228 Background: Seminoma constitutes the majority of testicular cancers and 75% percent of patients present with localized disease. Given that seminoma remains the most curable solid tumor, concerns of toxicity including late sequelae such as excess malignancies have given pause to the use of conventional adjuvant radiation therapy (RT) following orchiectomy for early-stage seminoma. As a result of the unique physical dose deposition characteristics of protons to avoid normal tissue, we evaluated both photon and proton beam therapy (PBT) treatment plans for para-aortic irradiation to assess dose distributions to organs at-risk and model rates of second cancers. Methods: Ten patients with stage I seminoma treated with conventional adjuvant para-aortic AP-PA photon RT to 25.5 Gy between 2004-2009 at Massachusetts General Hospital had PBT plans generated (AP-PA and PA alone). The dose differences to critical organs, as modeled by Equivalent Uniform Dose (EUD), were examined. The risks of second primary malignancies were calculated using validated methods and compared both to each other and to baseline population risks. Results: PBT plans were superior to photons in limiting dose to organs at-risk. The volume of whole body normal tissue spared 0.1 Gy was 9.0L and 7.8L for PA and AP-PA protons, respectively, compared to photons. The volume spared 1Gy was 5.0L and 3.8L for PA and AP-PA protons, respectively; while the volume spared 10Gy was 1.3L and 0.85L, respectively. PBT decreased the EUD by 46% (8.2 Gy) and 64% (10.2 Gy) to the stomach and large bowel, respectively (p<0.01), presumably translating into lower levels of nausea and fatigue. Notably, PBT was found to avert 612 excess second cancers among a population of 10,000 men diagnosed at age 35 and surviving to age 75 (p<0.01). Conclusions: In this comparative dosimetric and modeling 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 stage I seminoma patients. It is expected that this will translate into decreased acute toxicity and reduced risk of second cancers, for which prospective studies are warranted. No significant financial relationships to disclose.
Adjuvant radiation therapy in stage I seminoma: 20 years of oncologic results
