Radiation Therapy for Stage IIA/B Seminoma: Modeling Secondary Cancer Risk for Protons and VMAT versus 3D Photons

Simple Summary Radiation therapy is an effective treatment for stage IIA and select stage IIB testicular seminoma patients. However, given the young age and long life expectancy of these patients, there are concerns about the risk of secondary cancers from radiation therapy. This study assessed the expected differences in secondary cancer risk for stage II seminoma patients receiving proton pencil-beam scanning (PBS) and photon VMAT compared to 3D conformal photon treatment plans. For 10 seminoma patients treated at our institution, 3D, VMAT, and PBS plans were generated. The percentage increase in the lifetime risk of developing cancers secondary to the radiation was modeled for each plan. On average, PBS reduced the mean radiation dose to critical organs by 60% and reduced the increase in lifetime secondary cancer risk by 55% compared to photons. Proton therapy should be considered for this patient population as it shows the potential to reduce late side effects.Abstract Radiation therapy (RT) is an effective treatment for stage IIA and select stage IIB seminomas. However, given the long life expectancy of seminoma patients, there are concerns about the risk of secondary cancers from RT. This study assessed differences in secondary cancer risk for stage II seminoma patients following proton pencil-beam scanning (PBS) and photon VMAT, compared to 3D conformal photon RT. Ten seminoma patients, five with a IIA staging who received 30 GyRBE and five with a IIB staging who received 36 GyRBE, had three RT plans generated. Doses to organs at risk (OAR) were evaluated, and secondary cancer risks were calculated as the Excess Absolute Risk (EAR) and Lifetime Attributable Risk (LAR). PBS reduced the mean OAR dose by 60% on average compared to 3D, and reduced the EAR and LAR for all OAR, with the greatest reductions seen for the bowel, liver, and stomach. VMAT reduced high doses but increased the low-dose bath, leading to an increased EAR and LAR for some OAR. PBS provided superior dosimetric sparing of OAR compared to 3D and VMAT in stage II seminoma cases, with models demonstrating that this may reduce secondary cancer risk. Therefore, proton therapy shows the potential to reduce acute and late side effects of RT for this population.