Session Item

In the new protocol, recently implemented in our clinic, unresectable and partially resectable soft tissue sarcomas (STS) were treated with neoadjuvant chemotherapy with hypofractionated radiotherapy before surgery. Usually, STS is located under the skin which is often included in the PTV. That is why in many cases boluses are used. The purpose of this work was to investigate the possibility of using Gafchromic EBT-3 films (EBT-3) for measuring the skin dose and verification of treatment repeatability. 

Twenty-two patients with STS were treated with IMRT and VMAT plans (Eclipse v.13.6, Varian, Palo Alto). The dose of 25 Gy was delivered in five fractions with 6X or 15X photon beams. Due to shallow tumor location and skin infiltration all patients were irradiated with individual bolus. In-vivo measurements were done with EBT-3 at four points on the skin surface, all located under the bolus: (B: central axis, A, C: shifted in and opposite to gantry direction, respectively and D: shifted right or left from B). For every session, two pieces of 2x2 cm EBT-3 were placed together, one on another, in previously described points., The films were laid on skin before the bolus was placed and CBCT acquisition was performed. Films were scanned using EPSON V750PRO flatbed scanner. The multichannel method was used. The average signal in the 1x1 cm region of interest (ROI) was read. Measured dose values were compared with dose from adequate ROI calculated in TPS . To assess total uncertainty phantom measurements were performed . Ten 2x2cm films (5 pair) were irradiated 5 Gy dose. All films with blank and calibration film were scanned (next day). Map of doses was obtain using homemade software. It was repeated 10 times. From every map of dose, the mean signal from 1x1 cm ROI was obtained. For one dose map, mean signal from 1x1 cm ROI was read ten times.

The mean differences between the two film irradiated simultaneously in phantom measurement was 0,2%±0,9%. During treatment sessions mean differences between two films were: 0.8% ±1.3%. The mean difference between measured and calculated with TPS doses for all patients was 2.6%±6%, 4.8%±4.7%, 4.6% ±7,7%, 5.1±4,3% in A, B, C, D respectively. For single patients, the mean difference between measured and calculated dose in A, B, C, D points was in range from -15.5%±12.3% up to 25,5%±8.3%. To access the repeatability of treatment, the normalized value of the mean dose for every patient was used. Results fiveteen on eighty eight cases exceed 3SD total measurement uncertainty. Results for point located on the central axis (B) are presented in Fig 1.

Repeatability of dose reading was checked for EBT-3 in phantom measurements, confirming usability for in-vivo dosimetry. EBT-3 in vivo measurements allowed to verify patient skin dose and indirectly also treatment repeatability. Influence of difference between irradiated and planned dose in regions that underwent resection in further treatment should be investigated.