Proton beam therapy for medulloblastoma.

It is heartening to read a report of good functional outcomes following proton beam therapy in young patients with medulloblastoma, including the absence of many unpleasant side eff ects that can occur after conventional photon therapy. The authors quote UK research, which criticises the current proton therapy treatment policy of dividing the x-ray (photon) equivalent dose by 1·1. This is done in order to compensate for the enhanced eff ectiveness of proton therapy caused by the greater proximity of ionisation in proton beam therapy compared with photons. Radiobiological modelling studies, based on the high radiosensitivity of medulloblastoma cells, suggest a lower necessary dose reduction of 1·03–1·08 in order to maintain the same tumour control eff ect. A dose division by 1·1, compared with, for example, 1·05 (if correct), would incur a 4% overall change in eff ective dose, which could translate into a 4–8% reduction in tumour control. The number of patients required to detect such a change would be much larger than the 59 patients included in the study by Yock and colleagues, however: a minimum of several hundred and perhaps over a thousand patients might be necessary. This statistical power requirement indicates the need for national and international cooperation in the conduct and analysis of such rare treatments. The reported maintenance of cognitive function is important. With a predicted dose reduction for neural tissues of 1·2 or more, rather than the 1·1 used in practice, this outcome implies a suffi cient reserve of cortical neural radiation tolerance compared with the dose required for tumour control. The brainstem necrosis in one child is likely to be caused by a combination of the lower radiation tolerance of brainstem than cortical brain, enhanced ionisation density eff ects, previous surgery, and the chemotherapy regimen used. Another important factor is that the meticulous treatments in Boston are given to children who may not only be fi tter to travel, but are also from wealthier backgrounds, with better outcomes than those of less privileged children without such access to excellent care.