Abstract Book

S1032

ESTRO 37

Electron plans were generated with a single direct beam of energies 6, 9, 12, 15 or 18 MeV (4%, 18%, 38%, 33% and 7%). Photon plans were generated with three different techniques of 6 or 15 MV (82% and 18%) beams: 26% with two perpendicular beams (AP-LAT), 51% with two oblique beams (2-O), and 23% with 3 oblique beams (3-O). Plans were evaluated by means of conformity (CI) and homogeneity (HI) indeces for PTV. CI was calculated as the volume within 95% isodose line/volume of PTV. HI was calculated as D5%/D95%. Where D5% is the dose covered by 5% of PTV and D95% is the dose covered by 95% of PTV. For organs at risk (OARs) we analysed maximum dose (Dmax), mean dose (Dmean), V5(%) and V2(%) to heart and ipsilateral lung. We compared electron versus photon plans and 3 different photon plans among them. Results

Dosimetric parameters are summarized in Table 1 for 2 types of beam plans and in Table 2 for 3 types of photon plans. PTV coverage is best achieved with photon plans. Concerning CI, mean values are in average 0.95 per photons and 0.80 per electrons. For HI indeces mean values are about 1.07 and 1.14 respectively. All dosimetric values for OARs are lower with electrons but all techniques enable low and acceptable values. Major differences can be seen in very low doses. Mean values for V2 in ipsilateral lung are about 2 times higher for photons in left-sided breast, only 2 oblique photon plans give similar values. Mean values for V2 in heart can be as high as 30 times greater with photons in right-sided breast for two perpendicular beams. When comparing 3 different photon techniques the best coverage is reached with 3 oblique beams and the best OARs sparing with 2 oblique beams. Conclusion This work aimed to determine whether photon beams are always suitable for breast boost technique in order to diminish the number of electron beams in clinical use. Regarding our results, it is possible to use photon beams for all boosts. However, it is strongly advisable to have one linac per site with calibrated electron beams for patients with high OARs values especially in the first part of breast treatment. EP-1904 A comparison of contralateral breast dose from different breast radiotherapy techniques K.H. Tse 1 , C.Y.H. Cheng 2 , M.K.A. Chow 3 1 Pamela Youde Nethersole Eastern Hospital, Clinical Oncology, Hong Kong, Hong Kong SAR China 2 St. Teresa's Hospital, Oncology Centre, Hong Kong, Hong Kong SAR China 3 Queen Mary Hospital, Clinical Oncology, Hong Kong, Hong Kong SAR China