Equivalent square formula for determining the surface dose of rectangular field from 6 MV therapeutic photon beam

Abstract

The purpose of the study was to investigate the use of the equivalent square formula for determining the surface dose from a rectangular photon beam. A 6 MV therapeutic photon beam delivered from a Varian Clinac 23EX medical linear accelerator was modeled using the EGS4nrc Monte Carlo simulation package. It was then used to calculate the dose in the build‐up region from both square and rectangular fields. The field patterns were defined by various settings of the X‐ and Y‐collimator jaw ranging from 5 to 20 cm. Dose measurements were performed using a thermoluminescence dosimeter and a Markus parallel‐plate ionization chamber on the four square fields (5 × 5, 10 × 10, 15 × 15, and 20×20 cm2). The surface dose was acquired by extrapolating the build‐up doses to the surface. An equivalent square for a rectangular field was determined using the area‐to‐perimeter formula, and the surface dose of the equivalent square was estimated using the square‐field data. The surface dose of square field increased linearly from approximately 10% to 28% as the side of the square field increased from 5 to 20 cm. The influence of collimator exchange on the surface dose was found to be not significant. The difference in the percentage surface dose of the rectangular field compared to that of the relevant equivalent square was insignificant and can be clinically neglected. The use of the area‐to‐perimeter formula for an equivalent square field can provide a clinically acceptable surface dose estimation for a rectangular field from a 6 MV therapy photon beam.PACS number: 87.55.ne

Analytical Consideration of Surface Dose and Kerma for Megavoltage Photon Beams in Clinical Radiation Therapy

“…With an increasing energy of these radiations, the penetration power of photons and secondary electrons will be increased and as a result the point of maximum dose (d_max) is placed in more depth (Apipunyasopon et al, 2013).Depending on the amount of energy, dose build up region occurs for x-rays megavoltage in the first depths of the entrance surface of phantom or skin. Dose build up region are a causes the skin sparing effect in megavoltage beams and makes them suitable for delivering the lethal dose to deep tumors as well as reaching the lowest dose in the surface layers of the skin (Butson et al, 1997;Ravikumar and Ravichandran, 2000).…”