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

Abstract

<h3>Purpose/Objective(s)</h3> Radiation therapy to the para-aortic and iliac lymph nodes is an effective treatment for stage IIA and appropriately selected stage IIB seminoma patients. However, there are concerns for serious late complications, particularly an increased risk of secondary cancers due to the large areas of normal tissue irradiated. The purpose of this study was to quantify the expected differences in secondary cancer risk for stage II seminomas when evaluating proton pencil-beam scanning (PBS) and volumetric modulated arc therapy (VMAT) compared to conventional AP-PA photon (3D) treatment plans. <h3>Materials/Methods</h3> Five seminoma patients (four stage IIA receiving 30 Gy and one stage IIB receiving 36 Gy) ranging in age from 38 to 48 (mean 41.2) treated at our institution with PBS had two additional plans generated for comparison— one photon VMAT and one photon 3D. Dose differences to critical organs were examined and the risks of second primary cancers in those organs were calculated as the Excess Absolute Risk (EAR), reported in cancers per 10,000 person years, and Lifetime Attributable Risk (LAR), reported in percentage increase in risk, both projected to age 70. <h3>Results</h3> PBS reduced the mean organ dose compared to photons, on average by 60% but ranging from 30%-95% for different organs, except for the spinal cord which received the lowest mean dose with VMAT. PBS also reduced the EAR for all organs, with the greatest reduction for large and small bowel, liver, and stomach. Table 1 shows the average ratio of LAR between PBS, 3D, and VMAT for critical organs in five patients, and this ratio is greater than 1 for all organs for 3D compared to PBS. VMAT plans provide a mean dose reduction to some critical organs compared to 3D, particularly the spinal cord and pancreas, where VMAT also reduces the LAR compared to PBS. However, VMAT increases the low dose bath through other organs, leading to slightly increased LAR for the bladder, liver, and rectum compared to 3D. <h3>Conclusion</h3> The use of protons in stage IIA/B seminoma provides a favorable dose distribution with an expectation that this will transfer into decreased toxicity and reduced secondary cancer risk compared to conventional 3D photons. VMAT is another alternative to conventional 3D photons that provides a reduction in dose to some critical organs and comparable or reduced secondary cancer risk predictions for most organs.