Abstract Book

S955

ESTRO 37

Material and Methods Commercial CT phantom, Catphan®504, was CT scanned (pCT). In module CTP404 the materials of interest were outlined in the Eclipse treatment planning system v.11.0.13 (Varian Medical Systems) using CT ranger with the nominal HU-range and a cylindrical central volume (PTV) was drawn with a diameter of 5.25 cm. Two plans were created with eight 6 MV fields, each field aligned with one material. First plan was calculated with AAA and the other with AcurosXB algorithm. For five Varian OBI CBCT default protocols (standard-dose head, high-quality head, low-dose head, pelvis and low-dose thorax) five CBCT were acquired. Each CBCT was rigid-registered to pCT and structures were copied into it creating a set of 25 planning CBCTs (pCBCT). Plans were copied into pCBCT and calculated with AAA algorithm (pCBCTAAA) and AcurosXB (pCBCTAcu) maintaining the number of monitor units fixed and using the CT HU to electronic density table, generating two sets of 25 plans. As dose differences between AAA and AcurosXB are well known, and AAA calculates dose to water while AcurosXB dose to medium, every pCBCT was compared only with its correspondent pCT. Differences in the average dose for the structures outlined between pCBCT and pCT were analyzed. Results Graph1 and Graph 2 show dose differences between pCT and pCBCTAAA and pCBCTAcu respectively for all the structures studied. Maximum average difference observed was in PTV (-1.3% ± 0.1%) in a pCBCTAcu acquired with the thorax protocol. The maximum absolute difference of 2% was found in Acrylic and Air cavity in a pCBCTAcu also acquired with the thorax protocol. No statistically significant differences were found between average dose differences observed in pCBCTAAA and pCBCTAcu. Taking into account all the structures studied, pelvis protocol slightly increased average dose both for AAA (0.2 %, p=0.005) and AcurosXB (0.2%, p=0.01). On the contrary, in low-dose thorax, high-quality head and low- dose head average dose tended to decrease (-0.5%, p<0.001). Average dose differences were not statistically significant in standard dose head.

Conclusion Although Thorax-Low Dose presented the higher dose average variation, was lower than 1.3%. AAA and AcurosXB reproduce similar results while using a CBCT for dose calculation. These results suggest that CBCT-based dose calculation can be made with a reasonable difference in calculated dose. EP-1780 Volumetric EPID transit dosimetry for abdominal and pelvic stereotactic treatments M. Esposito 1 , A. Ghirelli 2 , S. Pini 2 , S. Russo 2 , G. Zatelli 2 , P. Alpi 3 , R. Barca 3 , M. Coppola 3 , S. Fondelli 3 , B. Grilli Leonulli 3 , L. Paoletti 3 , F. Rossi 3 , P. Bastiani 3 1 Esposito Marco, S.C. Fisica Sanitaria, Firenze, Italy 2 Azienda Sanitaria USL centro, S.C. Fisica Sanitaria, Firenze, Italy 3 Azienda Sanitaria USL centro, S. C. Radioterapia, Firenze, Italy Purpose or Objective Stereotactic body radiation therapy (SBRT) is one of the most complex treatment in common clinical use. This complexity, combined with the high dose prescribed, requires high level of dosimetric accuracy. EPID in vivo dosimetry measures the accuracy of the actual delivered treatment and is sensitive to detecting treatments errors that would be missed by pre-treatment QA. In this work we reported findings from routinely measured in vivo doses of pelvic and abdominal SBRT treatments. Different metrics for evaluation of dosimetric accuracy have been compared: punctual dose difference, gamma metric and DVH differences. Material and Methods 83 fractions of 50 stereotactic VMAT treatments with target in the abdomen or pelvis were analyzed (9 liver, 10 adrenal gland, 4 spine, 27 lymph nodes). Daily cone