Photon spectrum and absorbed dose in brain tumor

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• Accurate dose calculation is critical in radiation therapy, especially for brain tumors near critical organs like the pituitary gland.
• Understanding photon scatter is essential for predicting dose distribution and minimizing unintended radiation exposure.
• Monte Carlo methods provide a robust framework for simulating radiation transport and dose deposition.

Purpose of the Study:

• To quantify photon fluence and absorbed dose in a brain tumor and the adjacent pituitary gland using Monte Carlo simulations.
• To investigate the impact of Compton scattering on dose distribution when treating a brain tumor with a Varian 6MV linac.
• To assess the extent of unwanted radiation reaching the pituitary gland despite beam collimation.

Main Methods:

• Utilized Monte Carlo methods to simulate photon transport from a Varian 6MV linear accelerator.
• Modeled a BOMAB phantom representing a human head with a brain tumor near the pituitary gland.
• Calculated photon spectrum and absorbed dose within the tumor, pituitary gland, and surrounding head tissues.

Main Results:

• Photon fluence reaching the tumor was 15.7 times greater than that in the pituitary gland.
• Despite lower fluence, the absorbed dose in the tumor was 37.1 times higher than in the pituitary gland.
• Compton scattering within the head and tumor significantly contributed to the absorbed dose in the pituitary gland.

Conclusions:

• Even with precise beam collimation, scattered photons from Varian 6MV linac treatments can deliver a measurable dose to the pituitary gland.
• Compton scattering is a primary mechanism increasing the pituitary gland's absorbed dose, highlighting the need for careful treatment planning.
• Monte Carlo simulations are valuable for predicting and mitigating radiation dose to critical structures near brain tumors.