Accurate skin dose measurements using radiochromic film in clinical applications

Abstract

Megavoltage x‐ray beams exhibit the well‐known phenomena of dose buildup within the first few millimeters of the incident phantom surface, or the skin. Results of the surface dose measurements, however, depend vastly on the measurement technique employed. Our goal in this study was to determine a correction procedure in order to obtain an accurate skin dose estimate at the clinically relevant depth based on radiochromic film measurements. To illustrate this correction, we have used as a reference point a depth of . We used the new dosimetry films (HS, XR‐T, and EBT) that have effective points of measurement at depths slightly larger than . In addition to films, we also used an Attix parallel‐plate chamber and a home‐built extrapolation chamber to cover tissue‐equivalent depths in the range from to of water‐equivalent depth. Our measurements suggest that within the first millimeter of the skin region, the PDD for a photon beam and field size of increases from 14% to 43%. For the three dosimetry film models, the beam entrance skin dose measurement corrections due to their effective point of measurement are as follows: 15% for the EBT, 15% for the HS, and 16% for the XR‐T model films. The correction factors for the exit skin dose due to the build‐down region are negligible. There is a small field size dependence for the entrance skin dose correction factor when using the EBT film model. Finally, a procedure that uses EBT model film for an accurate measurement of the skin dose in a parallel‐opposed pair photon beam arrangement is described.