Surface dosimetry for oblique tangential photon beams: a Monte Carlo simulation study

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• Tangential photon beams are used in radiation therapy.
• Beam obliquity can affect dose distribution, particularly at the surface.
• Understanding these effects is crucial for accurate treatment planning.

Purpose of the Study:

• To investigate the impact of beam obliquity on surface relative dose profiles for tangential photon beams.
• To quantify the changes in relative depth dose due to varying gantry angles.
• To evaluate the phantom-air interface effect using a percent depth dose (PDD) ratio.

Main Methods:

• Monte Carlo simulations (EGSnrc code) to create phase-space models of 6 and 15 MV photon beams.
• Verification of simulations using film measurements.
• Calculation of relative dose profiles in half-phantom geometry (HPG) and full phantom geometry (FPG) at varying gantry angles.
• Analysis of the PDD ratio to assess the phantom-air interface effect.

Main Results:

• Beam obliquity increased surface relative depth dose in the HPG, but not significantly in the FPG.
• For 6 MV beams (10x10 cm2), a 1-3 degree angle increase raised depth doses from 68% to 79% at 10 cm.
• The PDD ratio increased significantly with obliquity, showing a greater effect for 6 MV beams and smaller field sizes (4x4 cm2).

Conclusions:

• Beam obliquity significantly alters surface dose profiles in HPG, impacting radiotherapy.
• The phantom-air interface plays a critical role in dose variations with oblique beams.
• Findings are valuable for physicists and dosimetrists in predicting dose variations for clinical tangential beams.