Determination of out-of-field doses and related risks from the photon beam radiotherapy

Abstract

This study investigates out-of-field radiation doses and associated risks in radiotherapy using experimental measurements and standard dosimetry techniques. Radiotherapy is a primary modality for cancer treatment, delivering therapeutic radiation doses to tumors while minimizing exposure to surrounding tissues. Although radiation outside the treatment field is small, it can still cause late radiogenic effects. This study investigates out-of-field doses and related risks using an Alderson Rando phantom and a ⁶⁰Co teletherapy beam. Absorbed doses in target organs and effective doses in peripheral organs were measured with thermoluminescence dosimeters (TLDs). Dosimetry was performed using an FC65G Farmer chamber, NE2575 ion chamber, and water phantom following IAEA TRS-398 protocols. TLDs were placed at different depths in target organs, including the left lung, right lung, stomach, and on the surface of peripheral organs. Irradiations were performed at 100 cGy and 200 cGy, and the TLD readings were obtained using a calibrated TLD reader. Measured organ doses were very close to or slightly lower than the expected values at most depths. The TLDs showed a linear dose response with an uncertainty of 1.54%.  Depth dose correction factors for ⁶⁰Co beams were calculated to ensure accurate treatment and maximum dose delivery. The depth-dose correction factors obtained in this study for the lung, stomach, kidney, and spleen are 0.897±0.028, 0.775±0.023, 0.765±0.027, and 0.723±0.118, respectively. Age dependent secondary cancer risks were also estimated from the measured out-of-field doses. The findings provide useful data to support the optimization of treatment planning and to minimize the risk of secondary cancers in radiotherapy patients.

The Chittagong Univ. J. Sci. 46(1): 85-100, 2025