149. Skin dose in radiotherapy: Results of in vivo measurements with Gafchromic EBT3 films

Purpose An accurate estimation of surface dose is becoming important in radiotherapy to avoid complications to skin [1] . However, the Treatment Planning Systems (TPS) do not accurately model skin dose. The aim of this study was to assess surface dose in vivo using EBT3 Gafchromic films. Methods The measurements were performed applying films (2 × 2 cm 2 ) directly on patient skin or inside the thermoplastic mask. Targets included head and neck (HN breasts at Linac and skin tumors in Plesio-Rontgen therapy. For each patient measurements were repeated for three fractions locating films in 1–3 reproducible points and compared with TPS-calculated doses. Films were read with a flatbed scanner and analyzed using red channel calibration. In vivo dose evaluations were compared with measurements performed on Cheese phantom both with and without thermoplastic mask. Results A total of 117 film measurements were performed on 21 patients. The absolute value of the mean difference between measured and TPS-calculated doses was (11.3 ± 6.5) % (17.7% for Linac plans, 11.6% in Tomotherapy and 4.6% in Plesio-Rontgen therapy). At Linac and in Plesio-Rontgen there was not a clear trend of overestimation of the TPS compared to measurements. Instead in Tomotherapy the TPS underestimated measurements (−9.1 %) for H&N and brain treatments (with thermoplastic mask) and overestimated for sarcoma (9.2%). This trend was confirmed by assessments on Cheese phantom in Tomotherapy, where TPS overestimated without mask (28.6% vs −0.7% with mask). An improvement of the agreement between measurements and estimations in Pinnacle and Tomotherapy was shown with mask (28.6% to −0.7% in Tomotherapy and −20.7% to −16.3% at Linac). Conclusions EBT3 films are suitable for skin measurements in radiotherapy and represent a useful detector for quality assurance, since TPS does not give accurate dose values in the first millimeters of skin.