Impact of Relative Biologic Effectiveness for Proton Therapy for Head and Neck and Skull-Base Tumors: A Technical and Clinical Review.

Proton therapy has emerged as a crucial tool in the treatment of head and neck and skull-base cancers, offering advantages over photon therapy in terms of decreasing integral dose and reducing acute and late toxicities, such as dysgeusia, feeding tube dependence, xerostomia, secondary malignancies, and neurocognitive dysfunction. Despite its benefits in dose distribution and biological effectiveness, the application of proton therapy is challenged by uncertainties in its relative biological effectiveness (RBE). Overcoming the challenges related to RBE is key to fully realizing proton therapy's potential, which extends beyond its physical dosimetric properties when compared with photon-based therapies. In this paper, we discuss the clinical significance of RBE within treatment volumes and adjacent serial organs at risk in the management of head and neck and skull-base tumors. We review proton RBE uncertainties and its modeling and explore clinical outcomes. Additionally, we highlight technological advancements and innovations in plan optimization and treatment delivery, including linear energy transfer/RBE optimizations and the development of spot-scanning proton arc therapy. These advancements show promise in harnessing the full capabilities of proton therapy from an academic standpoint, further technological innovations and clinical outcome studies, however, are needed for their integration into routine clinical practice.

Long-Term Outcomes Following Definitive or Adjuvant Proton Radiotherapy for Adenoid Cystic Carcinoma.

Purpose: Adenoid cystic carcinoma (ACC) is a rare malignancy accounting for 1% of all head and neck cancers. Treatment for ACC has its challenges and risks, yet few outcomes studies exist. We present long-term outcomes of patients with ACC of the head and neck treated with proton therapy (PT). Materials and Methods: Under an institutional review board-approved, single-institutional prospective outcomes registry, we reviewed the records of 56 patients with de novo, nonmetastatic ACC of the head and neck treated with PT with definitive (n = 9) or adjuvant PT (n = 47) from June 2007 to December 2021. The median dose to the primary site was 72.6 gray relative biological equivalent (range, 64-74.4) delivered as either once (n = 19) or twice (n = 37) daily treatments. Thirty patients received concurrent chemotherapy. Thirty-one patients received nodal radiation, 30 electively and 1 for nodal involvement. Results: With a median follow-up of 6.2 years (range, 0.9-14.7), the 5-year local-regional control (LRC), disease-free survival, cause-specific survival, and overall survival rates were 88%, 85%, 89%, and 89%, respectively. Intracranial extension (P = .003) and gross residual tumor (P = .0388) were factors associated with LRC rates. While the LRC rate for those with a gross total resection was 96%, those with subtotal resection or biopsy alone were 81% and 76%, respectively. The 5-year cumulative incidence of clinically significant grade ≥3 toxicity was 15%, and the crude incidence at the most recent follow-up was 23% (n = 13). Conclusion: This is the largest sample size with the longest median follow-up to date of patients with ACC treated with PT. PT can provide excellent disease control for ACC of the head and neck with acceptable toxicity. T4 disease, intracranial involvement, and gross residual disease at the time of PT following either biopsy or subtotal resection were significant prognostic features for worse outcomes.

Elective Irradiation of Retropharyngeal Lymph Nodes as an Indication for Adjuvant Radiation Therapy After Transoral Surgery for Tonsil Cancer.

Treatment of squamous cell carcinoma of the tonsil involves primary radiation therapy (RT) or surgical resection. Historically, if RT was the primary or adjuvant treatment modality, most of the bilateral retropharyngeal lymph nodes (RPLNs) were treated electively with a therapeutic dose for subclinical disease, regardless of whether radiographically pathologic lymph nodes were seen on initial diagnostic imaging. De-escalation strategies include the incorporation of transoral surgery with the goal to either eliminate or reduce the dose of adjuvant RT or chemotherapy. Transoral surgery does not include elective removal of the RPLNs, and no guideline or outcome paper recommends adjuvant RT specifically to electively treat RPLNs. In this Topic Discussion, we discuss pertinent literature and suggest management decisions. The management decisions discussed in this Topic Discussion pertain to only tonsillar primaries and not those of the soft palate or base of the tongue.

Outcomes following proton therapy for pediatric esthesioneuroblastoma.

BACKGROUND: Pediatric esthesioneuroblastoma (EN) can infiltrate skull base anatomy, presenting challenges due to high radiation doses and pediatric tissue sensitivity. This study reports outcomes of pediatric EN treated with proton radiotherapy (PT). PROCEDURE: Using an IRB-approved prospective outcomes registry, we evaluated patient, tumor, and treatment-related variables impacting disease control and toxicity in pediatric nonmetastatic EN treated with modern multimodality therapy, including PT. RESULTS: Fifteen consecutive patients (median age 16) comprising Kadish stage B (n = 2), C (n = 9), and D (n = 4) tumors were assessed, including six with intracranial involvement, four with cranial nerve deficits, and four with cervical lymphadenopathy. Before radiation, two had subtotal and 13 had gross total resections (endoscopic or craniofacial). Two underwent neck dissection. Eleven received chemotherapy before radiation (n = 5), concurrent with radiation (n = 4), or both (n = 2). Median total radiation dose (primary site) was 66 Gy/CGE for gross disease and 54 Gy/CGE (cobalt Gray equivalent) for microscopic disease. Median follow-up was 4.8 years. No patients were lost to follow-up. Five-year disease-free and overall survival rates were 86% (no local or regional recurrences). Two patients developed vertebral metastases and died. Two required a temporary feeding tube for oral mucositis/dysphagia. Late toxicities included symptomatic retinopathy, major reconstructive surgery, cataracts, chronic otitis media, chronic keratoconjunctivitis, hypothyroidism, and in-field basal cell skin cancer. CONCLUSIONS: A multimodality approach for pediatric EN results in excellent local control. Despite the moderate-dose PT, serious radiation toxicity was observed; further dose and target volume reductions may benefit select patients. Longer follow-up and comparative data from modern photon series are necessary to fully characterize any relative PT advantage.

Impact of mandated drug monitoring on opioid use during highly conformal radiotherapy for oropharynx cancer.

BACKGROUND: Prescription drug monitoring programs (PDMPs) have proliferated due to increasing opioid-related deaths. We evaluated acute opioid use changes for 64 patients treated with highly conformal radiotherapy (RT) following a state-mandated PDMP. METHODS: Patients receiving proton therapy (PT) (n=40), intensity-modulated RT (IMRT) (n=14), or both (n=10) were divided into preintervention (n=26) and postintervention cohorts (n=38); records were reviewed retrospectively under an institutional review board (IRB)-approved tracking protocol. Dosages prescribed during acute therapy (during RT-3 months post-RT) and patient-reported pain (Defense and Veterans Pain Rating Scale) were endpoints. Dosages were treated as responses in Chi-square tests (three-level ordinal response). RESULTS: Overall, 72% (n=46) received opioids; of which 22% (n=10) of all patients and 10% (n=2) of opioid-naive patients continued analgesic management 3 months post-RT. Median total doses were 975 and 1,025 morphine milligram equivalents (MME) in pre- and postintervention groups, with no significant differences in MME prescribed (P=0.8) or uncontrolled pain (P=0.3). Statistically significant factors were tonsil primaries (P<0.01) and alcohol use (P=0.02). Uncontrolled pain episodes during and post-RT did not vary per cohort (P=0.19). CONCLUSIONS: PDMP use was not associated with management changes in patient-reported acute pain during RT (IMRT or PT). Following highly conformal RT, few patients remained on narcotics 3 months post-RT.

Carbon Ion Radiotherapy: An Evidence-Based Review and Summary Recommendations of Clinical Outcomes for Skull-Base Chordomas and Chondrosarcomas.

Skull-base chordoma and chondrosarcoma are rare radioresistant tumors treated with surgical resection and/or radiotherapy. Because of the established dosimetric and biological benefits of heavy particle therapy, we performed a systematic and evidence-based review of the clinical outcomes of patients with skull-base chordoma and chondrosarcoma treated with carbon ion radiotherapy (CIRT). A literature review was performed using a MEDLINE search of all articles to date. We identified 227 studies as appropriate for review, and 24 were ultimately included. The published data illustrate that CIRT provides benchmark disease control outcomes for skull-base chordoma and chondrosarcoma, respectively, with acceptable toxicity. CIRT is an advanced treatment technique that may provide not only dosimetric benefits over conventional photon therapy but also biologic intensification to overcome mechanisms of radioresistance. Ongoing research is needed to define the magnitude of benefit, patient selection, and cost-effectiveness of CIRT compared to other forms of radiotherapy.

Proton Radiotherapy for Skull-Base Malignancies: Imaging Considerations of Radiotherapy and Complications.

Proton therapy (PT) is a form of highly conformal external-beam radiotherapy used to mitigate acute and late effects following radiotherapy. Indications for treatment include both benign and malignant skull-base and central nervous system pathologies. Studies have demonstrated that PT shows promising results in minimizing neurocognitive decline and reducing second malignancies with low rates of central nervous system necrosis. Future directions and advances in biologic optimization may provide additional benefits beyond the physical properties of particle dosimetry.

Long-term outcomes of fractionated proton beam therapy for benign or radiographic intracranial meningioma.

PURPOSE: Benign intracranial meningioma is one of the most common primary brain neoplasms. Proton therapy has been increasingly utilized for nonoperative management of this neoplasm, yet few long-term outcomes studies exist. METHODS: The medical records of a total of 59 patients with 64 lesions were reviewed under a prospective outcomes tracking protocol for histologically proven or radiographically benign meningioma. The patients were treated with proton therapy at the University of Florida Proton Therapy Institute between 2007 and 2019 and given a median dose of 50.4 GyRBE at 1.8 GyRBE (relative biological effectiveness) (range 48.6-61.2 GyRBE) in once-daily treatments. RESULTS: With a median clinical and imaging follow-up of 6.3 and 4.7 years, the rates of 5-year actuarial local progression and cumulative incidence of grade 3 or greater toxicity were 6% (95% confidence interval [CI] 1%-14%), and 2% (95% CI < 1%-15%), respectively. Two patients experienced local progression after 5 years. The 5-year actuarial overall survival rate was 87% (95% CI 74-94%). CONCLUSION: Fractionated PBT up to 50.4 GyRBE is a safe and highly effective therapy for treating benign intracranial meningioma.

Early-stage vocal cord cancer treated with hypofractionated radiotherapy to the larynx with or without concurrent chemotherapy.

BACKGROUND: We report outcomes among patients with T2 and select T3 glottic squamous cell carcinoma (SCC) treated with radiotherapy. METHODS: We reviewed T2 and T3 (only paraglottic space invasion) N0 M0 glottic SCC patients treated with curative-intent hypofractionated larynx radiotherapy, with or without concurrent systemic therapy. RESULTS: Of 71 patients, those who received concurrent chemotherapy (23/71; 32%) had worse prognostic factors, including impaired cord mobility (70% vs. 40%, p = 0.02) and larger median gross tumor volume (3.0 vs. 1.6 cm3 , p = 0.003). Over a median follow-up of 3.8 years, 2-year local control among patients with impaired cord mobility appeared higher for those who received chemotherapy (88% vs. 61%, p = 0.12), but the difference was not statistically significant. Acute and late toxicity rates were not higher among patients who received chemotherapy. CONCLUSIONS: The addition of concurrent platinum-based chemotherapy to hypofractionated larynx radiotherapy among patients with early-stage glottic SCC with impaired cord mobility appears safe and worthy of additional investigation.

The incidence of brainstem toxicity following high-dose conformal proton therapy for adult skull-base malignancies.

BACKGROUND: Dose escalation for skull-based malignancies often presents risks to critical adjacent neural structures, including the brainstem. We report the incidence of brainstem toxicity following fractionated high-dose conformal proton therapy and associated dosimetric parameters. MATERIAL AND METHODS: We performed a single-institution review of patients with skull-base chordoma or chondrosarcoma who were treated with proton therapy between February 2007 and January 2020 on a prospective outcomes-tracking protocol. The primary endpoint was grade ≥2 brainstem toxicity. No patients received concurrent chemotherapy, and brainstem toxicity was censored for analysis if it coincided with local disease progression. RESULTS: We analyzed 163 patients who received a minimum of 45 GyRBE to 0.03 cm3 of the brainstem. Patients were treated to a median total dose of 73.8 (range 64.5-74.4) GyRBE at 1.8 GyRBE per fraction with 17 patients undergoing twice-daily treatment at 1.2 GyRBE per fraction. With a median follow-up of 4 years, the 5-year cumulative incidence of grade ≥2 brainstem injury was 1.3% (95% CI 0.25-4.3%). There was one grade 2, one grade 3, and no grade 4 or 5 events, with all patients recovering function with medical management. CONCLUSION: In delivering curative-intent radiotherapy for skull-base chordoma and chondrosarcoma in adults, small volumes of the brainstem can safely receive at least 64 GyRBE with minimal risk of serious brainstem injury.

Impact of Type of Treatment Center and Access to Care on Mortality and Survival for Skull Base Chordoma and Chondrosarcoma.

Introduction In adults with skull base chordoma or chondrosarcoma, the impact of treatment center and access to care have not been well described in regard to perioperative mortality and survival. Methods A query of the National Cancer Database (NCDB) and review of 1,102 adults-488 with chordomas and 614 with chondrosarcomas-was performed. The Kaplan-Meier's product limit method and chi-square analysis, respectively, assessed overall survival and 30-day (30D) and 90-day (90D) mortalities. Results For 925 patients who had surgery and available mortality data, the 30D and 90D mortality rates were 0.9 and 1.5%. Lower education level ( p = 0.0185) and treatment at a nonacademic facility ( p = 0.016) were associated with increased risk of 90-day mortality. Median follow-up was 52 months and analysis was dichotomized by histology. For those with skull base chordoma, patients from a larger metro size ( p = 0.002), age below the median 52 years ( p ≤ 0.001), and private insurance (<0.001) were associated with prolonged survival, whereas for skull base chondrosarcoma, the factors were treatment at an academic medical center ( p = 0.001), high-volume center ( p = 0.007), age below the median 52 years ( p ≤ 0.001), higher income ( p = 0.043), higher education ( p = 0.017), and private insurance ( p ≤ 0.001). Comparing high-, medium-, and low-volume centers, high-volume centers were most likely to be academic, deliver radiotherapy, escalate doses >70 Gy, and utilize proton radiotherapy consistent across both disease subsets. Conclusion Higher educational attainment and treatment at an academic facility were associated with decreased 90D mortality for patients with skull base chordoma and chondrosarcoma. For those with skull base chordoma, larger metro size, younger age, and private insurance were associated with prolonged survival; for those with chondrosarcoma, it was treatment at a high-volume or academic medical center, younger age, higher income or education, and private insurance.

Disease Control after Radiotherapy for Adult Craniopharyngioma: Clinical Outcomes from a Large Single-Institution Series.

PURPOSE: To report disease control and treatment-related side effects among adult patients with craniopharyngioma treated with radiotherapy. METHODS: We performed a single-institution review of adult patients (> 21 years old) with craniopharyngioma treated with radiotherapy either definitively or postoperatively for gross residual disease. We report disease control, survival, and radiotherapy-related side effects. RESULTS: A total of 49 adult patients with craniopharyngioma were included, 27 of whom were treated at initial presentation and 22 for recurrent disease following initial surgery and observation. Overall, 77% received radiotherapy postoperatively (either after primary surgery or surgery for recurrence). With a median clinical and radiographic follow-up of 4.2 (range, 0.4-21.6) years and 3.0 (range, 0-21.5) years, the 5- and 10-year local control rates were 100 and 94%, respectively. The 5- and 10-year overall survival rates were 80 and 66%, respectively. Eleven percent of patients experienced grade 2 vision deterioration and 18% suffered grade 2 endocrinopathies following radiotherapy. CONCLUSIONS: Radiotherapy provides excellent disease control with acceptable toxicity among adult patients with craniopharyngioma. These data support the use of fractionated radiotherapy in adult patients with recurrent or gross residual disease after surgery. For inoperable patients or those with moderate or high surgical risk to neurologic and/or vascular structures, we advocate for limited surgical resection and postoperative radiotherapy to balance optimal tumor control with tumor- and treatment-related morbidity.

Establishing Cost-Effective Allocation of Proton Therapy for Patients With Mediastinal Hodgkin Lymphoma.

PURPOSE: For curative treatment of Hodgkin lymphoma, radiation therapy benefit must be weighed against toxicity. Although more costly, proton radiation therapy reduces dose to healthy tissue, potentially improving the therapeutic ratio compared with photons. We sought to determine the cost-effectiveness of proton versus photon therapy for mediastinal Hodgkin lymphoma (MHL) based on reduced heart disease. METHODS AND MATERIALS: Our model approach was 2-fold: (1) Use patient-level dosimetric information for a cost-effectiveness analysis using a Markov cohort model. (2) Use population-based data to develop guidelines for policymakers to determine thresholds of proton therapy favorability for a given photon dose. The HD14 trial informed relapse risk; coronary heart disease risk was informed by the Framingham risk calculator modified by the mean heart dose (MHD) from radiation. Sensitivity analyses assessed model robustness and identified the most influential model assumptions. A 30-year-old adult with MHL was the base case using 30.6-Gy proton therapy versus photon intensity modulated radiation therapy. RESULTS: Proton therapy was not cost-effective in the base case for male ($129,000/ quality-adjusted life years [QALYs]) or female patients ($196,000/QALY). A 5-Gy MHD decrease was associated with proton therapy incremental cost-effectiveness ratio <$100,000/QALY in 40% of scenarios. The hazard ratio associating MHD and heart disease was the most influential clinical parameter. CONCLUSIONS: Proton therapy may be cost-effective a select minority of patients with MHL based on age, sex, and MHD reduction. We present guidance for clinicians using MHD to aid decision-making for radiation therapy modality.

Clinical Outcomes Following Dose-Escalated Proton Therapy for Skull-Base Chordoma.

PURPOSE: To evaluate the effectiveness of external-beam proton therapy (PT) on local control and survival in patients with skull-base chordoma. MATERIALS AND METHODS: We reviewed the medical records of patients with skull-base chordoma treated with definitive or adjuvant high-dose PT and updated their follow-up when feasible. We assessed overall survival, disease-specific survival, local control, and freedom from distant metastasis. Radiotherapy toxicities were scored using the Common Terminology Criteria for Adverse Events, version 4.0. RESULTS: A total 112 patients were analyzed, of whom 105 (94%) received PT and 7 (6%) received combined proton-photon therapy between 2007 and 2019. Eighty-seven patients (78%) underwent a subtotal resection, 22 (20%) a gross total resection, and 3 (3%) a biopsy alone. The median radiotherapy dose was 73.8 Gy radiobiologic equivalent (GyRBE; range, 69.6-74.4). Ninety patients (80%) had gross disease at radiotherapy and 7 (6%) were treated for locally recurrent disease following surgery. Median follow-up was 4.4 years (range, 0.4-12.6); for living patients, it was 4.6 years (range, 0.4-12.6), and for deceased patients, 4.1 years (range, 1.2-11.2). At 5 years after radiotherapy, the actuarial overall survival, disease-specific survival, local control, and freedom from distant metastasis rates were 78% (n = 87), 83% (n = 93), 74% (n = 83), and 99% (n = 111), respectively. The median time to local progression was 2.4 years (range, 0.8-7). Local control and disease-specific survival by resection status was 95% versus 70% (P = 0.28) and 100% versus 80% (P = 0.06) for gross total, versus subtotal, resection or biopsy alone, respectively. There were no serious acute toxicities (grade ≥ 3) related to radiotherapy. CONCLUSION: High-dose PT alone or after surgical resection for skull-base chordoma reaffirms the favorable 5-year actuarial local control rate compared with conventional techniques with acceptable late-complication-free survival. Outcomes following gross total resection and adjuvant PT were excellent. Further follow-up of this cohort is necessary to better characterize long-term disease control and late toxicities.

Long-term Outcomes from Proton Therapy for Sinonasal Cancers.

PURPOSE: To report long-term disease control, survival, and toxicity after proton therapy for sinonasal cancer. PATIENTS AND METHODS: We reviewed 143 cases of adults with nonmetastatic sinonasal cancers treated with primary (18%; n = 26) or adjuvant (82%; n = 117) proton therapy. The most common histologies were squamous cell carcinoma (29%; n = 42), olfactory neuroblastoma (23%; n = 33), and adenoid cystic carcinoma (16%; n = 23). Patients had predominantly advanced-stage disease (T3, 24%, n = 35; T4, 66%, n = 94) and high-grade histology (52%; n = 74). Surgery included endoscopic resection alone (50%) with craniotomy (10%) or open resection (40%), and 31% had gross disease present at radiotherapy. Most (91%) received high-dose (median, 73.6 Gy radiobiological equivalent [GyRBE]; 84% >70 GyRBE) passive-scatter proton therapy using accelerated hyperfractionation (1.2 GyRBE twice daily) and concurrent chemotherapy (70%). Univariate and multivariate models assessed prognostic factors. Grade 3+ toxicities were recorded per Common Terminology Criteria, version 4. Median follow-up was 3.4 years (range, 0.1-12.5 years) overall and 4.9 years (range, 0.9-12.5 years) for living patients. RESULTS: The 5-year outcomes were as follows: local control (LC), 80%; neck control, 96%; local-regional control, 78%; freedom from distant metastases, 71%; and disease-free survival, 62%; cause-specific survival, 64%; and overall survival, 59%. Surgery improved LC, but only with gross total resection (5-year LC 87% versus subtotal resection 62.9%, and biopsy alone 55% (P < 0.001). Gross residual disease was the only significant prognostic factor for local-regional control on multivariate analysis. High-grade, T4, and local recurrence were associated with decreased overall survival. Late (G3+) toxicity occurred in 22% (32 of 143), including central nervous system necrosis and vision loss in 6% (9 of 143) and 3.5% (5 of 143), respectively. CONCLUSION: Proton therapy after gross-total resection provides excellent long-term LC in patients with locally advanced, high-grade sinonasal cancer. Moreover, LC remains strongly associated with long-term survival. With gross disease, about 60% of patients had long-term LC with proton therapy and induction or concurrent chemotherapy.

Passively Scattered Proton Therapy for Nonmelanoma Skin Cancer with Clinical Perineural Invasion.

PURPOSE: To report our experience with the delivery of passively scattered proton therapy in the management of nonmelanoma skin cancers with clinical perineural invasion. MATERIALS AND METHODS: We reviewed the medical records of patients who received definitive or postoperative proton therapy for nonmelanoma skin cancer with clinical perineural invasion at our institution and updated patient follow-up when possible. All patients were treated with curative intent with or without the delivery of concurrent systemic therapy. We report disease control rates and the rates of late toxicity among this cohort. RESULTS: Twenty-six patients treated between 2008 and 2017 were included in the analysis. Following proton therapy, the 3-year overall, cause-specific, and disease-free survival rates were 59%, 73%, and 60%, respectively. The 3-year local control, local regional control, and distant metastasis-free survival rates were 80%, 65%, and 96%, respectively. On univariate analysis, surgical resection before radiation therapy significantly improved local regional control rates at 3 years (55% versus 86%; P = .04). Grade 3+ late toxicities occurred in 13 patients (50%) and the most common toxicities included grade 3+ keratitis of the ipsilateral eye, which occurred in 4 patients (15%) and grade 3+ brain necrosis in 4 patients (15%). CONCLUSION: Proton therapy is effective in the management of nonmelanoma skin cancer with clinical perineural invasion. Although disease control and complication rates compare favorably to those previously published for photon-based radiation therapy, the risk for late toxicity is significant and patients should be appropriately counseled.

Proton therapy for adult medulloblastoma: Acute toxicity and disease control outcomes.

PURPOSE: We report disease control, survival outcomes, and treatment-related toxicity among adult medulloblastoma patients who received proton craniospinal irradiation (CSI) as part of multimodality therapy. METHODS: We reviewed 20 adults with medulloblastoma (≥ 22 years old) who received postoperative proton CSI ± chemotherapy between 2008 and 2020. Patient, disease, and treatment details and prospectively obtained patient-reported acute CSI toxicities were collected. Acute hematologic data were analyzed. RESULTS: Median age at diagnosis was 27 years; 45% of patients had high-risk disease; 75% received chemotherapy, most (65%) after CSI. Eight (40%) patients received concurrent vincristine with radiotherapy. Median CSI dose was 36GyE with a median tumor bed boost of 54GyE. Median duration of radiotherapy was 44 days. No acute ≥ grade 3 gastrointestinal or hematologic toxicities attributable to CSI occurred. Grade 2 nausea and vomiting affected 25% and 5% of patients, respectively, while 36% developed acute grade 2 hematologic toxicity (36% grade 2 leukopenia and 7% grade 2 neutropenia). Those receiving concurrent chemotherapy with CSI had a 38% rate of grade 2 hematologic toxicity compared to 33% among those not receiving concurrent chemotherapy. Among patients receiving adjuvant chemotherapy (n = 13), 100% completed ≥ 4 cycles and 85% completed all planned cycles. With a median follow-up of 3.1 years, 4-year actuarial local control, disease-free survival, and overall survival rates were 90%, 90%, and 95%, respectively. CONCLUSIONS: Proton CSI in adult medulloblastoma patients is very well tolerated and shows promising disease control and survival outcomes. These data support the standard use of proton CSI for adult medulloblastoma.