Encouraging Experience with Image-Guided Pencil Beam Scanning Proton Therapy in Craniopharyngioma-First Case Series From India

OBJECTIVE

We report our early clinical experience with image-guided, pencil beam scanning proton beam therapy (PBS-PBT) for residual and recurrent craniopharyngioma.

METHODS

Between September 2019 and January 2023, 19 consecutive patients with residual or recurrent craniopharyngioma, suitable for radiotherapy and treated with image-guided PBS-PBT were analyzed. We documented detailed dosimetric data, acute toxicities, early outcomes, and imaging response on follow-up magnetic resonance imaging scans.

RESULTS

A total of 19 patients (11 males and 8 females) with residual or recurrent craniopharyngioma were treated during the study period. The median age of the cohort was 14 years (range, 3-33 years). The histology of most lesions was the adamantinomatous subtype (95%). The most common clinical presentation (before PBT) and most common endocrine deficit was visual disturbance (79%) and hypocortisolism (74%), respectively. Of the 19 patients, 13 had recurrent craniopharyngioma, and 5 had undergone radiotherapy previously. Five patients (26%) had undergone surgery ≥3 times before proton therapy. The median dose delivered was 54 GyE. The most common acute toxicity was grade 1 alopecia (63%). No patient experienced grade ≥3 acute toxicity. With a median follow-up of 18 months (range, 3-40 months), 12 patients showed shrinkage of the residual tumor and/or cyst, and 4 showed a dramatic cyst reduction at 3-9 months of follow-up. Two patients experienced a reduction in both solid and cystic components, with the remaining experiencing a reduction in the cystic component only. The remaining 8 patients had stable disease on magnetic resonance imaging, with 100% disease control and overall survival. Visual function remained stable after treatment.

CONCLUSIONS

Our preliminary experience with modern PBS-PBT and image guidance for craniopharyngioma is encouraging. Proton therapy in our cohort was well tolerated, resulting in limited toxicity and promising early outcomes.

Advancing knowledge-based intensity modulated proton planning for adaptive treatment of high-risk prostate cancer

To assess the performance of a knowledge-based planning (KBP) model for generating intensity-modulated proton therapy (IMPT) treatment plans as part of an adaptive radiotherapy (ART) strategy for patients with high-risk prostate cancer. A knowledge-based planning (KBP) model for proton adaptive treatment plan generation was developed based on thirty patient treatment plans utilizing RapidPlanTM PT (Varian Medical Systems, Palo Alto, CA). The model was subsequently validated using an additional eleven patient cases. All patients in the study were administered a prescribed dose of 70.2 Gy to the prostate and seminal vesicle (CTV70.2), along with 46.8 Gy to the pelvic lymph nodes (CTV46.8) through simultaneous integrated boost (SIB) technique. To assess the quality of the validation knowledge-based proton plans (KBPPs), target coverage and organ-at-risk (OAR) dose-volume constraints were compared against those of clinically used expert plans using paired t-tests. The KBP model training statistics (R2) (mean ± SD, 0.763 ± 0.167, range, 0.406 to 0.907) and χ² values (1.162 ± 0.0867, 1.039-1.253) indicate acceptable model training quality. Moreover, the average total treatment planning optimization and calculation time for adaptive plan generation is approximately 10 minutes. The CTV70.2 D98% for the KBPPs (mean ± SD, 69.1 ± 0.08 Gy) and expert plans (69.9 ± 0.04 Gy) shows a significant difference (p < 0.05) but are both within 1.1 Gy of the prescribed dose which is clinically acceptable. While the maximum dose for some organs-at-risk (OARs) such as the bladder and rectum is generally higher in the KBPPs, the doses still fall within clinical constraints. Among all the OARs, most of them received comparable results to the expert plan, except the cauda equina Dmax, which shows statistical significance and was lower in the KBPPs than in expert plans (48.5 ± 0.06 Gy vs 49.3 ± 0.05 Gy). The generated KBPPs were clinically comparable to manually crafted plans by expert treatment planners. The adaptive plan generation process was completed within an acceptable timeframe, offering a quick same-day adaptive treatment option. Our study supports the integration of KBP as a crucial component of an ART strategy, including maintaining plan consistency, improving quality, and enhancing efficiency. This advancement in speed and adaptability promises more precise treatment in proton ART.

Breaking barriers: Stereotactic ablative proton and photon radiation therapy for renal cell carcinoma with extensive metastases: A case report

Patients with advanced renal cancer (RCC) often have limited success with systemic therapy due to tumor heterogeneity. However, stereotactic ablative radiotherapy (SABR) has been shown to have a beneficial therapeutic effect for oligometastatic disease when used early. Despite this, current guidelines recommend the use of tyrosine kinase inhibitors (TKIs) as the first-line therapeutic agent for patients with recurrent or metastatic kidney cancer. Additionally, there is limited data on the combination of systemic treatment and SABR for extensive metastatic RCC due to concerns about high toxicity. Proton therapy offers a promising treatment option as it emits energy at a specific depth, generating high target doses while minimizing damage to normal tissue. This allows for precise treatment of various tumor lesions. In this case report, we describe a high-risk 65-year-old male with extensive pleural and thoracic lymph node metastases and 2 bone metastases of clear cell renal cancer. While the targeted therapy and immunotherapy effectively treated the bone metastases, it was not effective in treating the chest metastases, including the pleural and lymph node metastases. Thus, the patient received full-coverage radiotherapy with photon for primary renal tumor and intensity-modulated proton therapy (IMPT) for thoracic metastases. The patient showed no evidence of disease for 1 year after the initial radiotherapy, and no severe SABR-related adverse effects were observed until now. The combination of targeted therapy and immunotherapy with full-coverage radiotherapy may be a promising treatment option for selected patients with extensive metastatic renal cancer, especially as proton therapy allows for more precise control of the beam and minimal damage to normal tissue. This case has motivated us to investigate the potential advantages of administering proton therapy concurrently with systemic therapy in the management of metastatic renal cell carcinoma patients.

Dose evaluation of inter- and intra-fraction prostate motion in extremely hypofractionated intensity-modulated proton therapy for prostate cancer

Inter- and intra-fractional prostate motion can deteriorate the dose distribution in extremely hypofractionated intensity-modulated proton therapy. We used verification CTs and prostate motion data calculated from 1024 intra-fractional prostate motion records to develop a voxel-wise based 4-dimensional method, which had a time resolution of 1 s, to assess the dose impact of prostate motion. An example of 100 fractional simulations revealed that motion had minimal impact on planning dose, the accumulated dose in 95 % of the scenarios fulfilled the clinical goals for target coverage (D95 > 37.5 Gy). This method can serve as a complementary measure in clinical setting to guarantee plan quality.

Dosimetric comparison of pencil beam scanning proton therapy with or without multi-leaf collimator versus volumetric-modulated arc therapy for treatment of malignant glioma

This study aimed to examine the dosimetric effect of intensity-modulated proton therapy (IMPT) with a multi-leaf collimator (MLC) in treating malignant glioma. We compared the dose distribution of IMPT with or without MLC (IMPT_MLC+ or IMPT_MLC-, respectively) using pencil beam scanning and volumetric-modulated arc therapy (VMAT) in simultaneous integrated boost (SIB) plans for 16 patients with malignant gliomas. High- and low-risk target volumes were assessed using D_2%, V_90%, V_95%, homogeneity index (HI), and conformity index (CI). Organs at risk (OARs) were evaluated using the average dose (D_mean) and D_2%. Furthermore, the dose to the normal brain was evaluated using from V_5Gy to V_40Gy at 5 Gy intervals. There were no significant differences among all techniques regarding V_90%, V_95%, and CI for the targets. HI and D_2% for IMPT_MLC+ and IMPT_MLC- were significantly superior to those for VMAT (p < 0.01). The D_mean and D_2% of all OARs for IMPT_MLC+ were equivalent or superior to those of other techniques. Regarding the normal brain, there was no significant difference in V_40Gy among all techniques whereas V_5Gy to V_35Gy in IMPT_MLC+ were significantly smaller than those in IMPT_MLC- (with differences ranging from 0.45% to 4.80%, p < 0.05) and VMAT (with differences ranging from 6.85% to 57.94%, p < 0.01). IMPT_MLC+ could reduce the dose to OARs, while maintaining target coverage compared to IMPT_MLC- and VMAT in treating malignant glioma.

Clinical 3D/4D cumulative proton dose assessment methods for thoracic tumours with large motion

PURPOSE

Despite the anticipated clinical benefits of intensity-modulated proton therapy (IMPT), plan robustness may be compromised due to its sensitivity to patient treatment uncertainties, especially for tumours with large motion. In this study, we investigated treatment course-wise plan robustness for intra-thoracic tumours with large motion comparing a 4D pre-clinical evaluation method (4DREM) to our clinical 3D/4D dose reconstruction and accumulation methods.

MATERIALS AND METHODS

Twenty patients with large target motion (>10 mm) were treated with five times layered rescanned IMPT. The 3D-robust optimised plans were generated on the averaged planning 4DCT. Using multiple 4DCTs, treatment plan robustness was assessed on a weekly and treatment course-wise basis through the 3D robustness evaluation method (3DREM, based on averaged 4DCTs), the 4D robustness evaluation method (4DREM, including the time structure of treatment delivery and 4DCT phases) and 4D dose reconstruction and accumulation (4DREAL, based on fraction-wise information).

RESULTS

Baseline target motion for all patients ranged from 11-17 mm. For the offline adapted course-wise dose assessment, adequate target dose coverage was found for all patients. The target volume receiving 95% of the prescription dose was consistent between methods with 16/20 patients showing differences < 1%. 4DREAL showed the highest target coverage (99.8 ± 0.6%, p < 0.001), while no differences were observed between 3DREM and 4DREM (99.3 ± 1.3% and 99.4 ± 1.1%, respectively).

CONCLUSION

Our results show that intra-thoracic tumours can be adequately treated with IMPT in free breathing for target motion amplitudes up to 17 mm employing any of the accumulation methods. Anatomical changes, setup and range errors demonstrated a more severe impact on target coverage than motion in these patients treated with fractionated proton radiotherapy.

High-dose Intensity-modulated proton therapy versus Standard-dose Intensity-modulated RadIation therapy for esophageal squamous cell carcinoma (HI-SIRI): study protocol for a randomized controlled clinical trial

Background

Chemoradiotherapy is the standard of care for esophageal cancer as a neoadjuvant treatment before surgery, or as a definitive treatment for unresectable disease. Intensity-modulated radiotherapy (IMRT) has been considered the standard radiation technique. However, patients suffer from treatment-related toxicities, and most die from disease progression or recurrence. With emerging technological advancement, proton therapy has theoretical advantages over IMRT because it offers apparent dosimetric benefits to allow dose escalation to the target while better sparing surrounding tissues such as the lungs, heart, liver, and spinal cord. The purpose of this study protocol is to investigate the survival benefit of proton therapy using modern intensity-modulated proton therapy (IMPT) compared to standard IMRT for esophageal cancer.

Methods

This is a two-arm open phase II/III multi-institution randomized controlled trial. Eligible patients will have histologically confirmed squamous cell carcinoma of the thoracic esophagus with no evidence of tracheoesophageal/esophagobronchial fistula or distant metastasis. After stratification according to resectability status (resectable vs. borderline resectable/unresectable), a total of 232 patients will be randomized to receive IMPT or IMRT using a 1:1 allocation ratio. In resectable cases, surgical resection following concurrent chemoradiation will be attempted for the patients who are medically fit at the time of surgery. In those with initially borderline resectable/unresectable disease, definitive concurrent chemoradiation will be performed. The phase II study will assess safety (toxicity and postoperative complications) and feasibility (recruitment rate and chemoradiation dose modification) in 40 patients into each arm. The study will then continue into phase III, further recruit 76 patients into each arm, and compare progression-free survival between IMPT vs IMRT groups. The secondary endpoints will be overall survival, local and distant control, toxicities, health-related quality of life, and cost-utility. This protocol describes a detailed radiotherapy and chemotherapy.

Discussion

This randomized clinical trial will demonstrate the clinical benefit of IMPT in esophageal cancer treatment in terms of survival and toxicity outcomes which will further establish high-level evidence for radiation modality in squamous cell carcinoma of the thoracic esophagus.

Trial registration

TCTR20200310006. Registered 10 March 2020.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13063-022-06822-8.

Training and validation of a knowledge-based dose-volume histogram predictive model in the optimisation of intensity-modulated proton and volumetric modulated arc photon plans for pleural mesothelioma patients

BACKGROUND

To investigate the performance of a narrow-scope knowledge-based RapidPlan (RP) model for optimisation of intensity-modulated proton therapy (IMPT) and volumetric modulated arc therapy (VMAT) plans applied to patients with pleural mesothelioma. Second, estimate the potential benefit of IMPT versus VMAT for this class of patients.

METHODS

A cohort of 82 patients was retrospectively selected; 60 were used to "train" a dose-volume histogram predictive model; the remaining 22 provided independent validation. The performance of the RP models was benchmarked, comparing predicted versus achieved mean and near-to-maximum dose for all organs at risk (OARs) in the training set and by quantitative assessment of some dose-volume metrics in the comparison of the validation RP-based data versus the manually optimised training datasets. Treatment plans were designed for a prescription dose of 44 Gy in 22 fractions (proton doses account for a fixed relative biological effectiveness RBE = 1.1).

RESULTS

Training and validation RP-based plans resulted dosimetrically similar for both VMAT and IMPT groups, and the clinical planning aims were met for all structures. The IMPT plans outperformed the VMAT ones for all OARs for the contra-lateral and the mean and low dose regions for the ipsilateral OARs. Concerning the prediction performance of the RP models, the linear regression for the near-to-maximum dose resulted in D_achieved = 1.03D_predicted + 0.58 and D_achieved = 1.02D_predicted + 1.46 for VMAT and IMPT, respectively. For the mean dose it resulted: D_achieved = 0.99D_predicted + 0.34 and D_achieved = 1.05D_predicted + 0.27 respectively. In both cases, the linear correlation between prediction and achievement is granted with an angular coefficient deviating from unity for less than 5%. Concerning the dosimetric comparison between manual plans in the training cohort and RP-based plans in the validation cohort, no clinical differences were observed for the target volumes in both the VMAT and IMPT groups. Similar consistency was observed for the dose-volume metrics analysed for the OAR. This proves the possibility of achieving the same quality of plans with manual procedures (the training set) or with automated RP-based methods (the validation set).

CONCLUSION

Two models were trained and validated for VMAT and IMPT plans for pleural mesothelioma. The RP model performance resulted satisfactory as measured by the agreement between predicted and achieved (after full optimisation) dose-volume metrics. The IMPT plans outperformed the VMAT plans for all the OARs (with different intensities for contra- or ipsilateral structures). RP-based planning enabled the automation of part of the optimisation and the harmonisation of the dose-volume results between training and validation. The IMPT data showed a systematic significant dosimetric advantage over VMAT. In general, using an RP-based approach can simplify the optimisation workflow in these complex treatment indications without impacting the quality of plans.

Improving workflow for adaptive proton therapy with predictive anatomical modelling: A proof of concept

PURPOSE

To demonstrate predictive anatomical modelling for improving the clinical workflow of adaptive intensity-modulated proton therapy (IMPT) for head and neck cancer.

METHODS

10 radiotherapy patients with nasopharyngeal cancer were included in this retrospective study. Each patient had a planning CT, weekly verification CTs during radiotherapy and predicted weekly CTs from our anatomical model. Predicted CTs were used to create predicted adaptive plans in advance with the aim of maintaining clinically acceptable dosimetry. Adaption was triggered when the increase in mean dose (Dmean) to the parotid glands exceeded 3 Gy(RBE). We compared the accumulated dose of two adaptive IMPT strategies: 1) Predicted plan adaption: One adaptive plan per patient was optimised on a predicted CT triggered by replan criteria. 2) Standard replan: One adaptive plan was created reactively in response to the triggering weekly CT.

RESULTS

Statistical analysis demonstrates that the accumulated dose differences between two adaptive strategies are not significant (p > 0.05) for CTVs and OARs. We observed no meaningful differences in D95 between the accumulated dose and the planned dose for the CTVs, with mean differences to the high-risk CTV of -1.20 %, -1.23 % and -1.25 % for no adaption, standard and predicted plan adaption, respectively. The accumulated parotid Dmean using predicted plan adaption is within 3 Gy(RBE) of the planned dose and 0.31 Gy(RBE) lower than the standard replan approach on average.

CONCLUSION

Prediction-based replanning could potentially enable adaptive therapy to be delivered without treatment gaps or sub-optimal fractions, as can occur during a standard replanning strategy, though the benefit of using predicted plan adaption over the standard replan was not shown to be statistically significant with respect to accumulated dose in this study. Nonetheless, a predictive replan approach can offer advantages in improving clinical workflow efficiency.

Feasibility study of fast intensity-modulated proton therapy dose prediction method using deep neural networks for prostate cancer

PURPOSE

Compared to the pencil-beam algorithm, the Monte-Carlo (MC) algorithm is more accurate for dose calculation but time-consuming in proton therapy. To solve this problem, this study uses deep learning to provide fast 3D dose prediction for prostate cancer patients treated with intensity-modulated proton therapy (IMPT).

METHODS

A novel recurrent U-net (RU-net) architecture was trained to predict the 3D dose distribution. Doses, CT images, and beam spot information from IMPT plans were used to train the RU-net with a 5-fold cross-validation. However, predicting the complicated dose properties of the IMPT plan is difficult for neural networks. Instead of the Peak-MU model, this work develops the Multi-MU model that adopted more comprehensive inputs and was trained with a combinational loss function. The dose difference between the prediction dose and MC dose was evaluated with gamma analysis, dice similarity coefficient (DSC), and dose-volume histogram (DVH) metrics. The Monte-Carlo dropout was also added to the network to quantify the uncertainty of the model.

RESULTS

Compared to the Peak-MU model, the Multi-MU model led to smaller mean absolute errors (3.03% vs. 2.05%, p = 0.005), higher gamma-passing rate (2mm, 3%: 97.42% vs. 93.69%, p = 0.005), higher dice similarity coefficient, and smaller relative DVH metrics error (CTV D_98% : 3.03% vs. 6.08%, p = 0.017; in Bladder V30: 3.08% vs. 5.28%, p = 0.028; and in Bladder V20: 3.02% vs. 4.42%, p = 0.017). Considering more prior knowledge, the Multi-MU model had better-predicted accuracy with a prediction time of less than half a second for each fold. The mean uncertainty value of the Multi-MU model is 0.46%, with a dropout rate of 10%.

CONCLUSION

This method was a nearly real-time IMPT dose prediction algorithm with accuracy comparable to the PB analytical algorithms used in prostate cancer. This RU-net might be used in plan robustness optimization and robustness evaluation in the future. This article is protected by copyright. All rights reserved.

Evaluation of the clinical value of automatic online dose restoration for adaptive proton therapy of head and neck cancer

INTRODUCTION

Intensity modulated proton therapy (IMPT) is highly sensitive to anatomical variations which can cause inadequate target coverage during treatment. This study compares not-adapted (NA) robust plans to two adaptive IMPT methods - a fully-offline adaptive (FOA) and a simplified automatic online adaptive strategy (dose restoration (DR)) to determine the benefit of DR, in head and neck cancer (HNC).

MATERIAL/METHODS

Robustly optimized clinical IMPT doses in planning-CTs (pCTs) were available for a cohort of 10 HNC patients. During robust re-optimization, DR used isodose contours, generated from the clinical dose on pCTs, and patient specific objectives to reproduce the clinical dose in every repeated-CT(rCT). For each rCT(n=50), NA, DR and FOA plans were robustly evaluated.

RESULTS

An improvement in DVH-metrics and robustness was seen for DR and FOA plans compared to NA plans. For NA plans, 74%(37/50) of rCTs did not fulfill the CTV coverage criteria (D98%>95%Dprescription). DR improved target coverage, target homogeneity and variability on critical risk organs such as the spinal cord. After DR, 52%(26/50) of rCTs met all clinical goals. Because of large anatomical changes and/or inaccurate patient repositioning, 48%(24/50) of rCTs still needed full offline adaptation to ensure an optimal treatment since dose restoration was not able to re-establish the initial plan quality.

CONCLUSION

Robust optimization together with fully-automatized DR avoided offline adaptation in 52% of the cases. Implementation of dose restoration in clinical routine could ensure treatment plan optimality while saving valuable human and material resources to radiotherapy departments.

Volumetric modulated arc therapy versus intensity-modulated proton therapy in the irradiation of infra diaphragmatic Hodgkin Lymphoma in female patients

PURPOSE

To investigate the role of infra diaphragmatic intensity-modulated proton therapy (IMPT) compared to volumetric modulated arc therapy (VMAT) for female Hodgkin Lymphoma (HL) patients and to estimate the risk of secondary cancer and ovarian failure.

METHODS

A comparative treatment planning study was performed on 14 patients, and the results were compared according to conventional dose-volume metrics. In addition, estimates of the excess absolute risk (EAR) of secondary cancer induction were determined for the bowel, the bladder and the rectum. For the ovaries, the risk of ovarian failure was estimated.

RESULTS

The dosimetric findings demonstrate the equivalence between VMAT and IMPT in terms of target coverage. A statistically significant reduction of the mean and near-to-maximum doses was proven for the organs at risk. The EAR ratio estimated for IMPT to VMAT was 0.51 ± 0.32, 0.32 ± 0.35 and 0.05 ± 0.11 for the bowel, bladder and rectum, respectively. Concerning the risk of ovarian failure for the chronologic age ranging from 18 to 46 years, the expected net loss in fertility years ranged from 4.8 to 3.0 years for protons and 12.0 to 5.7 years for photons.

CONCLUSION

This in-silico study confirmed the beneficial role of IMPT from a dosimetric point of view. Mathematical models suggested that the use of protons might be further advantageous due to the expected reduction of the risk of secondary cancer induction and its milder impact on the reduction of fertility.

Evaluation of alternative parameter settings for dose restoration and full plan adaptation in IMPT for prostate cancer

BACKGROUND/PURPOSE

Intensity-modulated proton therapy is highly sensitive to anatomical variations. A dose restoration method and a full plan adaptation method have been developed earlier, both requiring several parameter settings. This study evaluates the validity of the previously selected settings by systematically comparing them to alternatives.

MATERIALS/METHODS

The dose restoration method takes a prior plan and uses an energy-adaptation followed by a spot-intensity re-optimization to restore the plan to its initial state. The full adaptation method uses an energy-adaptation followed by the addition of new spots and a spot-intensity optimization to fit the new anatomy. We varied: 1) The margins and robustness settings of the prior plan, 2) the spot-addition sample size, i.e. the number of added spots, 3) the spot-addition stopping criterion, and 4) the spot-intensity optimization approach. The last three were evaluated only for the full plan adaptation. Evaluations were done on 88 CT scans of 11 prostate cancer patients. Dose was prescribed as 55 Gy(RBE) to the lymph nodes and seminal vesicles with a boost to 74 Gy(RBE) to the prostate.

RESULTS

For the dose restoration method, changing the applied CTV-to-PTV margins and plan robustness in the prior plans yielded insufficient target coverage or increased OAR doses. For the full plan adaptation, more spot-addition iterations and using a different optimization approach resulted in lower OAR doses compared to the default settings while maintaining target coverage. However, the calculation times increased by up to 20 times, making these variations infeasible for online-adaptation.

CONCLUSION

We recommend maintaining the default setting for the dose restoration approach. For the full plan adaptation we recommend to focus on fine-tuning the optimization-parameters, and apart from this using the default settings.

A treatment planning study of urethra-sparing intensity-modulated proton therapy for localized prostate cancer

•

US-IMPT can potentially reduce the risk of genitourinary toxicities.

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The urethral NTCP value in US-IMPT is significantly lower than in the clinical plan.

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TCP for CTV did not differ significantly between the clinical and US-IMPT plans.

Background and Purpose

Urethra-sparing radiation therapy for localized prostate cancer can reduce the risk of radiation-induced genitourinary toxicity by intentionally underdosing the periurethral transitional zone. We aimed to compare the clinical impact of a urethra-sparing intensity-modulated proton therapy (US-IMPT) plan with that of conventional clinical plans without urethral dose reduction.

Materials and Methods

This study included 13 patients who had undergone proton beam therapy. The prescribed dose was 63 GyE in 21 fractions for 99% of the clinical target volume. To compare the clinical impact of the US-IMPT plan with that of the conventional clinical plan, tumor control probability (TCP) and normal tissue complication probability (NTCP) were calculated with a generalized equivalent uniform dose-based Lyman–Kutcher model using dose volume histograms. The endpoints of these model parameters for the rectum, bladder, and urethra were fistula, contraction, and urethral stricture, respectively.

Results

The mean NTCP value for the urethra in US-IMPT was significantly lower than that in the conventional clinical plan (0.6% vs. 1.2%, p < 0.05). There were no statistically significant differences between the conventional and US-IMPT plans regarding the mean minimum dose for the urethra with a 3-mm margin, TCP value, and NTCP value for the rectum and bladder. Additionally, the target dose coverage of all plans in the robustness analysis was within the clinically acceptable range.

Conclusions

Compared with the conventional clinically applied plans, US-IMPT plans have potential clinical advantages and may reduce the risk of genitourinary toxicities, while maintaining the same TCP and NTCP in the rectum and bladder.

Anatomic changes in head and neck intensity-modulated proton therapy: Comparison between robust optimization and online adaptation

BACKGROUND/PURPOSE

Setup variations and anatomical changes can severely affect the quality of head and neck intensity-modulated proton therapy (IMPT) treatments. The impact of these changes can be alleviated by increasing the plan's robustness a priori, or by adapting the plan online. This work compares these approaches in the context of head and neck IMPT.

MATERIALS/METHODS

A representative cohort of 10 head and neck squamous cell carcinoma (HNSCC) patients with daily cone-beam computed tomography (CBCT) was evaluated. For each patient, three IMPT plans were created: 1- a classical robust optimization (cRO) plan optimized on the planning CT, 2- an anatomical robust optimization (aRO) plan additionally including the two first daily CBCTs and 3- a plan optimized without robustness constraints, but online-adapted (OA) daily, using a constrained spot intensity re-optimization technique only.

RESULTS

The cumulative dose following OA fulfilled the clinical objective of both the high-risk and low-risk clinical target volumes (CTV) coverage in all 10 patients, compared to 8 for aRO and 4 for cRO. aRO did not significantly increase the dose to most organs at risk compared to cRO, although the integral dose was higher. OA significantly reduced the integral dose to healthy tissues compared to both robust methods, while providing equivalent or superior target coverage.

CONCLUSION

Using a simple spot intensity re-optimization, daily OA can achieve superior target coverage and lower dose to organs at risk than robust optimization methods.

Intensity-modulated proton therapy for oropharyngeal cancer reduces rates of late xerostomia

BACKGROUND AND PURPOSE

To determine rates of xerostomia after intensity-modulated radiotherapy (IMRT) or intensity-modulated proton therapy (IMPT) for oropharyngeal cancer (OPC) and identify dosimetric factors associated with xerostomia risk.

MATERIALS AND METHODS

Patients with OPC who received IMRT (n = 429) or IMPT (n = 103) from January 2011 through June 2015 at a single institution were studied retrospectively. Every 3 months after treatment, each patient completed an eight-item self-reported xerostomia-specific questionnaire (XQ; summary XQ score, 0-100). An XQ score of 50 was selected as the demarcation value for moderate-severe (XQs ≥ 50) and no-mild (XQs < 50) xerostomia. The mean doses and percent volumes of organs at risk receiving various doses (V5-V70) were extracted from the initial treatment plans. The dosimetric variables and xerostomia risk were compared using an independent-sample t-test or chi-square test.

RESULTS

The median follow-up time was 36.2 months. The proportions of patients with moderate-severe xerostomia were similar in the two treatment groups up to 18 months after treatment. However, moderate-severe xerostomia was less common in the IMPT group than in the IMRT group at 18-24 months (6% vs. 20%; p = 0.025) and 24-36 months (6% vs. 20%; p = 0.01). During the late xerostomia period (24-36 months), high dose/volume exposures (V25-V70) in the oral cavity were associated with high proportions of patients with moderate-severe xerostomia (all p < 0.05), but dosimetric variables regarding the salivary glands were not associated with late xerostomia.

CONCLUSION

IMPT was associated with less late xerostomia than was IMRT in OPC patients. Oral cavity dosimetric variables were related to the occurrence of late xerostomia.

Knowledge-based intensity-modulated proton planning for gastroesophageal carcinoma

PURPOSE

To investigate the performance of a narrow-scope knowledge-based RapidPlan (RP) model, for optimisation of intensity-modulated proton therapy (IMPT) plans applied to patients with locally advanced carcinoma in the gastroesophageal junction.

METHODS

A cohort of 60 patients was retrospectively selected; 45 were used to 'train' a dose-volume histogram predictive model; the remaining 15 provided independent validation. The performance of the RP model was benchmarked against manual optimisation. Quantitative assessment was based on several dose-volume metrics.

RESULTS

Manual and RP-optimised IMPT plans resulted dosimetrically similar, and the planning dose-volume objectives were met for all structures. Concerning the validation set, the comparison of the manual vs RP-based plans, respectively, showed for the target (PTV): the homogeneity index was 6.3 ± 2.2 vs 5.9 ± 1.2, and V98% was 89.3 ± 2.9 vs 91.4 ± 2.2% (this was 97.2 ± 1.9 vs 98.8 ± 1.1 for the CTV). Regarding the organs at risk, no significant differences were reported for the combined lungs, the whole heart, the left anterior descending artery, the kidneys, the spleen and the spinal canal. The D_0.1 cm3 for the left ventricle resulted in 40.3 ± 3.4 vs 39.7 ± 4.3 Gy(RBE). The mean dose to the liver was 3.4 ± 1.3 vs 3.6 ± 1.5 Gy(RBE).

CONCLUSION

A narrow-scope knowledge-based RP model was trained and validated for IMPT delivery in locally advanced cancer of the gastroesophageal junction. The results demonstrate that RP can create models for effective IMPT. Furthermore, the equivalence between manual interactive and unattended RP-based optimisation could be displayed. The data also showed a high correlation between predicted and achieved doses in support of the valuable predictive power of the RP method.

The Role of Plan Robustness Evaluation in Comparing Protons and Photons Plans - An Application on IMPT and IMRT Plans in Skull Base Chordomas

Purpose:

To analyze robustness of treatment plans optimized using different approaches in intensity modulated proton therapy (IMPT) and investigate the necessity of robust optimization and evaluation in intensity modulated radiotherapy (IMRT) plans for skull base chordomas.

Materials and Methods:

Two photon plans, standard IMRT and robustly optimized IMRT (RB-IMRT), and two IMPT plans, robustly optimized multi field optimization (MFO) and hybrid-MFO (HB-MFO), were created in RayStation TPS for five patients previously treated using single field uniform optimization (SFO). Both set-up and range uncertainties were incorporated during robust optimization of IMPT plans whereas only set-up uncertainty was used in RB-IMRT. The dosimetric outcomes from the five planning techniques were compared for every patient using standard dose volume indices and integral dose (ID) estimated for target and organs at risk (OARs). Robustness of each treatment plan was assessed by introducing set-up uncertainties of ±3 mm along the three translational axes and, only in protons, an additional range uncertainty of ±3.5%.

Results:

All the five nominal plans provided comparable and clinically acceptable target coverage. In comparison to nominal plans, worst case decrease in D_95% of clinical target volume-high risk (CTV-HR) were 11.1%, 13.5%, and 13.6% for SFO, MFO, and HB-MFO plans respectively. The corresponding values were 13.7% for standard IMRT which improved to 11.5% for RB-IMRT. The worst case increased in high dose (D_1%) to CTV-HR was highest in IMRT (2.1%) and lowest in SFO (0.7%) plans. Moreover, IMRT showed worst case increases in D_1% for all neurological OARs and were lowest for SFO plans. The worst case D_1% for brainstem, chiasm, spinal cord, optic nerves, and temporal lobes were increased by 29%, 41%, 30%, 41% and 14% for IMRT and 18%, 21%, 21%, 24%, and 7% for SFO plans, respectively. In comparison to IMRT, RB-IMRT improved D_1% of all neurological OARs ranging from 5% to 14% in worst case scenarios.

Conclusion:

Based on the five cases presented in the current study, all proton planning techniques (SFO, MFO and HB-MFO) were robust both for target coverage and OARs sparing. Standard IMRT plans were less robust than proton plans in regards to high doses to neurological OARs. However, robust optimization applied to IMRT resulted in improved robustness in both target coverage and high doses to OARs. Robustness evaluation may be considered as a part of plan evaluation procedure even in IMRT.

Dose-escalated simultaneously integrated boost photon or proton therapy in pancreatic cancer in an in-silico study: Gastrointestinal organs remain critical

•

Robustly optimized proton plans (rMFO-IMPT) with simultaneously integrated boost (SIB) were clinically applicable.

•

Gastrointestinal organs reached critical dose values in rMFO-IMPT, VMAT and Tomotherapy techniques.

•

rMFO-IMPT significantly reduced the low and intermediate dose to organs at risk.

•

No clinically significant differences on results depending on tumor location or surgical status were observed.

Purpose

To compare the dosimetric results of an in-silico study among intensity-modulated photon (IMRT) and robustly optimized intensity-modulated proton (IMPT) treatment techniques using a dose-escalated simultaneously integrated boost (SIB) approach in locally recurrent or advanced pancreatic cancer patients.

Material and methods

For each of 15 locally advanced pancreatic cancer patients, a volumetric-modulated arc therapy (VMAT), a Tomotherapy (TOMO), and an IMPT treatment plan was optimized on free-breathing treatment planning computed tomography (CT) images. For the photon treatment plans, doses of 66 Gy and 51 Gy, both as SIB in 30 fractions, were prescribed to the gross tumor volume (GTV) and to the planning target volume (PTV), respectively. For the proton plans, a dose prescription of 66 Gy(RBE) to the GTV and of 51 Gy(RBE) to the clinical target volume (CTV) was planned. For each SIB-treatment plan, doses to the targets and OARs were evaluated and statistically compared.

Results

All treatment techniques reached the prescribed doses to the GTV and CTV or PTV. The stomach and the bowel, in particular the duodenum and the small bowel, were found to be frequently exposed to doses exceeding 50 Gy, irrespective of the treatment technique. For doses below 50 Gy, the IMPT technique was statistically significant superior to both IMRT techniques regarding decreasing dose to the OARs, e.g. volume of the bowel receiving 15 Gy (V_15Gy) was reduced for IMPT compared to VMAT (p = 0.003) and TOMO (p < 0.001).

Conclusion

With all photon and proton techniques investigated, the radiation dose to gastrointestinal OARs remained critical when treating patients with unresectable locally recurrent or advanced pancreatic cancer using a dose-escalated SIB approach.

Intensity-modulated proton therapy (IMPT) interplay effect evaluation of asymmetric breathing with simultaneous uncertainty considerations in patients with non-small cell lung cancer

PURPOSE

Intensity-modulated proton therapy (IMPT) is sensitive to uncertainties from patient setup and proton beam range, as well as interplay effect. In addition, respiratory motion may vary from cycle to cycle, and also from day to day. These uncertainties can severely degrade the original plan quality and potentially affect patient's outcome. In this work, we developed a new tool to comprehensively consider the impact of all these uncertainties and provide plan robustness evaluation under them.

METHODS

We developed a comprehensive plan robustness evaluation tool that considered both uncertainties from patient setup and proton beam range, as well as respiratory motion simultaneously. To mimic patients' respiratory motion, the time spent in each phase was randomly sampled based on patient-specific breathing pattern parameters as acquired during the four-dimensional (4D)-computed tomography (CT) simulation. Spots were then assigned to one specific phase according to the temporal relationship between spot delivery sequence and patients' respiratory motion. Dose in each phase was calculated by summing contributions from all the spots delivered in that phase. The final 4D dynamic dose was obtained by deforming all doses in each phase to the maximum exhalation phase. Three hundred (300) scenarios (10 different breathing patterns with 30 different setup and range uncertainty scenario combinations) were calculated for each plan. The dose-volume histograms (DVHs) band method was used to assess plan robustness. Benchmarking the tool as an application's example, we compared plan robustness under both three-dimensional (3D) and 4D robustly optimized IMPT plans for 10 nonrandomly selected patients with non-small cell lung cancer.

RESULTS

The developed comprehensive plan robustness tool had been successfully applied to compare the plan robustness between 3D and 4D robustly optimized IMPT plans for 10 lung cancer patients. In the presence of interplay effect with uncertainties considered simultaneously, 4D robustly optimized plans provided significantly better CTV coverage (D_95% , P = 0.002), CTV homogeneity (D_5% -D_95% , P = 0.002) with less target hot spots (D_5% , P = 0.002), and target coverage robustness (CTV D_95% bandwidth, P = 0.004) compared to 3D robustly optimized plans. Superior dose sparing of normal lung (lung D_mean , P = 0.020) favoring 4D plans and comparable normal tissue sparing including esophagus, heart, and spinal cord for both 3D and 4D plans were observed. The calculation time for all patients included in this study was 11.4 ± 2.6 min.

CONCLUSION

A comprehensive plan robustness evaluation tool was successfully developed and benchmarked for plan robustness evaluation in the presence of interplay effect, setup and range uncertainties. The very high efficiency of this tool marks its clinical adaptation, highly practical and versatile nature, including possible real-time intra-fractional interplay effect evaluation as a potential application for future use.

Patient-specific quality assurance and plan dose errors on breast intensity-modulated proton therapy

PURPOSE

To investigate, in proton therapy, whether the Gamma passing rate (GPR) is related to the patient dose error and whether MU scaling can improve dose accuracy.

METHODS

Among 20 consecutively treated breast patients selected for analysis, two IMPT plans were retrospectively generated: (1) the pencil-beam (PB) plan and (2) the Monte Carlo (MC) plan. Patient-specific QA was performed. A 3%/3-mm Gamma analysis was conducted to compare the TPS-calculated PB algorithm dose distribution with the measured 2D dose. Dose errors were compared between the plans that passed the Gamma testing and those that failed. The MU was then scaled to obtain a better GPR. MU-scaled PB plan dose errors were compared to the original PB plan.

RESULTS

Of the 20 PB plans, 8 were passed Gamma testing (G_pass_group) and 12 failed (G_fail_group). Surprisingly, the G_pass_group had a greater dose error than the G_fail_group. The median (range) of the PTV DVH RMSE and PTV ΔDmean were 1.36 (1.00-1.91) Gy vs 1.18 (1.02-1.80) Gy and 1.23 (0.92-1.71) Gy vs 1.10 (0.87-1.49) Gy for the G_pass_group and the G_fail_group, respectively. MU scaling reduced overall dose error. However, for PTV D99 and D95, MU scaling worsened some cases.

CONCLUSION

For breast IMPT, the PB plans that passed the Gamma testing did not show smaller dose errors compared to the plans that failed. For individual plans, the MU scaling technique leads to overall smaller dose errors. However, we do not suggest use of the MU scaling technique to replace the MC plans when the MC algorithm is available.

Online-adaptive versus robust IMPT for prostate cancer: How much can we gain?

BACKGROUND/PURPOSE

Intensity-modulated proton therapy (IMPT) is highly sensitive to anatomical variations which can cause inadequate target coverage during treatment. Available mitigation techniques include robust treatment planning and online-adaptive IMPT. This study compares a robust planning strategy to two online-adaptive IMPT strategies to determine the benefit of online adaptation.

MATERIALS/METHODS

We derived the robustness settings and safety margins needed to yield adequate target coverage (V_95%≥98%) for >90% of 11 patients in a prostate cancer cohort (88 repeat CTs). For each patient, we also adapted a non-robust prior plan using a simple restoration and a full adaptation method. The restoration uses energy-adaptation followed by a fast spot-intensity re-optimization. The full adaptation uses energy-adaptation followed by the addition of new spots and a range-robust spot-intensity optimization. Dose was prescribed as 55 Gy(RBE) to the low-dose target (lymph nodes and seminal vesicles) with a boost to 74 Gy(RBE) to the high-dose target (prostate). Daily patient set-up was simulated using implanted intra-prostatic markers.

RESULTS

Margins of 4 and 8 mm around the high- and low-dose target regions, a 6 mm setup error and a 3% range error were found to obtain adequate target coverage for all repeat CTs of 10/11 patients (94.3% of all 88 repeat CTs). Both online-adaptive strategies yielded V_95%≥98% and better OAR sparing in 11/11 patients. Median OAR improvements up to 11%-point and 16%-point were observed when moving from robust planning to respectively restoration and full adaption.

CONCLUSION

Both full plan adaptation and simple dose restoration can increase OAR sparing besides better conforming to the target criteria compared to robust treatment planning.

The role of a knowledge based dose-volume histogram predictive model in the optimisation of intensity-modulated proton plans for hepatocellular carcinoma patients : Training and validation of a novel commercial system

PURPOSE

To investigate the performance of a knowledge-based RapidPlan, for optimisation of intensity-modulated proton therapy (IMPT) plans applied to hepatocellular cancer (HCC) patients.

METHODS

A cohort of 65 patients was retrospectively selected: 50 were used to "train" the model, while the remaining 15 provided independent validation. The performance of the RapidPlan model was benchmarked against manual optimisation and was also compared to volumetric modulated arc therapy (RapidArc) photon plans. A subanalysis appraised the performance of the RapidPlan model applied to patients with lesions ≤300 cm³ or larger. Quantitative assessment was based on several metrics derived from the constraints of the NRG-GI003 clinical trial.

RESULTS

There was an equivalence between manual plans and RapidPlan-optimised IMPT plans, which outperformed the RapidArc plans. The planning dose-volume objectives were met on average for all structures except for D_0.5 cm3 ≤30 Gy in the bowels. Limiting the results to the class-solution proton plans (all values in Gy), the data for manual plans vs RapidPlan-based IMPT plans, respectively, showed the following: D_99% to the target of 47.5 ± 1.4 vs 47.2 ± 1.2; for organs at risk, the mean dose to the healthy liver was 6.7 ± 3.6 vs 6.7 ± 3.7; the mean dose to the kidneys was 0.2 ± 0.5 vs 0.1 ± 0.2; D_0.5 cm3 for the bowels was 33.4 ± 16.4 vs 30.2 ± 16.0; for the stomach was 17.9 ± 19.9 vs 14.9 ± 18.8; for the oesophagus was 17.9 ± 15.1 vs 14.9 ± 13.9; for the spinal cord was 0.5 ± 1.6 vs 0.2 ± 0.7. The model performed similarly for cases with small or large lesions.

CONCLUSION

A knowledge-based RapidPlan model was trained and validated for IMPT. The results demonstrate that RapidPlan can be trained adequately for IMPT in HCC. The quality of the RapidPlan-based plans is at least equivalent compared to what is achievable with manual planning. RapidPlan also confirmed the potential to optimise the quality of the proton therapy results, thus reducing the impact of operator planning skills on patient results.

Radiobiological model-based approach to determine the potential of dose-escalated robust intensity-modulated proton radiotherapy in reducing gastrointestinal toxicity in the treatment of locally advanced unresectable pancreatic cancer of the head

Background

The purpose of this study was to determine the potential of escalated dose radiation (EDR) robust intensity-modulated proton radiotherapy (ro-IMPT) in reducing GI toxicity risk in locally advanced unresectable pancreatic cancer (LAUPC) of the head in term of normal tissue complication probability (NTCP) predictive model.

Methods

For 9 patients, intensity-modulated radiotherapy (IMRT) was compared with ro-IMPT. For all plans, the prescription dose was 59.4GyE (Gray equivalent) in 33 fractions with an equivalent organ at risk (OAR) constraints. Physical dose distribution was evaluated. GI toxicity risk for different endpoints was estimated using published NTCP Lyman Kutcher Burman (LKB) models for stomach, duodenum, small bowel, and combine stomach and duodenum (Stoduo). A Wilcoxon signed-rank test was used for dosimetry parameters and NTCP values comparison.

Result

The dosimetric results have shown that, with similar target coverage, ro-IMPT achieves a significant dose-volume reduction in the stomach, small bowel, and stoduo in low to high dose range in comparison to IMRT. NTCP evaluation for the endpoint gastric bleeding of stomach (10.55% vs. 13.97%, P = 0.007), duodenum (1.87% vs. 5.02%, P = 0.004), and stoduo (5.67% vs. 7.81%, P = 0.008) suggest reduced toxicity by ro-IMPT compared to IMRT. ∆NTCP _{IMRT – ro-IMPT} (using parameter from Pan et al. for gastric bleed) of ≥5 to < 10% was seen in 3 patients (33%) for stomach and 2 patients (22%) for stoduo. An overall GI toxicity relative risk (NTCP_ro-IMPT/NTCP_IMRT) reduction was noted (0.16–0.81) for all GI-OARs except for duodenum (> 1) with endpoint grade ≥ 3 GI toxicity (using parameters from Holyoake et al.).

Conclusion

With similar target coverage and better conformity, ro-IMPT has the potential to substantially reduce the risk of GI toxicity compared to IMRT in EDR of LAUPC of the head. This result needs to be further evaluated in future clinical studies.

Volumetric modulated arc therapy vs intensity-modulated proton therapy in the postoperative irradiation of thymoma. J Cancer Res Clin Oncol. 2020;146:2267-2276. [PMID: 32514629 DOI: 10.1007/s00432-020-03281-z]

BACKGROUND

To investigate the role of intensity-modulated proton therapy (IMPT) compared to volumetric modulated arc therapy (VMAT) for the radiation treatment of thymoma cancer.

METHODS

Twenty patients were retrospectively planned for IMPT [with (IMPT_R1 or IMPT_R2 according to the approach adopted) and without robust optimization] and VMAT. The results were compared according to dose-volume metrics on the clinical and planning target volumes (CTV and PTV) and the main organs at risk (heart, breasts, lungs, spinal cord and oesophagus). Estimates of the excess absolute risk (EAR) of secondary cancer induction were determined for the oesophagus, the breasts and the composite lungs. For the heart, the relative risk (RR) of chronic heart failure (CHF) was assessed.

RESULTS

IMPT and VMAT plans resulted equivalent in terms of target coverage for both the CTV and the PTV. The CTV homogeneity index resulted in 0.03 ± 0.01 and 0.04 ± 0.01 for VMAT and all IMPT plans, respectively. The conformality index resulted in 1.1 ± 0.1 and 1.2 ± 0.1 for VMAT and all IMPT plans. The mean dose to the breasts resulted in 10.5 ± 5.0, 4.5 ± 3.4, 4.7 ± 3.5 and 4.6 ± 3.4 Gy for VMAT, IMPT, IMPT_R1 and IMPT_R2. For the lungs, the mean dose was 9.6 ± 2.3, 3.5 ± 1.5, 3.6 ± 1.6 and 3.8 ± 1.4 Gy; for the heart: 8.7 ± 4.4, 4.3 ± 1.9, 4.5 ± 2.0 and 4.4 ± 2.4 Gy and for the oesophagus 8.2 ± 3.5, 2.2 ± 3.4, 2.4 ± 3.6 and 2.5 ± 3.5 Gy. The RR for CHF was 1.6 ± 0.3 for VMAT and 1.3 ± 0.2 for IMPT (R1 or R2). The EAR was 3.6 ± 0.v vs 1.0 ± 0.6 or 1.2 ± 0.6 (excess cases/10,000 patients year) for the oesophagus; 17.4 ± 6.5 vs 5.7 ± 3.2 or 6.1 ± 3.8 for the breasts and 24.8 ± 4.3 vs 8.1 ± 2.7 or 8.7 ± 2.3 for the composite lungs for VMAT and IMPT_R, respectively.

CONCLUSION

The data from this in-silico study suggest that intensity-modulated proton therapy could be significantly advantageous in the treatment of thymoma patients with particular emphasis to a substantial reduction of the risk of cardiac failure and secondary cancer induction. Robust planning is a technical pre-requisite for the safety of the delivery.

Volumetric modulated arc therapy versus intensity-modulated proton therapy in neoadjuvant irradiation of locally advanced oesophageal cancer

Background

To investigate the role of intensity-modulated proton therapy (IMPT) compared to volumetric modulated arc therapy (VMAT), realised with RapidArc and RapidPlan methods (RA_RP) for neoadjuvant radiotherapy in locally advanced oesophagal cancer.

Methods

Twenty patients were retrospectively planned for IMPT (with two fields, (IMPT_2F) or with three fields (IMPT_3F)) and RA_RP and the results were compared according to dose-volume metrics. Estimates of the excess absolute risk (EAR) of secondary cancer induction were determined for the lungs. For the cardiac structures, the relative risk (RR) of coronary artery disease (CAD) and chronic heart failure (CHF) were estimated.

Results

Both the RA_RP and IMPT approached allowed to achieve the required coverage for the gross tumour volume, (GTV) and the clinical and the planning target volumes, CTV and PTV (V_98% > 98 for CTV and GTV and V_95% > 95 for the PTV)). The conformity index resulted in 0.88 ± 0.01, 0.89 ± 0.02 and 0.89 ± 0.02 for RA_RP, IMPT_2F and IMPT_3F respectively. With the same order, the homogeneity index for the PTV resulted in 5.6 ± 0.6%, 4.4 ± 0.9% and 4.5 ± 0.8%. Concerning the organs at risk, the IMPT plans showed a systematic and statistically significant incremental sparing when compared to RA_RP, especially for the heart. The mean dose to the combined lungs was 8.6 ± 2.9 Gy for RA_RP, 3.2 ± 1.5 Gy and 2.9 ± 1.2 Gy for IMPT_2F and IMPT_3F. The mean dose to the whole heart resulted to 9.9 ± 1.9 Gy for RA_RP compared to 3.7 ± 1.3 Gy or 4.0 ± 1.4 Gy for IMPT_2F or IMPT_3F; the mean dose to the left ventricle resulted to 6.5 ± 1.6 Gy, 1.9 ± 1.5 Gy, 1.9 ± 1.6 Gy respectively. Similar sparing effects were observed for the liver, the kidneys, the stomach, the spleen and the bowels.

The EAR per 10,000 patients-years of secondary cancer induction resulted in 19.2 ± 5.7 for RA_RP and 6.1 ± 2.7 for IMPT_2F or 5.7 ± 2.4 for IMPT_3F. The RR for the left ventricle resulted in 1.5 ± 0.1 for RA_RP and 1.1 ± 0.1 for both IMPT sets. For the coronaries, the RR resulted in 1.6 ± 0.4 for RA_RP and 1.2 ± 0.3 for protons.

Conclusion

With regard to cancer of the oesophagogastric junction type I and II, the use of intensity-modulated proton therapy seems to have a clear advantage over VMAT. In particular, the reduction of the heart and abdominal structures dose could result in an optimised side effect profile. Furthermore, reduced risk of secondary neoplasia in the lung can be expected in long-term survivors and would be a great gain for cured patients.

Dosimetric consequences of image guidance techniques on robust optimized intensity-modulated proton therapy for treatment of breast Cancer

PURPOSE

To investigate the consequences of residual setup error on target dose distribution using various image registration strategies for breast cancer treated with intensity-modulated proton therapy (IMPT).

MATERIALS AND METHODS

Among 11 post-lumpectomy patients who received IMPT, 44 dose distributions were computed. For each patient, the original plan (Plan-O) and three verification plans were calculated using different alignments: bony anatomy (VPlan-B), breast tissue (VPlan-T), and skin (VPlan-S). The target coverage were evaluated for each alignment technique. Additionally, 2 subvolumes-BreastNearSkin (1-cm rim of anterior CTV) and BreastNearCW (1-cm rim of posterior CTV)-were created to help localize CTV underdosing. Furthermore, we divided the setup error into the posture error and breast error. Patients with a large posture error and those with good posture setup but a large breast error were identified to evaluate the effect of posture error and breast error.

RESULTS

For Plan-O, VPlan-B, VPlan-T, and VPlan-S, respectively, the median (interquartile range) breast CTV D95 was 95.7%(94.7-96.3%), 95.1% (93.9-95.7%), 95.2% (94.8-95.6%), and 95.2% (94.9-95.7%); BreastNearCW D95 was 96.9% (95.6-97.3%), 94.8% (93.5-97.0%), 95.6% (94.8-97.0%), 95.6% (94.8-97.1%); and BreastNearSkin D95 was 94.1% (92.7-94.4%), 93.6% (92.2-94.5%), 93.5% (92.4-94.5%), and 94.4% (92.2-94.5%) of the prescription dose. 4/11 patients had ≥1% decrease in breast CTV D95, 1 of whom developed breast edema while the other 3 all had a > 2^o posture error. The CTV D95 variation was within 1% for the patients with good posture setup but >2^o breast error.

CONCLUSION

Acceptable target coverage was achieved with all three alignment strategies. Breast tissue and skin alignment maintained the breast target coverage marginally better than bony alignment, with which the posterior CTV along the chest wall is the predominant area affected by under-dosing. For target dose distribution, posture error appears more influential than breast error.

Multiple-CT optimization: An adaptive optimization method to account for anatomical changes in intensity-modulated proton therapy for head and neck cancers

PURPOSE

We aimed to determine whether multiple-CT (MCT) optimization of intensity-modulated proton therapy (IMPT) could improve plan robustness to anatomical changes and therefore reduce the additional need for adaptive planning.

METHODS AND MATERIALS

Ten patients with head and neck cancer who underwent IMPT were included in this retrospective study. Each patient had primary planning CT (PCT), a first adaptive planning CT (ACT1), and a second adaptive planning CT (ACT2). Selective robust IMPT plans were generated using each CT data set (PCT, ACT1, and ACT2). Moreover, a MCT optimized plan was generated using the PCT and ACT1 data sets together. Dose distributions optimized using each of the four plans (PCT, ACT1, ACT2, and MCT plans) were re-calculated on ACT2 data. The doses to the target and to organs at risk were compared between optimization strategies.

RESULTS

MCT plans for all patients met all target dose and organs-at-risk criteria for all three CT data sets. Target dose and organs-at-risk dose for PCT and ACT1 plans re-calculated on ACT2 data set were compromised, indicating the need for adaptive planning on ACT2 if PCT or ACT1 plans were used. The D98% of CTV1 and CTV3 of MCT plan re-calculated on ACT2 were both above the coverage criteria. The CTV2 coverage of the MCT plan re-calculated on ACT2 was worse than ACT2 plan. The MCT plan re-calculated on ACT2 data set had lower chiasm, esophagus, and larynx doses than did PCT, ACT1, or ACT2 plans re-calculated on ACT2 data set.

CONCLUSIONS

MCT optimization can improve plan robustness toward anatomical change and may reduce the number of plan adaptation for head and neck cancers.

Dose distribution of intensity-modulated proton therapy with and without a multi-leaf collimator for the treatment of maxillary sinus cancer: a comparative effectiveness study

BACKGROUND

Severe complications, such as eye damage and dysfunciton of salivary glands, have been reported after radiotherapy among patients with head and neck cancer. Complications such as visual impairment have also been reported after proton therapy with pencil beam scanning (PBS). In the case of PBS, collimation can sharpen the penumbra towards surrounding normal tissue in the low energy region of the proton beam. In the current study, we examined how much the dose to the normal tissue was reduced by when intensity-modulated proton therapy (IMPT) was performed using a multi-leaf collimator (MLC) for patients with maxillary sinus cancer.

METHODS

Computed tomography findings of 26 consecutive patients who received photon therapy at Okayama University Hospital were used in this study. We compared D2% of the region of interest (ROI; ROI-_D2%) and the mean dose of ROI (ROI-_mean) with and without the use of an MLC. The organs at risk (OARs) were the posterior retina, lacrimal gland, eyeball, and parotid gland. IMPT was performed for all patients. The spot size was approximately 5-6 mm at the isocenter. The collimator margin was calculated by enlarging the maximum outline of the target from the beam's eye view and setting the margin to 6 mm. All plans were optimized with the same parameters.

RESULTS

The mean of ROI-_D2% for the ipsilateral optic nerve was significantly reduced by 0.48 Gy, and the mean of ROI-_mean for the ipsilateral optic nerve was significantly reduced by 1.04 Gy. The mean of ROI-_mean to the optic chiasm was significantly reduced by 0.70 Gy. The dose to most OARs and the planning at risk volumes were also reduced.

CONCLUSIONS

Compared with the plan involving IMPT without an MLC, in the dose plan involving IMPT using an MLC for maxillary sinus cancer, the dose to the optic nerve and optic chiasm were significantly reduced, as measured by the ROI-_D2% and the ROI-_mean. These findings demonstrate that the use of an MLC during IMPT for maxillary sinus cancer may be useful for preserving vision and preventing complications.

Statistical evaluation of worst-case robust optimization intensity-modulated proton therapy plans using an exhaustive sampling approach

PURPOSE

To assess the worst-case robust optimization IMPT plans with setup and range uncertainties and to test the hypothesis that the worst-case robust optimization strategies could cover most possible setup and range uncertainties in the real scenarios.

METHODS

We analyzed the nominal and worst-case robust optimization IMPT plans of seven patients with head and neck cancer patients. To take uncertainties into account for the dose calculation, we performed a comprehensive simulation in which the dose was recalculated 625 times per given plan using Gaussian systematic setup and proton range uncertainties. Subsequently, based on the simulation results, we calculated the target coverage in all perturbation scenarios, as well as the ratios of target coverage located within the threshold of eight worst-case scenarios. We set the criteria for the optimized plan to be the ratios of 1) the dose delivered to 95% (D95%) of clinical target volumes 1 and 2 (CTV1 and CTV2) above 95% of the prescribed dose, and 2) the D95% of clinical target volume 3 (CTV3) above 90% of the prescribed dose in worst-case situations.

RESULTS

The probability that the perturbed-dose indices of the CTVs in each scenario were within the worst-case scenario limits ranged from 89.51 to 91.22% for both the nominal and worst-case robust optimization IMPT plans. A quartile analysis showed that the selective robust optimization IMPT plans all had higher D95% values for CTV1, CTV2, and CTV3 than did the nominal IMPT plans.

CONCLUSIONS

The worst-case strategy for robust optimization is adequately models and covers most of the setup and range uncertainties for the IMPT treatment of head and neck patients in our center.

Effect of setup and inter-fraction anatomical changes on the accumulated dose in CT-guided breath-hold intensity modulated proton therapy of liver malignancies

PURPOSE

To evaluate the effect of setup uncertainties including uncertainties between different breath holds (BH) and inter-fractional anatomical changes under CT-guided BH with intensity-modulated proton therapy (IMPT) in patients with liver cancer.

METHODS AND MATERIALS

This retrospective study considered 17 patients with liver tumors who underwent feedback-guided BH (FGBH) IMRT treatment with daily CT-on-rail imaging. Planning CT images were acquired at simulation using FGBH, and FGBH CT-on-rail images were also acquired prior to each treatment. Selective robust IMPT plans were generated using planning CT and re-calculated on each daily CT-on-rail image. Subsequently, the fractional doses were deformed and accumulated onto the planning CT according to the deformable image registration between daily and planning CTs. The doses to the target and organs at risk (OARs) were compared between IMRT, planned IMPT, and accumulated IMPT doses.

RESULTS

For IMPT plans, the mean of D_98% of CTV for all 17 patients was slightly reduced from the planned dose of 68.90 ± 1.61 Gy to 66.48 ± 1.67 Gy for the accumulated dose. The target coverage could be further improved by adjusting planning techniques. The dose-volume histograms of both planned and accumulated IMPT doses showed better sparing of OARs than that of the IMRT.

CONCLUSIONS

IMPT with FGBH and CT-on-rail guidance is a robust treatment approach for liver tumor cases.

A Pilot Study on the Comparison between Planning Target Volume-based Intensity-Modulated Proton Therapy Plans and Robustly Optimized Intensity-Modulated Proton Therapy Plans

The objective of this work is to compare the planning target volume (PTV)-based intensity-modulated proton therapy (IMPT) plans with robustly optimized IMPT plans using the robust optimization tools available in Pinnacle Treatment Planning System. We performed the study in five cases of different anatomic sites (brain, head and neck, lung, pancreas, and prostate). Pinnacle IMPT nonclinical version was used for IMPT planning. Two types of IMPT plans were created for each case. One is PTV-based conventionally optimized IMPT plan and the other is robustly optimized plan considering setup uncertainties. For the PTV-based plans, margins were on top of clinical target volume (CTV) to account for the setup errors, whereas in the robustly optimized plan, the setup errors were directly incorporated into the optimization process. The plan evaluation included target (CTV) coverage and dose uniformity. Our interest was to see how the target coverage and dose uniformity were perturbed on imposing setup errors in +X, −X, +Y, −Y, +Z, and −Z directions for both PTV-based and robust optimization (RO)-based plans. On the average, RO-based IMPT plans have shown a good consistency of target coverage and dose uniformity for all six setup errors scenarios as compared to PTV-based plans. In addition, RO-based plans have a better target coverage and dose uniformity under uncertainty conditions as compared to the PTV-based plans. The study demonstrates the superiority of robustly optimized IMPT plans over the PTV-based IMPT plans in terms of dose distribution under the uncertainty conditions.

Proton Therapy for Head and Neck Cancers

Because of its sharp lateral penumbra and steep distal fall-off, proton therapy offers dosimetric advantages over photon therapy. In head and neck cancer, proton therapy has been used for decades in the treatment of skull-base tumors. In recent years the use of proton therapy has been extended to numerous other disease sites, including nasopharynx, oropharynx, nasal cavity and paranasal sinuses, periorbital tumors, skin, and salivary gland, or to reirradiation. The aim of this review is to present the physical properties and dosimetric benefit of proton therapy over advanced photon therapy; to summarize the clinical benefit described for each disease site; and to discuss issues of patient selection and cost-effectiveness.

3D treatment planning system-Varian Eclipse for proton therapy planning

The capabilities of the Eclipse treatment planning system (TPS) (Varian Medical Systems, Palo Alto, CA) for proton therapy treatment planning are described. Various steps involved in the planning process to produce a 3-dimensional (3D) dose distribution both for the passive scattering and pencil beam scanning proton beam therapy are outlined. Mitigation of range and setup uncertainties through robust optimization is discussed. Use of verification plans for patient treatment field dosimetry quality assurance (QA) is presented. Limitations of the Eclipse TPS and future developments are discussed.

Multiple-CT optimization of intensity-modulated proton therapy - Is it possible to eliminate adaptive planning?

BACKGROUND AND PURPOSE

We hypothesized that a plan's robustness to anatomical changes can be improved by optimizing with multiple CT scans of a patient. The purpose of this study was to determine whether an intensity modulated proton therapy (IMPT) plan could be developed to meet dose criteria on both planning and adaptive CT plans.

MATERIAL AND METHODS

Eight lung cancer patients who underwent adaptive IMPT were retrospectively selected. Each patient had two CTs: a primary planning CT (PCT) and an adaptive planning CT (ACT), and IMPT plans associated with the scans. PCT and ACT were then used in combination to optimize one plan (MCT plan). The doses to the target and organs at risk from the PCT plan, ACT plan, P-ACT plan (PCT plan calculated on ACT data), and MCT plans calculated on both CTs were compared.

RESULTS

The MCT plan maintained the D_95% on both CTs (mean, 65.99 Gy on PCT and 66.02 Gy on ACT). Target dose coverage on ACT was significantly better with the MCT plan than with the P-ACT plan (p = 0.01). MCT plans had slightly higher lung V₂₀ (0.6%, p = 0.02) than did PCT plans. The various plans showed no statistically significant difference in heart and spinal cord dose.

CONCLUSIONS

The results of this study indicate that an IMPT plan can meet the dose criteria on both PCT and ACT, and that MCT optimization can improve the plan's robustness to anatomical change.

Proton therapy - Present and future

In principle, proton therapy offers a substantial clinical advantage over conventional photon therapy. This is because of the unique depth-dose characteristics of protons, which can be exploited to achieve significant reductions in normal tissue doses proximal and distal to the target volume. These may, in turn, allow escalation of tumor doses and greater sparing of normal tissues, thus potentially improving local control and survival while at the same time reducing toxicity and improving quality of life. Protons, accelerated to therapeutic energies ranging from 70 to 250MeV, typically with a cyclotron or a synchrotron, are transported to the treatment room where they enter the treatment head mounted on a rotating gantry. The initial thin beams of protons are spread laterally and longitudinally and shaped appropriately to deliver treatments. Spreading and shaping can be achieved by electro-mechanical means to treat the patients with "passively-scattered proton therapy" (PSPT) or using magnetic scanning of thin "beamlets" of protons of a sequence of initial energies. The latter technique can be used to treat patients with optimized intensity modulated proton therapy (IMPT), the most powerful proton modality. Despite the high potential of proton therapy, the clinical evidence supporting the broad use of protons is mixed. It is generally acknowledged that proton therapy is safe, effective and recommended for many types of pediatric cancers, ocular melanomas, chordomas and chondrosarcomas. Although promising results have been and continue to be reported for many other types of cancers, they are based on small studies. Considering the high cost of establishing and operating proton therapy centers, questions have been raised about their cost effectiveness. General consensus is that there is a need to conduct randomized trials and/or collect outcomes data in multi-institutional registries to unequivocally demonstrate the advantage of protons. Treatment planning and plan evaluation of PSPT and IMPT require special considerations compared to the processes used for photon treatment planning. The differences in techniques arise from the unique physical properties of protons but are also necessary because of the greater vulnerability of protons to uncertainties, especially from inter- and intra-fractional variations in anatomy. These factors must be considered in designing as well as evaluating treatment plans. In addition to anatomy variations, other sources of uncertainty in dose delivered to the patient include the approximations and assumptions of models used for computing dose distributions for planning of treatments. Furthermore, the relative biological effectiveness (RBE) of protons is simplistically assumed to have a constant value of 1.1. In reality, the RBE is variable and a complex function of the energy of protons, dose per fraction, tissue and cell type, end point, etc. These uncertainties, approximations and current technological limitations of proton therapy may limit the achievement of its true potential. Ongoing research is aimed at better understanding the consequences of the various uncertainties on proton therapy and reducing the uncertainties through image-guidance, adaptive radiotherapy, further study of biological properties of protons and the development of novel dose computation and optimization methods. However, residual uncertainties will remain in spite of the best efforts. To increase the resilience of dose distributions in the face of uncertainties and improve our confidence in dose distributions seen on treatment plans, robust optimization techniques are being developed and implemented. We assert that, with such research, proton therapy will be a commonly applied radiotherapy modality for most types of solid cancers in the near future.

Toward a model-based patient selection strategy for proton therapy: External validation of photon-derived normal tissue complication probability models in a head and neck proton therapy cohort

OBJECTIVE

To externally validate head and neck cancer (HNC) photon-derived normal tissue complication probability (NTCP) models in patients treated with proton beam therapy (PBT).

METHODS

This prospective cohort consisted of HNC patients treated with PBT at a single institution. NTCP models were selected based on the availability of data for validation and evaluated by using the leave-one-out cross-validated area under the curve (AUC) for the receiver operating characteristics curve.

RESULTS

192 patients were included. The most prevalent tumor site was oropharynx (n=86, 45%), followed by sinonasal (n=28), nasopharyngeal (n=27) or parotid (n=27) tumors. Apart from the prediction of acute mucositis (reduction of AUC of 0.17), the models overall performed well. The validation (PBT) AUC and the published AUC were respectively 0.90 versus 0.88 for feeding tube 6months PBT; 0.70 versus 0.80 for physician-rated dysphagia 6months after PBT; 0.70 versus 0.68 for dry mouth 6months after PBT; and 0.73 versus 0.85 for hypothyroidism 12months after PBT.

CONCLUSION

Although a drop in NTCP model performance was expected for PBT patients, the models showed robustness and remained valid. Further work is warranted, but these results support the validity of the model-based approach for selecting treatment for patients with HNC.

Intensity-modulated proton beam therapy (IMPT) versus intensity-modulated photon therapy (IMRT) for patients with oropharynx cancer - A case matched analysis

BACKGROUND

Owing to its physical properties, intensity-modulated proton therapy (IMPT) used for patients with oropharyngeal carcinoma has the ability to reduce the dose to organs at risk compared to intensity-modulated radiotherapy (IMRT) while maintaining adequate tumor coverage. Our aim was to compare the clinical outcomes of these two treatment modalities.

METHODS

We performed a 1:2 matching of IMPT to IMRT patients. Our study cohort consisted of IMPT patients from a prospective quality of life study and consecutive IMRT patients treated at a single institution during the period 2010-2014. Patients were matched on unilateral/bilateral treatment, disease site, human papillomavirus status, T and N status, smoking status, and receipt of concomitant chemotherapy. Survival analyzes were performed using a Cox model and binary toxicity endpoints using a logistic regression analysis.

RESULTS

Fifty IMPT and 100 IMRT patients were included. The median follow-up time was 32months. There were no imbalances in patient/tumor characteristics except for age (mean age 56.8years for IMRT patients and 61.1years for IMPT patients, p-value=0.010). Statistically significant differences were not observed in overall survival (hazard ratio (HR)=0.55; 95% confidence interval (CI): 0.12-2.50, p-value=0.44) or in progression-free survival (HR=1.02; 95% CI: 0.41-2.54; p-value=0.96). The age-adjusted odds ratio (OR) for the presence of a gastrostomy (G)-tube during treatment for IMPT vs IMRT were OR=0.53; 95% CI: 0.24-1.15; p-value=0.11 and OR=0.43; 95% CI: 0.16-1.17; p-value=0.10 at 3months after treatment. When considering the pre-planned composite endpoint of grade 3 weight loss or G-tube presence, the ORs were OR=0.44; 95% CI: 0.19-1.0; p-value=0.05 at 3months after treatment and OR=0.23; 95% CI: 0.07-0.73; p-value=0.01 at 1year after treatment.

CONCLUSION

Our results suggest that IMPT is associated with reduced rates of feeding tube dependency and severe weight loss without jeopardizing outcome. Prospective multicenter randomized trials are needed to validate such findings.

Advances in radiation oncology for the management of oropharyngeal tumors

The major benefits of modern radiation therapy (eg, intensity-modulated [x-ray] radiation therapy [IMRT]) for oropharyngeal cancer are reduced xerostomia and better quality of life. Intensity-modulated proton therapy may provide additional advantages over IMRT by reducing radiation beam-path toxicities. Several acute and late treatment-related toxicities and symptom constellations must be kept in mind when designing and comparing future treatment strategies, particularly because currently most patients with oropharyngeal carcinoma present with human papillomavirus-positive disease and are expected to have a high probability of long-term survival after treatment.