LAUSR

Abstract Purpose The Treatment Planning Systems (TPS) do not accurately model skin dose, and several studies in vivo have demonstrated that the TPS overestimates skin dose in tomotherapy treatments [1] . The aim of this study was to report results of in vivo measurements of skin dose in tomotherapy using EBT3 Gafchromic films with and without thermoplastic mask. Methods Surface dose measurements were performed using EBT3 pieces (2 × 2 cm2) that were read with a flatbed scanner and analyzed using red channel calibration. Preliminarily, reproducibility of surface dose measurements and the bolus effect were assessed on “cheese phantom” (Fig. 1). A total of 72 measuements were performed in vivo on 8 patients. Skin doses were measured applying films directly on the skin or in the inner side of thermoplastic mask (Fig. 2). The target lesions included H&N, brain and sarcoma. For each patient measurements were performed for three fractions locating films in 1 to 3 reproducible points, then compared with calculated doses (Fig. 3). Results The reproducibility of film measurements was on average 2%. Measurements performed on phantom without mask overestimated TPS values of 28.6%. In presence of mask there was an improvement of the agreement between measurements and TPS-calculated doses (0.7%). The mean absolute value of the difference between measured and TPS dose was 11.6% ± 2.8% for all treatments. The average differences were 9.1% for brain and H&N (measurements made with mask), and −9.2% for sarcoma ( Table 1 ). Conclusions In vivo surface dose measurements with EBT3 are a useful tool for quality assurance in tomotherapy, since the TPS does not give accurate dose values in the first millimiters of skin. Measurements performed both on phantom and in vivo have shown a bolus effect due to the thermoplastic mask, that compensates for the overestimation of the skin dose calculated by the TPS.