INTRODUCTION: 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.
MATERIAL AND METHODS: 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.
RESULTS: 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.
CONCLUSION: 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.