gms | German Medical Science

Background: Adjuvant photon radiation therapy has been the standard practice for the treatment of stage I seminoma following orchiectomy. More recently, single dose carboplatin or observation have emerged as alternative adjuvant courses due to concerns of acute toxicities and secondary malignancies reported in patients treated with radiation. In this study, we conducted a comparative analysis of photon and proton dose distributions in the adjuvant treatment of stage I seminoma and predict the rates of excess secondary malignancies by organ sites for photon and proton irradiation.

Material and methods: Ten consecutive patients treated between 6/2006–6/2008 at the Walter Reed Army Medical Center with photon irradiation for stage I seminoma had proton plans generated. Organs in the treatment field previously reported to be at increased risk for secondary malignancies, including the bladder, stomach, pancreas and large bowel, were contoured. In-field critical structures, including the liver and bilateral kidneys, were also contoured. Reported models of organ-specific radiation-induced cancer incidence rates based on organ equivalent dose were used to determine the absolute excess risk of secondary malignancies for the photon and proton plans targeting the para-aortic lymph nodes.

Results: Photon and proton plans provided equally acceptable target volume coverage. Improved dose conformality was observed with proton therapy, with a significant sparing of all examined normal tissues except the left kidney. Significantly more excess secondary cancers per 10,000 patients/year were predicted with photons compared with protons for the stomach (4.1; 95% confidence interval 3.2-5.1), large bowel (0.79; 0.39-1.0), and bladder (0.03; 0.0060-0.58), while no significant difference was observed for the pancreas (0.03; -0.013 to 0.062).

Conclusion: For patients with stage I seminoma following orchiectomy, proton therapy was demonstrated to reduce the predicted secondary cancer risk compared with photon irradiation. We predict a reduction of one additional secondary cancer for every 50 patients with a life expectancy of 40 years from the time of therapy treated with proton irradiation instead of photon irradiation. Superior normal tissue sparing was also observed with proton irradiation compared with photon irradiation. Lower doses to critical organs observed in this study with proton therapy may reduce acute toxicities previously reported with adjuvant irradiation, including nausea, lethargy and delay in return to work. Longitudinal studies are needed to confirm the clinical significance of our findings.

This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-04-2-0022. The views, opinions, interpretations, conclusions and recommendations expressed in this abstract are the authors and do not reflect the official policy of the Department of Army, Navy, Department of Defense, or US government.