and position verification in lymphoma radiotherapy.

In the last decades, the substantial technical progress in radiation oncology offered the opportunity for more accurate planning and delivery of treatment. At the same time, the evolution of systemic treatment and the advent of modern diagnostic tools allowed for more accurate staging and consequently a safe reduction of radiotherapy (RT) target volumes and RT doses in the treatment of lymphomas. As a result, incidental irradiation of organs at risk was reduced, with a consequent reduction of severe late toxicity in long-term lymphoma survivors. Nevertheless, these innovations warrant that professionals pay attention to concurrently ensure precise planning and dose delivery to the target volume and safe sparing of the organs at risk. In particular, target and organ motion should be carefully managed in order to prevent any compromise of treatment efficacy. Several aspects should be taken into account during the treatment pathway to minimise uncertainties and to apply a valuable motion management strategy, when needed. These include: reliable image registration between diagnostic and planning radiologic exams to facilitate the contouring process, image guidance to limit positioning uncertainties and to ensure the accuracy of dose delivery and management of lung motion through procedures of respiratory gating and breath control. In this review, we will cover the current clinical approaches to minimise these uncertainties in patients treated with modern RT techniques, with a particular focus on mediastinal lymphoma. In addition, since uncertainties have a different impact on the dose deposition of protons compared to conventional x-rays, the role of motion management and position verification in proton beam therapy (PBT) will be discussed in a separate section.

Adjuvant chemoradiation for high-grade cardiac leiomyosarcoma in a child: Case report and review of literature.

A 13-year-old healthy girl presented with dizziness and palpitations, found to have a left atrial mass. An 8-cm tumor was removed en bloc. Pathology confirmed grade 3 leiomyosarcoma with multifocal positive margins. She received adjuvant ifosfamide and doxorubicin, followed by concurrent proton radiotherapy and ifosfamide. Radiotherapy included 66 Gy (RBE) in 33 fractions to the operative bed. Prospectively graded toxicities included Grade 2 esophagitis and Grade 1 anorexia, dermatitis, and fatigue. She completed six cycles of ifosfamide. Two years post operation, she had no evidence of disease, intermittent palpitations with normal cardiac function, and no other cardiopulmonary or esophageal symptoms.

Selection of Mediastinal Lymphoma Patients for Proton Therapy Within the Proton Collaborative Group Registry: Concordance With the ILROG Guidelines.

PURPOSE: As patients with mediastinal lymphoma are typically young with curable disease, advanced radiation techniques such as proton therapy are often considered to minimize subacute and late toxicity. However, it is unclear which mediastinal lymphoma patients are treated with proton therapy. Within a prospective, multi-institutional proton registry, we characterized mediastinal lymphoma patients treated with proton therapy and assessed concordance with consensus recommendations published in 2018 by the International Lymphoma Radiation Oncology Group (ILROG). METHODS: Eligible patients included those with lymphoma of the mediastinum treated exclusively with proton therapy for whom digital imaging and communications in medicine (DICOM) treatment data were available for review. Given the challenge with reliably visualizing the left mainstem coronary artery, the inferior-most aspect of the left pulmonary artery (PA) was used as a surrogate. Extent of disease was characterized as upper mediastinum (above level of left PA), middle mediastinum (below left PA but at or above level of T8), or low mediastinum (below T8). RESULTS: Between November 2012 and April 2019, 56 patients were treated and met inclusion criteria. Patients treated with proton therapy were young (median, 24 y; range: 12 to 88), with over half being female (55%). Patients were most commonly treated at initial diagnosis (86%) and had Hodgkin lymphoma (79%). Most patients (96%) had mediastinal disease that extended down to the level of the heart: 48% had middle and 48% had low mediastinal involvement. Nearly all patients (96%) met the ILROG consensus recommendations: 95% had lower mediastinal disease, 46% were young females, and 9% were heavily pretreated. Heart (mean) and lung dose (mean, V5, V20) were significantly associated with lowest extent of mediastinal disease. CONCLUSIONS: Mediastinal lymphoma patients treated with proton therapy are typically young with lower mediastinal involvement. Within a prospective, multi-institutional proton registry, nearly all treated patients fit the ILROG consensus recommendations regarding which mediastinal lymphoma patients may most benefit from proton therapy.

Image-Guided Hypofractionated Proton Therapy in Early-Stage Non-Small Cell Lung Cancer: A Phase 2 Study.

PURPOSE: Due to the excellent outcomes with image-guided stereotactic body radiotherapy for patients with early-stage non-small cell lung cancer (NSCLC) and the low treatment-related toxicities using proton therapy (PT), we investigated treatment outcomes and toxicities when delivering hypofractionated PT. MATERIALS AND METHODS: Between 2009 and 2018, 22 patients with T1 to T2 N0M0 NSCLC (45% T1, 55% T2) received image-guided hypofractionated PT. The median age at diagnosis was 72 years (range, 58-90). Patients underwent 4-dimensional computed tomography simulation following fiducial marker placement, and daily image guidance was performed. Nine patients (41%) were treated with 48 GyRBE in 4 fractions for peripheral lesions, and 13 patients (59%) were treated with 60 GyRBE in 10 fractions for central lesions. Patients were assessed for CTCAEv4 toxicities with computed tomography imaging for tumor assessment. The primary endpoint was grade 3 to 5 toxicity at 1 year. RESULTS: The median follow-up for all patients was 3.5 years (range, 0.2-8.8 years). The overall survival rates at 3 and 5 years were 81% and 49%, respectively. Cause-specific survival rates at 3 and 5 years were 100% and 75%, respectively. The 3-year local, regional, and distant control rates were 86%, 85%, and 95%, respectively. Four patients experienced in-field recurrences between 18 and 45 months after treatment. One patient (5%) developed a late grade 3 bronchial stricture requiring hospitalization and stent. CONCLUSION: Image-guided hypofractionated PT for early-stage NSCLC provides promising local control and long-term survival with a low likelihood of toxicity. Regional nodal and distant relapses remain a problem.

Irradiating Residual Disease to 30 Gy with Proton Therapy in Pediatric Mediastinal Hodgkin Lymphoma.

BACKGROUND: Local relapse is a predominant form of recurrence among pediatric patients with Hodgkin lymphoma (PHL). Although PHL radiotherapy doses have been approximately 20 Gy, adults with Hodgkin lymphoma receiving 30 to 36 Gy experience fewer in-field relapses. We investigated the dosimetric effect of such a dose escalation to the organs at risk (OARs). MATERIALS AND METHODS: Ten patients with PHL treated with proton therapy to 21 Gy involved-site radiation therapy (ISRT21Gy) were replanned to deliver 30 Gy by treating the ISRT to 30 Gy (ISRT30Gy), delivering 21 Gy to the ISRT plus a 9-Gy boost to postchemotherapy residual volume (rISRTboost), and delivering 30 Gy to the residual ISRT target only (rISRT30Gy). Radiation doses to the OARs were compared. RESULTS: The ISRT30Gy escalated the dose to the target by 42% but also to the OARs. The rISRTboost escalated the residual target dose by 42%, and the OAR dose by only 17% to 26%. The rISRT30Gy escalated the residual target dose by 42% but reduced the OAR dose by 25% to 46%. CONCLUSION: Boosting the postchemotherapy residual target dose to 30Gy can allow for dose escalation with a slight OAR dose increase. Treating the residual disease for the full 30Gy, however, would reduce the OAR dose significantly compared with ISRT21Gy. Studies should evaluate these strategies to improve outcomes and minimize the late effects.

Hypofractionated Proton Therapy with Concurrent Chemotherapy for Locally Advanced Non-Small Cell Lung Cancer: A Phase 1 Trial from the University of Florida and Proton Collaborative Group.

PURPOSE: We report the safety data from the first multicenter phase 1 trial investigating the use of hypofractionated proton therapy with concurrent chemotherapy for patients with stage II or III non-small cell lung cancer. METHODS AND MATERIALS: From 2013 through 2018, patients with newly diagnosed stage II or III non-small cell lung cancer were enrolled in a multicenter phase 1 clinical trial evaluating concurrent chemotherapy with increasing dose-per-fraction proton therapy. This was a stepwise 5 + 2 dose-intensification protocol with the following dose arms: (1) 2.5 GyRBE per fraction to 60 GyRBE; (2) 3.0 GyRBE per fraction to 60 GyRBE; (3) 3.53 GyRBE per fraction to 60.01 GyRBE; and (4) 4.0 GyRBE per fraction to 60 GyRBE. A dose arm was considered tolerable if no radiation therapy-attributable severe adverse event (SAE) occurred within 90 days of treatment among 5 patients enrolled on the arm or if 1 SAE occurred among 7 patients enrolled. Dose constraints to the heart, brachial plexus, and spinal cord were more conservative at higher doses per fraction. RESULTS: The study closed early because of slow accrual and competing enrollment in NRG 1308 before accrual was met, with no maximum tolerated dose identified. Eighteen patients were treated, including 5 patients on arms 1 and 2, 7 patients on arm 3, and 1 patient on arm 4. Two SAEs occurred among 7 patients treated at 3.53 GyRBE per fraction; however, per outside expert review, both were attributed to chemotherapy and unrelated to radiation therapy. CONCLUSIONS: Hypofractionated proton therapy delivered at 2.5 to 3.53 GyRBE per fraction to a dose of 60 GyRBE with concurrent chemotherapy has an acceptable toxicity profile. Further exploration of this regimen is warranted on a phase 2 clinical trial.

The Meaningless Meaning of Mean Heart Dose in Mediastinal Lymphoma in the Modern Radiation Therapy Era.

PURPOSE: Mean heart dose (MHD) correlates with late cardiac toxicity among survivors of lymphoma receiving involved-field radiation therapy (IFRT). We investigated MHD and cardiac substructure dose across older and newer radiation fields and techniques to understand the value of evaluating MHD alone. METHODS AND MATERIALS: After institutional review board approval, we developed a database of dosimetry plans for 40 patients with mediastinal lymphoma, which included IFRT (anterior-posterior and posterior-anterior), involved-site radiation therapy (ISRT) + 3-dimensional conformal radiation therapy (3DCRT), ISRT + intensity modulated radiation therapy, and ISRT + proton therapy plans for each patient. Each plan was evaluated for dose to the heart and cardiac substructures, including the right and left ventricles (RV, LV) and atria (RA, LA); tricuspid, mitral (MV), and aortic valves; and left anterior descending coronary artery (LAD). Correlation between MHD and cardiac substructure dose was assessed with linear regression. A correlation was considered very strong, strong, moderate, or weak if the r was ≥0.8, 0.6-0.79, 0.4-0.59, or <0.4, respectively. RESULTS: A very strong correlation was observed between MHD and the mean cardiac substructure dose for each plan as follows: IFRT-LV, RV, LA, MV and LAD; ISRT + 3DCRT-LV, RV, MV, TV, and LA; ISRT + intensity modulated radiation therapy-LV and RV; ISRT + proton therapy-none. The following strong correlations were observed: IFRT-RA; ISRT + 3DCRT-LAD, RA, AV; ISRT + IMRT-LA, RA, LAD, AV, TV, and MV; ISRT + proton therapy-LV only. CONCLUSIONS: In the management of mediastinal lymphoma, more conformal treatment techniques can lead to more heterogeneous dose distributions across the heart, which translate into weaker relationships between mean heart dose and mean cardiac substructure doses. Consequently, models for assessing the risk of cardiac toxicity after radiation therapy that rely on MHD can be misleading when using modern treatment fields and techniques. Contouring the cardiac substructures and evaluating their dose is important when using contemporary RT.

Cross-modality applicability of rectal normal tissue complication probability models from photon- to proton-based radiotherapy.

BACKGROUND AND PURPOSE: Proton therapy (PT) is currently being studied to improve normal tissue (NT) sparing beyond what can be achieved with conventional photon-based therapy. Compared to photons, PT dose distributions have a reduced NT low-to-intermediate 'dose bath' and a different biological effectiveness, questioning the applicability of photon-based NT complication probability (NTCP) models to PT. The aim of this study was to assess the applicability of photon-based NTCP models to rectum morbidity outcomes following PT. MATERIALS AND METHODS: Treatment planning and morbidity data from 1151 prostate cancer patients treated with passive scattering PT and from 159 patients treated with conventional 3D conformal four-field photon therapy were analysed. Prospectively scored gastrointestinal morbidities (grade >=2) were analysed, with a total of 184 events (protons; medical and procedural) and 12 events (photons; procedural only), respectively. Rectal dose volume histograms were extracted for all patients in both cohorts and used as input to two different NTCP models, with up to six different published photon-based parameter sets. RESULTS: Photon-based rectal NTCP models either over- or underestimated the clinically observed gastrointestinal morbidity when used on the proton cohort, depending on the choice of endpoint (p < 0.05 for all parameter sets, for both morbidity classifications). Four of the six photon-based NTCP models showed a good fit to the photon outcome data (p > 0.05). CONCLUSION: There were large differences in morbidity predictions between cohorts and modalities, indicating that the validity of NTCP models and parameters across institutions and treatment modalities should be carefully investigated prior to clinical application.

Impact of intrafraction prostate motion on clinical target coverage in proton therapy: A simulation study of dosimetric differences in two delivery techniques.

PURPOSE: To investigate the dosimetric impact of prostate intrafraction motion on proton double-scattering (DS) and uniform scanning (US) treatments using electromagnetic transponder-based prostate tracking data in simulated treatment deliveries. METHODS: In proton DS delivery, the spread-out Bragg peak (SOBP) is created almost instantaneously by the constant rotation of the range modulator. US, however, delivers each entire energy layer of the SOBP sequentially from distal to proximal direction in time, which can interplay with prostate intrafraction motion. This spatiotemporal interplay during proton treatment was simulated to evaluate its dosimetric impact. Prostate clinical target volume (CTV) dose was obtained by moving CTV through dose matrices of the energy layers according to prostate-motion traces. Fourteen prostate intrafraction motion traces of each of 17 prostate patients were used in the simulated treatment deliveries. Both single fraction dose-volume histograms (DVHs) and fraction-cumulative DVHs were obtained for both 2 Gy per fraction and 7.25 Gy per fraction stereotactic body radiotherapy (SBRT). RESULTS: The simulation results indicated that CTV dose degradation depends on the magnitude and direction of prostate intrafraction motion and is patient specific. For some individual fractions, prescription dose coverage decreased in both US and DS treatments, and hot and cold spots inside the CTV were observed in the US results. However, fraction-cumulative CTV dose coverage showed much reduced dose degradation for both DS and US treatments for both 2 Gy per fraction and SBRT simulations. CONCLUSIONS: This study indicated that CTV dose inhomogeneity may exist for some patients with severe prostate intrafraction motion during US treatments. However, there are no statistically significant dose differences between DS and US treatment simulations. Cumulative dose of multiple-fractions significantly reduced dose uncertainties.

Radiation doses to brain substructures associated with cognition in radiotherapy of pediatric brain tumors.

Background: Several brain substructures associated with cognition (BSCs) are located close to typical pediatric brain tumors. Pediatric patients therefore have considerable risks of neurocognitive impairment after brain radiotherapy. In this study, we investigated the radiation doses received by BSCs for three common locations of pediatric brain tumor entities. Material and methods: For ten patients in each group [posterior fossa ependymoma (PFE), craniopharyngioma (CP), and hemispheric ependymoma (HE)], the cumulative fraction of BSCs volumes receiving various dose levels were analyzed. We subsequently explored the differences in dose pattern between the three groups and used available dose response models from the literature to estimate treatment-induced intelligence quotient (IQ) decline. Results: Doses to BSCs were found to differ considerably between the groups, depending on their position relative to the tumor. Large inter-patient variations were observed in the ipsilateral structures of the HE groups, and at low doses for all three groups. IQ decline estimates differed depending on the model applied, presenting larger variations in the HE group. Conclusion: While there were notable differences in the dose patterns between the groups, the extent of estimated IQ decline depended more on the model applied. This inter-model variability should be considered in dose-effect assessments on cognitive outcomes of pediatric patients.