Evaluation of Internal Scatter Contribution to Gonadal Dose in Colorectal Radiotherapy.

Colorectal cancer is among the most common cancers in terms of both incidence and mortality. External beam radiotherapy is frequently used to treat colorectal cancer. In colorectal radiotherapy, the gonad is considered as an organ that must be spared owing to its radiosensitivity. However, only a few medical facilities use gonadal shields. In addition, the contribution of internal scatter to the gonadal dose should be considered because it is unavoidable. Therefore, this study aimed to investigate the effectiveness of the gonad shield according to its thickness and to evaluate the internal scatter contribution during the rectal radiotherapy. First, a Monte Carlo simulation was performed using a simplified patient phantom that consisted of the patient's body and gonad. The dose to the gonad was assessed according to the thickness of the gonad shield, beam direction, and the distance between the gonad and irradiation field. The internal scatter contribution was calculated as the ratio of the gonadal doses with and without a shield. Second, a mesh-type reference computational phantom (MRCP) was employed to calculate the internal scatter contribution to the gonadal dose in rectal radiotherapy. Subsequently, the contribution of internal scatter was investigated by analyzing the dosimetry results in actual patient cases. The gonad shield reduced 50-75% of the gonadal dose according to its thickness from 0.5-3 cm. The internal scatter contribution ranged from 55-65% for the simplified phantom, whereas it was 75% on average for MRCP. The dosimetry results from the clinical case were similar to those of both the simplified phantom and MRCP, that is, the internal scatter contribution varied from 55-77% in most cases. These results are expected to improve the accuracy of rectal radiotherapy and benefit radiation shielding of the gonads.