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 70 micron. We used the new GAFCHROMIC dosimetry films (HS, XR-T, and EBT) that have effective points of measurement at depths slightly larger than 70 micron. 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 4 micron to 1 mm of water-equivalent depth. Our measurements suggest that within the first millimeter of the skin region, the PDD for a 6 MV photon beam and field size of 10 x 10 cm2 increases from 14% to 43%. For the three GAFCHROMIC dosimetry film models, the 6 MV 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 GAFCHROMIC 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 GAFCHROMIC film model. Finally, a procedure that uses EBT model GAFCHROMIC film for an accurate measurement of the skin dose in a parallel-opposed pair 6 MV photon beam arrangement is described.

Experimental evaluation of the accuracy of skin dose calculation for a commercial treatment planning system

“…That depth corresponds approximately to the interface between the epidermis and dermis layers of the skin ( 8 , 16 ) (0.05 – 1.5 mm, depending on the anatomic location). That depth is very difficult to measure or to calculate using most TPSs.…”

Beam perturbation characteristics of a 2D transmission silicon diode array, Magic Plate

“…However, for the real‐time QA system, the MP is placed in the accessory slot on the head of the linac and must be operated as a transmission detector during the patient treatment with minimal perturbation of the incident radiation field, requiring a detailed study of its influence on the treatment beam. The surface dose in particular can be one of the limiting factors in treatment plans with high doses to a target and is the focus of this work (38) . Similar studies in the past have been completed for beam modifiers such as wedges, MLC, and block trays 39 , 40 , 41 , 42 .…”

A simple technique to improve calculated skin dose accuracy in a commercial treatment planning system

“…The EGSnrc MC code used in this study has been validated before on entrance dose calculations 2, 29. A further experimental validation was performed in this study where the MC calculated entrance dose was compared to that of phantom measurements using the nanoDot™ detector (Landauer Inc., Glenwood, IL, USA), the optically stimulated luminescence (OSL) dosimeters, which were pre‐screened and have a measurement uncertainty of 3–5% in this study.…”

“…Skin dose and its resultant toxicity, radiation dermatitis, has long been a concern of the radiation oncologist and is often a dose limiting toxicity of high‐dose treatments, particularly in head‐and‐neck and thoracic cancer patients 1, 2, 3, 4, 5. Improvements in radiation therapy treatment technique and immobilization devices reduce patient setup uncertainty but have exacerbated this clinical dilemma.…”