Given concerns of excess malignancies following adjuvant radiation for seminoma, we evaluated photon and proton beam therapy (PBT) treatment plans to assess dose distributions to organs at risk and model rates of second cancers.

Ten stage I seminoma patients who were treated with conventional para-aortic AP-PA photon radiation to 25.5 Gy at Massachusetts General Hospital had PBT plans generated (AP-PA, PA alone). Dose differences to critical organs were examined. Risks of second primary malignancies were calculated.

PBT plans were superior to photons in limiting dose to organs at risk. PBT decreased dose by 46% (8.2 Gy) and 64% (10.2 Gy) to the stomach and large bowel, respectively (p<0.01). Notably, PBT was found to avert 300 excess second cancers among 10,000 men treated at a median age of 39 and surviving to 75 (p<0.01).

In this study, the use of protons provided a favorable dose distribution with an ability to limit unnecessary exposure to critical normal structures in the treatment of early-stage seminoma. It is expected that this will translate into decreased acute toxicity and reduced risk of second cancers, for which prospective studies are warranted.