Protons, the brainstem, and toxicity: ingredients for an emerging dialectic

Evolution of Proton Radiation Therapy Brainstem Constraints on the Pediatric Proton/Photon Consortium Registry

PURPOSE

Increasing concern that brainstem toxicity incidence after proton radiation therapy might be higher than with photons led to a 2014 University of Florida (UF) landmark paper identifying its risk factors and proposing more conservative dose constraints. We evaluated how practice patterns changed among the Pediatric Proton/Photon Consortium Registry (PPCR).

MATERIAL AND METHODS

This prospective multicenter cohort study gathered data from patients under the age of 22 years enrolled on the PPCR, treated between 2002 and 2019 for primary posterior fossa brain tumors. After standardizing brainstem contours, we garnered dosimetry data and correlated those meeting the 2014 proton-specific brainstem constraint guidelines by treatment era, histology, and extent of surgical resection.

RESULTS

A total of 467 patients with evaluable proton radiation therapy plans were reviewed. Median age was 7.1 years (range: <1-21.9), 63.0% (n = 296) were men, 76.0% (n = 357) were White, and predominant histology was medulloblastoma (55.0%, n = 256), followed by ependymoma (27.0%, n = 125). Extent of resection was mainly gross total resection (GTR) (67.0%, n = 312), followed by subtotal resection (STR) or biopsy (20.0%, n = 92), and near total resection (NTR) (9.2%, n = 43). The UF brainstem constraint metrics most often exceeded were the goal D50% of 52.4 gray relative biological equivalents (43.3%, n = 202) and maximal D50% of 54 gray relative biological equivalents (12.6%, n = 59). The compliance rate increased after the new guidelines (2002-2014: 64.0% vs 2015-2019: 74.6%, P = .02), except for ependymoma (46.3% pre- vs 50.0% post-guidelines, P = .86), presenting lower compliance (48.8%) in comparison to medulloblastoma/ primitive neuroectodermal tumors/pineoblastoma (77.7%), glioma (89.1%), and atypical teratoid/rhabdoid tumors (90.9%) (P < .001). Degree of surgical resection did not affect compliance rates (GTR/NTR 71.0% vs STR/biopsy 72.8%, P = .45), even within the ependymoma subset (GTR/NTR 50.5% vs STR/biopsy 38.1%, P = .82).

CONCLUSION

Since the publication of the UF guidelines, the pediatric proton community has implemented more conservative brainstem constraints in all patients except those with ependymoma, irrespective of residual disease after surgery. Future work will evaluate if this change in practice is associated with decreased rates of brainstem toxicity.

Fatal brainstem injury following proton radiation in a patient with medulloblastoma and a germline variant in RNF213

Brainstem injury occurs secondary to radiation to the posterior fossa in up to 2% of pediatric patients. It may occur after months to years after treatment. It has been associated with age less than 5 years and with comorbid conditions such as cerebrovascular disease, diabetes mellitus, and hypertension. Radiation necrosis is often symptomatic and can be fatal. A pathogenic variant in RNF213 was found in a patient who suffered fatal radiation necrosis. This mutation has been associated with moyamoya disease and may predispose to radiation necrosis.

The Role of Reirradiation in Childhood Progressive Diffuse Intrinsic Pontine Glioma (DIPG): An Ongoing Challenge beyond Radiobiology

To investigate the clinical impact of multiple courses of irradiation on pediatric patients with progressive diffuse intrinsic pontine glioma (DIPG), we conducted a retrospective case series on three children treated at our institution from 2018 to 2022. All children were candidates to receive systemic therapy with vinorelbine and nimotuzumab. Radiotherapy was administered to a total dose of 54 Gy. At any disease progression, our local tumor board evaluated the possibility of offering a new course of radiotherapy. To determine feasibility and assess toxicity rates, all children underwent clinical and hematological evaluation both during and after the treatment. To assess efficacy, all children performed contrast-enhanced MRI almost quarterly after the end of the treatment. In all children, following any treatment course, neurological improvement (>80%) was associated with a radiological response (41.7-46%). The longest overall survival (24 months) was observed in the child who underwent three courses of radiotherapy, without experiencing significant side effects. Even though it goes beyond the understanding of conventional radiobiology, first and second reirradiation in pediatric patients with progressive DIPG may represent a feasible and safe approach, capable of increasing overall survival and disease-free survival in selected patients and improving their quality of life.

Radiotherapy for Diffuse Intrinsic Pontine Glioma: Insufficient but Indispensable

Diffuse intrinsic pontine gliomas (DIPGs) account for 10%-20% of all central nervous system tumors in children and are the leading cause of death in children with brain tumors. Although many clinical trials have been conducted over the past decades, the survival outcome has remained unchanged. Over 90% of children die within 2 years of the diagnosis, and radiotherapy remains the standard treatment to date. To improve the prognosis, hyperfractionated and hypofractionated radiotherapy and/or addition of radiosensitizers have been investigated. However, none of the radiotherapy approaches have shown a survival benefit, and the overall survival of patients with DIPG is approximately 11 months. Here, we comprehensively review the management of DIPG with focus on radiotherapy.

Radiation Necrosis with Proton Therapy in a Patient with Aarskog-Scott Syndrome and Medulloblastoma

Purpose

Medulloblastoma is known to be associated with multiple cancer-predisposition syndromes. In this article, we explore a possible association among a patient's Aarskog-Scott syndrome, development of medulloblastoma, and subsequent brainstem radiation necrosis.

Case Presentation

A 5-year-old male with Aarskog-Scott syndrome initially presented to his pediatrician with morning emesis, gait instability, and truncal weakness. He was ultimately found to have a posterior fossa tumor with pathology consistent with group 3 medulloblastoma. After receiving a gross total resection and standard proton beam radiation therapy with concurrent vincristine, he was noted to develop brainstem radiation necrosis, for which he underwent therapy with high-dose dexamethasone, bevacizumab, and hyperbaric oxygen therapy with radiographic improvement and clinical stabilization.

Conclusion

Based on several possible pathologic correlates in the FDG1 pathway, there exists a potential association between this patient's Aarskog-Scott syndrome and medulloblastoma, which needs to be investigated further. In patients with underlying, rare genetic syndromes, further caution should be taken when evaluating chemotherapy and radiation dosimetry planning.

Normal tissue tolerance amongst paediatric brain tumour patients- current evidence in proton radiotherapy

BACKGROUND

Proton radiotherapy (PT) is used increasingly for paediatric brain cancer patients. However, as demonstrated here, the knowledge on normal tissue dose constraints, to minimize side-effects, for this cohort is limited.

METHODS

A search strategy was systematically conducted on MEDLINE® database. 65 papers were evaluated ranging from 2013 to 2021.

RESULTS

Large variations in normal tissue tolerance and toxicity reporting across PT studies makes estimation of normal tissue dose constraints difficult, with the potential for significant late effects to go unmeasured. Mean dose delivered to the pituitary gland varies from 20 to 30 Gy across literature. Similarly, the hypothalamic dose delivery ranges from 20 to 54.6 Gy for paediatric patients.

CONCLUSION

There is a significant lack of radiobiological data for paediatric brain cancer patients undergoing proton therapy, often using data from x-ray radiotherapy and adult populations. The way forward is through standardisation of reporting in order to validate relevant dose constraints.

Proton Therapy for Pediatric Ependymoma: Mature Results From a Bicentric Study

PURPOSE

To report the long-term efficacy and toxicity of proton therapy for pediatric ependymoma.

METHODS AND MATERIALS

Between 2000 and 2019, 386 children with nonmetastatic grade 2/3 intracranial ependymoma received proton therapy at 1 of 2 academic institutions. Median age at treatment was 3.8 years (range, 0.7-21.3); 56% were male. Most (72%) tumors were in the posterior fossa and classified as World Health Organization grade 3 (65%). Eighty-five percent had a gross total or near total tumor resection before radiation therapy; 30% received chemotherapy. Median radiation dose was 55.8 Gy relative biologic effectiveness (RBE) (range, 50.4-59.4).

RESULTS

Median follow-up was 5.0 years (range, 0.4-16.7). The 7-year local control, progression-free survival, and overall survival rates were 77.0% (95% confidence interval [CI], 71.9%-81.5%), 63.8% (95% CI, 58.0%-68.8%), and 82.2% (95% CI, 77.2%-86.3%), respectively. Subtotal resection was associated with inferior local control (59% vs 80%; P < .005), progression-free survival (48% vs 66%; P < .001), and overall survival (70% vs 84%; P < .05). Male sex was associated with inferior progression-free (60% vs 69%; P < .05) and overall survival (76% vs 89%; P < .05). Posterior fossa tumor site was also associated with inferior progression-free (59% vs 74%; P < .05) and overall survival (79% vs 89%; P < .01). Twenty-one patients (5.4%) required hearing aids; of these, 13 received cisplatin, including the 3 with bilateral hearing loss. Forty-five patients (11.7%) required hormone replacement, typically growth hormone (38/45). The cumulative incidence of grade 2+ brain stem toxicity was 4% and occurred more often in patients who received >54 GyRBE. Two patients (0.5%) died of brain stem necrosis. The second-malignancy rate was 0.8%.

CONCLUSION

Proton therapy offers disease control commensurate with modern photon therapy without unexpected toxicity. The high rate of long-term survival justifies efforts to reduce radiation exposure in this young population. Independent of radiation modality, this large series confirms extent of resection as the most important modifiable factor for survival.

Clinical outcomes in a large pediatric cohort of patients with ependymoma treated with proton radiotherapy

BACKGROUND

Treatment for pediatric ependymoma includes surgical resection followed by local radiotherapy (RT). Proton RT (PRT) enables superior sparing of critical structures compared with photons, with potential to reduce late effects. We report mature outcomes, patterns of failure, and predictors of outcomes in patients treated with PRT.

METHODS

One hundred fifty patients (<22 y) with World Health Organization grades II/III ependymoma were treated with PRT between January 2001 and January 2019 at Massachusetts General Hospital. Demographic, tumor, and treatment-related characteristics were analyzed. Event-free survival (EFS), overall survival (OS), and local control (LC) were assessed.

RESULTS

Median follow-up was 6.5 years. EFS, OS, and LC for the intracranial cohort (n = 145) at 7 years were 63.4%, 82.6%, and 76.1%. Fifty-one patients recurred: 26 (51.0%) local failures, 19 (37.3%) distant failures, and 6 (11.8%) synchronous failures. One hundred sixteen patients (77.3%) underwent gross total resection (GTR), 5 (3.3%) underwent near total resection (NTR), and 29 (19.3%) underwent subtotal resection (STR). EFS for the intracranial cohort at 7 years for GTR/NTR and STR was 70.3% and 35.2%. With multivariate analysis, the effect of tumor excision persisted after controlling for tumor location. There was no adverse effect on disease control if surgery to RT interval was within 9 weeks of GTR/NTR.

CONCLUSION

PRT is effective and safe in pediatric ependymoma. Similar to previous studies, GTR/NTR was the most important prognostic factor. Intervals up to 9 weeks from surgery to PRT did not compromise disease outcomes. There was no LC benefit between patients treated with >54 Gray relative biological effectiveness (GyRBE) versus ≤54 GyRBE.

Modern Radiotherapy for Pediatric Brain Tumors

Cancer is a leading cause of death in children with tumors of the central nervous system, the most commonly encountered solid malignancies in this population. Radiotherapy (RT) is an integral part of managing brain tumors, with excellent long-term survival overall. The tumor histology will dictate the volume of tissue requiring treatment and the dose. However, radiation in developing children can yield functional deficits and/or cosmetic defects and carries a risk of second tumors. In particular, children receiving RT are at risk for neurocognitive effects, neuroendocrine dysfunction, hearing loss, vascular anomalies and events, and psychosocial dysfunction. The risk of these late effects is directly correlated with the volume of tissue irradiated and dose delivered and is inversely correlated with age. To limit the risk of developing these late effects, improved conformity of radiation to the target volume has come from adopting a volumetric planning process. Radiation beam characteristics have also evolved to achieve this end, as exemplified through development of intensity modulated photons and the use of protons. Understanding dose limits of critical at-risk structures for different RT modalities is evolving. In this review, we discuss the physical basis of the most common RT modalities used to treat pediatric brain tumors (intensity modulated radiation therapy and proton therapy), the RT planning process, survival outcomes for several common pediatric malignant brain tumor histologies, RT-associated toxicities, and steps taken to mitigate the risk of acute and late effects from treatment.

The role of proton therapy in pediatric malignancies: Recent advances and future directions

Proton radiotherapy has promised an advantage in safely treating pediatric malignancies with an increased capability to spare normal tissues, reducing the risk of both acute and late toxicity. The past decade has seen the proliferation of more than 30 proton facilities in the United States, with increased capacity to provide access to approximately 3,000 children per year who will require radiotherapy for their disease. We provide a review of the initial efforts to describe outcomes after proton therapy across the common pediatric disease sites. We discuss the main attempts to assess comparative efficacy between proton and photon radiotherapy concerning toxicity. We also discuss recent efforts of multi-institutional registries aimed at accelerating research to better define the optimal treatment paradigm for children requiring radiotherapy for cure.

Proton radiotherapy and treatment delay in head and neck squamous cell carcinoma

OBJECTIVE

For patients with head and neck squamous cell carcinoma (HNSCC), delays in the initiation of radiotherapy (RT) have been closely associated with worse outcomes. We sought to investigate whether RT modality (proton vs. photon) is associated with differences in the time to initiation of RT.

METHODS

The National Cancer Database was queried for patients diagnosed with nonmetastatic HNSCC between 2004 and 2015 who received either proton or photon RT as part of their initial treatment. Wilcoxon rank-sum and chi-square tests were used to compare continuous and categorical variables, respectively. Multivariable logistic regression was used to determine the association between use of proton RT and delayed RT initiation.

RESULTS

A total of 175,088 patients with HNSCC receiving either photon or proton RT were identified. Patients receiving proton RT were more likely to be white, reside in higher income areas, and have private insurance. Proton RT was associated with delayed RT initiation compared to photon RT (median 59 days vs. 45, P < 0.001). Receipt of proton therapy was independently associated with RT initiation beyond 6 weeks after diagnosis (adjusted OR [aOR, definitive RT] = 1.69; 95% confidence interval [CI] 1.26-2.30) or surgery (aOR [adjuvant RT] = 4.08; 95% CI 2.64-6.62). In the context of adjuvant proton RT, increases in treatment delay were associated with worse overall survival (weeks, adjusted hazard ratio = 1.099, 95% CI 1.011-1.194).

CONCLUSION

Use of proton therapy is associated with delayed RT in both the definitive and adjuvant settings for patients with HNSCC and could be associated with poorer outcomes.

LEVEL OF EVIDENCE

2b Laryngoscope, 122:0000-0000, 2019 Laryngoscope, 130:E598-E604, 2020.

Pediatric Cancer

In pediatric brain tumors, the intensification of chemotherapy has allowed for a reduction in radiotherapy (RT) volume to an involved field approach, particularly in patients with medulloblastoma. For patients with low-grade gliomas, the trend has remained to delay RT with chemotherapy; however, when RT is used, typically smaller clinical target volume margins are used. For patients with extracranial tumors, intensive chemotherapy to address systemic disease with local control is considered standard. Proton beam therapy shows significant promise in addressing both short-term and long-term toxicities in both central nervous system (CNS) and non-CNS pediatric tumors.

National Cancer Institute Workshop on Proton Therapy for Children: Considerations Regarding Brainstem Injury

PURPOSE

Proton therapy can allow for superior avoidance of normal tissues. A widespread consensus has been reached that proton therapy should be used for patients with curable pediatric brain tumor to avoid critical central nervous system structures. Brainstem necrosis is a potentially devastating, but rare, complication of radiation. Recent reports of brainstem necrosis after proton therapy have raised concerns over the potential biological differences among radiation modalities. We have summarized findings from the National Cancer Institute Workshop on Proton Therapy for Children convened in May 2016 to examine brainstem injury.

METHODS AND MATERIALS

Twenty-seven physicians, physicists, and researchers from 17 institutions with expertise met to discuss this issue. The definition of brainstem injury, imaging of this entity, clinical experience with photons and photons, and potential biological differences among these radiation modalities were thoroughly discussed and reviewed. The 3 largest US pediatric proton therapy centers collectively summarized the incidence of symptomatic brainstem injury and physics details (planning, dosimetry, delivery) for 671 children with focal posterior fossa tumors treated with protons from 2006 to 2016.

RESULTS

The average rate of symptomatic brainstem toxicity from the 3 largest US pediatric proton centers was 2.38%. The actuarial rate of grade ≥2 brainstem toxicity was successfully reduced from 12.7% to 0% at 1 center after adopting modified radiation guidelines. Guidelines for treatment planning and current consensus brainstem constraints for proton therapy are presented. The current knowledge regarding linear energy transfer (LET) and its relationship to relative biological effectiveness (RBE) are defined. We review the current state of LET-based planning.

CONCLUSIONS

Brainstem injury is a rare complication of radiation therapy for both photons and protons. Substantial dosimetric data have been collected for brainstem injury after proton therapy, and established guidelines to allow for safe delivery of proton radiation have been defined. Increased capability exists to incorporate LET optimization; however, further research is needed to fully explore the capabilities of LET- and RBE-based planning.

Proton beam therapy in pediatric oncology

PURPOSE OF REVIEW

The advent of proton beam therapy (PBT) has initiated a paradigm shift in the field of pediatric radiation oncology, with increasing promise to alleviate both short-term and long-term toxicities. Given the dramatic rise in proton therapy centers in the United States, a discussion of the quality of evidence supporting its use in pediatric cancers is warranted.

RECENT FINDINGS

Proton radiotherapy appears to decrease the incidence and severity of late effects with the strongest evidence in pediatric brain tumor cohorts that shows benefits in neurocognitive, hearing, and endocrine outcomes. However, emerging data has shown that more conservative brainstem dose limits with protons compared with photons are required to limit brainstem toxicity; these modified recommendations have been incorporated into national cooperative group studies. Decreased toxicity in tumors outside of the CNS for PBT have also been reported in sarcomas, Hodgkin disease and neuroblastoma. Similarly, QoL outcomes are improved in brain tumor and other cohorts of patients treated with PBT.

SUMMARY

The collective findings demonstrate improved understanding and refinement of PBT in pediatric cancers. Data on QOL, toxicity and disease outcomes with PBT should continue to be collected and reported in order to understand the full extent of the risks and benefits associated with PBT.

Computational models and tools

In this chapter, we describe two different methods, analytical (pencil beam) algorithms and Monte Carlo simulations, used to obtain the intended dose distributions in patients and evaluate their strengths and shortcomings. We discuss the difference between the prescribed physical dose and the biologically effective dose, the relative biological effectiveness (RBE) between ions and photons and the dependence of RBE on the linear energy transfer (LET). Lastly, we show how LET- or RBE-based optimization can be used to improve treatment plans and explore how the availability of multimodality ion beam facilities can be used to design a tumor-specific optimal treatment.

New Agents, Emerging Late Effects, and the Development of Precision Survivorship

Incremental improvements in the treatment of children and adolescents with cancer have led to 5-year survival rates reaching nearly 85%. In the past decade, impressive progress has been made in understanding the biology of many pediatric cancers. With that understanding, multiple new agents have become available that offer the promise of more-effective and less-toxic treatment. These include agents that target various cell surface antigens and engage the adaptive immune system, as well as those that interfere with key signaling pathways involved in tumor development and growth. For local control, surgery and radiation techniques also have evolved, becoming less invasive or featuring new techniques and particles that more precisely target the tumor and limit the dose to normal tissue. Nevertheless, targeted agents, like conventional chemotherapy, radiotherapy, and surgery, may have off-target effects and deserve long-term follow-up of their safety and efficacy. These include injury to the endocrine, cardiovascular, and immunologic systems. New radiation and surgical techniques that theoretically reduce morbidity and improve long-term quality of life must also be validated with actual patient outcomes. Finally, with advances in genomics, information on host susceptibility to late effects is beginning to emerge. Such knowledge, coupled with improved metrics that better describe the spectrum of potential late effects across the entire lifespan, can lead to the development of decision models that project the potential long-term health outcomes associated with various treatment and follow-up strategies. These developments will help extend the current focus on precision medicine to precision survivorship, where clinicians, patients, and families will have a better grasp of the potential risks, benefits, and tradeoffs associated with the growing number of cancer treatment options.

Linear energy transfer distributions in the brainstem depending on tumour location in intensity-modulated proton therapy of paediatric cancer

BACKGROUND

For tumours near organs at risk, there is concern about unintended increase in biological dose from elevated linear energy transfer (LET) at the distal end of treatment fields. The objective of this study was therefore to investigate how different paediatric posterior fossa tumour locations impact LET and biological dose to the brainstem during intensity-modulated proton therapy (IMPT).

MATERIAL AND METHODS

Multiple IMPT plans were generated for four different simulated tumour locations relative to the brainstem for a five-year-old male patient. A prescribed dose of 59.4 Gy(RBE) was applied to the planning target volumes (PTVs). Plans with two lateral and one posterior non-coplanar fields were created, along with plans with modified field arrangements. The dose-averaged LET (LET_d) and the physical dose × RBE_LET (D × RBE_LET), where RBE_LET=1+c × LET_d, were calculated using the FLUKA Monte Carlo code. A scaling parameter c was applied to make the RBE_LET represent variations in the biological effect due to LET.

RESULTS

High LET_d values surrounded parts of the PTV and encompassed portions of the brainstem. Mean LET_d values in the brainstem were 3.2-6.6 keV/μm. The highest absolute brainstem LET_d values were seen with the tumour located most distant from the brainstem, whereas lower and more homogeneous LET_d values were seen when the tumour invaded the brainstem. In contrast, the highest mean D × RBE_LET values were found in the latter case (54.0 Gy(RBE)), while the case with largest distance between tumour and brainstem had a mean D × RBE_LET of 1.8 Gy(RBE).

CONCLUSIONS

Using IMPT to treat posterior fossa tumours may result in high LET_d values within the brainstem, particularly if the tumour volume is separated from the brainstem. However, the D × RBE_LET was greater for tumours that approached or invaded the brainstem. Changing field angles showed a reduction of LET_d and D × RBE_LET in the brainstem.

Relative biological effectiveness of carbon ions for tumor control, acute skin damage and late radiation-induced fibrosis in a mouse model

BACKGROUND

The aim of the present study was to compare the biological effectiveness of carbon ions relative to x-rays between tumor control, acute skin reaction and late RIF of CDF1 mice.

MATERIAL AND METHODS

CDF1 mice with a C3H mouse mammary carcinoma implanted subcutaneously on the foot of the right hind limb were irradiated with single fractions of either photons, or (12)C ions using a 30-mm spread-out Bragg peak. The endpoint of the study was local control (no tumor recurrence within 90 days). For the acute skin reaction, non-tumor bearing CDF1 mice were irradiated with a comparable radiation scheme, and monitored for acute skin damage between Day 7 and 40. Late RIF was assessed in the irradiated mice.

RESULTS

The TCD50 (dose producing tumor control in 50% of mice) values with 95% confidence interval were 29.7 (25.4-34.8) Gy for C ions and 43.9 (39.2-49.2) Gy for photons, with a corresponding Relative biological effectiveness (RBE) value of 1.48 (1.28-1.72). For acute skin damage the MDD50 (dose to produce moist desquamation in 50% of mice) values with 95% confidence interval were 26.3 (23.0-30.1) Gy for C ions and 35.8 (32.9-39.0) Gy for photons, resulting in a RBE of 1.36 (1.20-1.54). For late radiation-induced fibrosis the FD50 (dose to produce severe fibrosis in 50% of mice) values with 95% confidence interval were 26.5 (23.1-30.3) Gy for carbon ions and 39.8 (37.8-41.8) Gy for photons, with a RBE of 1.50 (1.33-1.69).

CONCLUSION

The observed RBE values were very similar for tumor response, acute skin damage and late RIF when irradiated with large doses of high- linear energy transfer (LET) carbon ions. This study adds information to the variation in biological effectiveness in different tumor and normal tissue models.

The use of proton therapy in the treatment of benign or low-grade pediatric brain tumors

Radiation therapy (RT) plays a critical role in the local tumor control of benign and low-grade central nervous system tumors in children but is not without the risk of long-term treatment-related sequelae. Proton therapy (PRT) is an advanced RT modality with a unique dose-deposition pattern that allows for treatment of a target volume with reduced scatter dose delivered to normal tissues compared with conventional photon RT and is now increasingly utilized in children with the hope of mitigating radiation-induced late effects. This article reviews the current literature evaluating the use of PRT in benign and low-grade pediatric central nervous system tumors such as low-grade glioma, craniopharyngioma, and ependymoma. Multiple dosimetric studies support the use of PRT by demonstrating the ability of PRT to better spare critical structures important for cognitive development, endocrine function, and hearing preservation and to reduce the total body dose associated with second malignancy risk. Early clinical data demonstrate that PRT is well tolerated with rates of local tumor control comparable to conventional photon RT series, and long-term clinical data are awaited.