Evaluation of Risk of Secondary Thyroid Cancer Caused by Neutron Dose Equivalent from Brain Tumor 3D-Conformal Radiation Therapy (3D-CRT)

Thyroid cancer is one of the most common secondary malignancies as a result of receiving therapeutic doses to the head and neck. In this study, the probability of secondary thyroid cancer risk due to neutron contamination of 15 MV Siemens Linear Accelerator (LINAC) in brain tumor 3D-Conformal Radiation Therapy (3D-CRT) was calculated.

Neutron fluence and neutron dose were measured at different points at the treatment table using an energy-independent neutron detector consisting of a sphere moderator and a Boron Trifluoride (BF3</sub>) counter, and the neutron dose equivalent to the thyroid was determined to calculate the probability of secondary cancer risk.

The neutron dose equivalent was obtained at the central axis (0.304 mSv/Gy) and at 4 cm (0.285 mSv/Gy), 15 cm (0.229 mSv/Gy), 45 cm (0.125 mSv/Gy), and 150 cm (0.02 mSv/Gy) inferior. The neutron dose equivalent reaching the thyroid for the total prescribed dose was 12.366 mSv. According to that, the probability of secondary thyroid cancer risk was obtained as 0.001%.

The thyroid dose in high-energy radiation therapy of brain tumor cannot cause significant biological damage. Therefore, the risk of secondary thyroid cancer due to neutron contamination is relatively low.