Successful Implementation of Image-Guided Pencil-Beam Scanning Proton Therapy in Medulloblastomas

Abstract

Medulloblastoma is the most common malignant brain tumour in children, while much rarer in adults. Although the prognosis and outcomes have greatly improved in the era of modern multidisciplinary management, long-term treatment-induced toxicities are common. Craniospinal irradiation followed by a boost to the primary and metastatic tumour sites forms the backbone of treatment. Proton therapy has been endorsed over conventional photon-based radiotherapy due to its superior dosimetric advantages and subsequently lower incidence and severity of toxicities. We report here our experience from South-East Asia's first proton therapy centre of treating 40 patients with medulloblastoma (38 children and adolescents, 2 adults) who received image-guided, intensity-modulated proton therapy with pencil-beam scanning between 2019 and 2023, with a focus on dosimetry, acute toxicities, and early survival outcomes. All patients could complete the planned course of proton therapy, with mostly mild acute toxicities that were manageable on an outpatient basis. Haematological toxicity was not dose-limiting and did not prolong the overall treatment time. Preliminary data on early outcomes including overall survival and disease-free survival are encouraging, although a longer follow-up and data on long-term toxicities are needed.

Keywords: craniospinal irradiation; intensity-modulated proton therapy; medulloblastoma; molecular subgroups; pencil-beam scanning; risk stratification in medulloblastoma.

Figure 1

Kaplan–Meier survival curve for all patients, n = 40.

Figure 2

Kaplan–Meier curves depicting survival functions in medulloblastoma patients stratified according to their molecular group: (a) Overall survival, (b) disease-specific survival, (c) progression-free survival.

Figure 3

Kaplan–Meier curves depicting survival functions in medulloblastoma patients stratified according to their risk group: (a) overall survival, (b) disease-specific survival.

Figure 4

Representative sagittal and axial sections showing the dose colour wash 98% (red) and 95% (cyan) for target coverage and anterior mucosal structures (oesophagus—yellow, midline mucosa—magenta, bowel bag—yellow): (a1,b1) vertebral-sparing CSI (VS-CSI) plan for skeletally mature patients. The spinal target canal includes the spinal canal and exiting nerve roots with 5 mm margin. Anterior structures are spared well. (a2,b2) Traditional vertebral-inclusive CSI (VI-CSI) plan for skeletally immature patients (children). The target includes the spinal canal, exiting nerve roots, and entire vertebral body with 5 mm margin. The axial section shows significant spillage of prescription doses into the anterior mucosal structures. (a3,b3) Modified vertebral-inclusive CSI (VI-CSI) plan for skeletally immature patients (children). The target includes the spinal canal, exiting nerve roots, with 5 mm margin, and the entire vertebral body without any additional PTV margin. The axial section shows adequate and homogenous coverage of the vertebral body between 95–98% isodose, while the anterior mucosal structures are spared well.