Plan robustness in proton beam therapy of a childhood brain tumour

The concern of secondary cancer induction and normal tissue complications have motivated a more frequent use of protons in radiotherapy (RT) of children. However, proton RT is likely to be less robust to anatomical changes occurring during therapy. In this study we present a recent clinical case to illustrate this issue.

A five-year-old boy with a highly proliferating malignant intracranial nerve sheath tumour underwent a partial resection prior to RT and developed a post-surgery oedema close to the surgical cavity. RT was delivered with volumetric modulated arc therapy (VMAT) to a total tumour dose of 61.2 Gy. The most critical organs at risk (ORs) were the right optical nerve, brainstem and chiasm. Proton plans were constructed for the purpose of this study. In order to simulate a worst-case scenario, the extent of the oedema observed in the last part of the treatment was used to modify the oedema on the planning computed tomography (CT). Both the photon and proton plans were then re-calculated, as follows: Scenario A: Treatment planning based on the planning CT with oedema and dose calculated as if it was delivered without oedema. Scenario B: Treatment planning on the modified planning CT without oedema, but re-calculated with oedema. These two scenarios were compared to the situation where the oedema was present at treatment planning and unchanged during RT.

Total dose to critical ORs remained unchanged for the photon plans, with changes within 0.3 Gy for the normal tissues and nearly identical target coverage. For protons, scenario A led to increased maximal doses in all critical ORs, 5.1 Gy in the brainstem, 6.1 Gy in the chiasm and 6.4 Gy in the right optical nerve. For scenario B the proton plans resulted in a loss in target coverage.

This case study shows that RT with protons were far less robust to anatomical changes than when treated with photons, emphasising the increased need for adaptive approaches in RT with protons.