Out-of-field organ doses and associated radiogenic risks from para-aortic radiotherapy for testicular seminoma.

PURPOSE

The aims of this study were to (a) calculate the radiation dose to out-of-field organs from radiotherapy for stage I testicular seminoma and (b) estimate the associated radiogenic risks.

METHODS

Monte Carlo methodology was employed to model radiation therapy with typical anteroposterior and posteroanterior para-aortic fields on an anthropomorphic phantom simulating an average adult. The radiation dose received by all main and remaining organs that defined by the ICRP publication 103 and excluded from the treatment volume was calculated. The effect of field dimensions on each organ dose was determined. Additional therapy simulations were generated by introducing shielding blocks to protect the kidneys from primary radiation. The gonadal dose was employed to assess the risk of heritable effects for irradiated male patients of reproductive potential. The lifetime attributable risks (LAR) of radiotherapy-induced cancer were estimated using gender- and organ-specific risk coefficients for patient ages of 20, 30, 40, and 50 years old. The risk values were compared with the respective nominal risks.

RESULTS

Para-aortic irradiation to 20 Gy resulted in out-of-field organ doses of 5.0-538.6 mGy. Blocked field treatment led to a dose change up to 28%. The mean organ dose variation by increasing or decreasing the applied field dimensions was 18.7% ± 3.9% and 20.8% ± 4.5%, respectively. The out-of-field photon doses increased the lifetime intrinsic risk of developing thyroid, lung, bladder, prostate, and esophageal cancer by (0.1-1.4)%, (0.4-1.1)%, (2.5-5.4)%, (0.2-0.4)%, and (6.4-9.2)%, respectively, depending upon the patient age at exposure and the field size employed. A low risk for heritable effects of less than 0.029% was found compared with the natural incidence of these defects.

CONCLUSIONS

Testicular cancer survivors are subjected to an increased risk for the induction of bladder and esophageal cancer following para-aortic radiotherapy. The probability for the appearance of any other malignant disease to out-of-field organs was slightly elevated in respect to the nominal cancer incidence rates.