Comprehensive analysis of electron beam central axis dose for a radiotherapy linear accelerator

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Dosimetry

Background:

• The American Association of Medical Physics (AAMP) Task Group 25 (TG25) provides a methodology for central axis depth dose determination.
• Recent modifications to a radiotherapy linear accelerator's dual scattering foil system and applicators necessitate re-evaluation of established dosimetry protocols.

Purpose of the Study:

• To evaluate the applicability of the AAMP TG25 methodology for central axis depth dose determination on a modified radiotherapy linear accelerator.
• To assess the accuracy of different methods for calculating percent depth dose (%DD) and output factors for square and rectangular fields at various source-to-surface distances (SSDs).

Main Methods:

• Measurements of %DD and dose output factors were performed for square and rectangular fields at 100-cm and 110-cm SSDs.
• The square-root method was compared against the TG25-recommended equivalent-square method for calculating rectangular field %DD and output factors.
• The inverse square law was used to calculate %DD at 110-cm SSD from 100-cm SSD data for comparison with measured values.

Main Results:

• At 100-cm SSD, %DD varied with applicator, with a maximum spread of +/- 2.5% or +/- 3 mm for the 20-MeV, 10x10-cm field.
• The square-root method provided more accurate %DD calculations for rectangular fields (within 1%) compared to the equivalent-square method.
• At 110-cm SSD, calculated %DD using an inverse square factor showed discrepancies (up to 5.5% or 10 mm) with measured values for certain fields.
• The square-root method proved suitable for determining air-gap correction factors for rectangular fields at 110-cm SSD.

Conclusions:

• The AAMP TG25 recommendations are largely suitable for central axis electron beam dosimetry on the modified linear accelerator.
• Clinically significant discrepancies can arise in limited cases due to applicator-scattered electrons, particularly affecting %DD measurements.
• The square-root method offers improved accuracy for calculating rectangular field parameters compared to the TG25 equivalent-square method.