Radiation dose to radiotherapy technologists due to induced activity in high energy medical electron linear accelerators

摘要

After delivering X-ray beam of 15 MV energy, measurable radiation level near the treatment head of medical electron linear accelerator (LINAC) and in its surrounding is observed due to induced activity generated in target, flattening filter, monitor chamber, cooling tube, collimating jaws, etc., The radiation generated in the head of LINAC has the potential of exposing the radiotherapy staff/personnel (technologists) if he/she comes in close proximity of the machine head immediately after delivering the planned treatment. A systematic study was carried out to quantify the radiation levels near LINAC head, isocenter and 0.5 m lateral to isocenter for planned beam delivery of 50,100,200, 300,400, 500 and 1000 MU (Monitor Units) for field sizes 5 cm × 5 cm, 10 cm × 10 cm, 20 cm × 20 cm, 30 cm × 30 cm and 40 cm × 40 cm. The measured maximum radiation level near LINAC head, isocenter and 0.5 m lateral to isocenter after 1 min of completion of treatment was about 10 μSv/h when the accelerator was operated for ＜ 500 MU. However, the radiation levels at these locations after 1 min of completion of treatment were found to be ＞ 10 μSv/h when the accelerator was operated for 800-1000 MU. It was also observed that the radiation level due to induced activity increases with increasing field size and number of monitor units and shows saturation characteristics when the field size approaches to 30 cm × 30 cm. This study indicates that the radiation dose received by the radiotherapy technologists while standing below the treatment head of the LINAC is significant even after 1 min of completion of the dose delivery when the accelerator is operated for higher number of monitor units (e.g., 1000 MU) in high energy photon mode.