ESTRO 37 Abstract book

S1033

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

It can be seen clearly that when calculating with the modulated density – which correlates to the act ual dose distribution in the patient – the PTV gets less dose compared to the case with the original density from the treatment-planning CT which correlates to the dose distribution predicted by the treatment-planning system. The dose deposited in the trachea is underestimated by the treatment-planning System. Conclusion We are able to analyse the effects of Bragg curve degradation due to human lung parenchyma in the treatment planning of lung cancer patients. Doing so we could show that the Bragg peak degradation must be considered in the treatment planning of lung cancer patients to avoid an underdose of the PTV and an overdose of distal normal tissue. [1] Baumann K-S, Witt M, Weber U, Engenhart-Cabillic R and Zink K 2017 An efficient method to predict and include Bragg curve degradation due to lung-equivalent materials in Monte Carlo codes by applying a density modulation Phys. Med. Biol. 62 3997-4016 [2] Perl J, Shin J, Schuemann J, Faddegon B and Paganetti H 2012 TOPAS: an innovative proton Monte Carlo platform for research and clinical applications Med. Phys. 39 6818–37 EP-1903 Is it always possible to boost breast tumour bed with conformal photon beams? M. Colomer 1 , D. Amat 1 , A. López 1 , G. Frontera 1 , D. Navarro 1 , R. Gómez 1 , E. Ambroa 1 , J. García-Miguel 1 , M. Parceisa 1 , R. Pujol 1 , T. Ramírez 1 1 Consorci Sanitari de Terrassa, Medical Physics Unit - Radiation Oncology Department, Terrassa, Spain Purpose or Objective In a very busy department with three linacs, one in a satellite location, commissioning and measuring all available electron beams is very time consuming. More than 70% of electron beams are used for breast boost radiotherapy and we would like to compare, in terms of PTV coverage and low doses to heart and ipsilateral lung, if we can give priority to photon plans in order to not measure all electron beams and gradually reduce them. Material and Methods We retrospectively selected 293 patients who underwent radiotherapy to the whole breast at a dose of 50 Gy in 25 sessions plus a tumour bed boost of 16 Gy in 8 sessions with either photons or electrons from 2013 to 2017. Patients treated with other fractionations were not considered. Only dosimetric boost data was taken into account. 192 patients were treated with electrons (110 left-sided breast and 82 right) and 101 with photons (56 left and 45 right).