OBJECTIVE: To improve medulloblastoma proton therapy. Although considered ideal 
for proton therapy, there are potential disadvantages. Expected benefits include 
reduced radiation-induced cancer and circulatory complications, while avoiding 
small brain volumes of dose in-homogeneity when compared with conventional 
X-rays. Several aspects of proton therapy might contribute to reduced tumour 
control due to (a) the use of more homogenous dose levels which can result in 
under-dosage, (b) differences in relative biological effectiveness (RBE) between 
that prescription RBE of 1.1 and the RBE of brain and spinal cord (likely to 
exceed 1.1) and in medulloblastoma cells (where RBE is likely to be below 1.1). 
Such changes, although speculative for RBE, might result in potential 
underdosage of tumour cells and a higher bio-effect in brain tissue.
METHODS: Dose distributions for X-ray and proton treatment are compared, with 
allocation of likely RBE values for fast growing medullolastoma cells and stable 
central nervous system tissue.
RESULTS: These physical and radiobiological factors are shown to combine to give 
a higher risk of tumour recurrence with further risks on tumour control when 
dose reduction schedules used for X-ray therapy are replicated for proton 
therapy for "low-risk" patients.
CONCLUSION: The dose distributions and prescribed doses of proton therapy, 
taking into account RBE, in children and adults with medulloblastoma, need to be 
reconsidered.