Abstract Book

S1071

ESTRO 37

Changes in mean, minimum and maximum dose (D mean , D min , D max ) are listed in Table 1. For the IMRT plans, changes in mean prescribed dose for the PTV were clinically irrelevant for both PTV and boost volumes, with a maximum observed change of 0.25%. Changes in minimum and maximum dose and dose to the organs at risk could reach up to 3%, which is still acceptable in virtually all clinical settings. Treated STX volumes ranged from 4.95 to 183.6 cm 3 . While the mean change of D 99% was 2.1 to 4.6% depending on the linac, this increased drastically for smaller volumes, resulting in a change of up to 11.1% for the smallest volume. The change in D mean was less pronounced, with a mean deviation of 0.34 to 0.68% from the originally planned dose.

Conclusion During the process of hippocampus sparing, HAR_D mean decreased linearly as well. However, dosimetry variation of PTV presented three different regions: plateau region (> 20Gy), gradient region (~20 to ~10Gy) and falling region (<10Gy). There’s almost no significant impact on the PTV dosimetry factors when hippo_D mean was not lower than 15Gy. With the hippocampus dose decreasing, CI decreased linearly and slightly, however, HI was impaired severely in the falling region. Plan delivery duration time was significantly prolonged during the hippocampus sparing process (from 13min to 23 min). EP-1969 The impact of isocenter movement on treatment plan delivery – a planning study L.J.M. Wack 1 , O.A. Sauer 1 1 Klinik und Poliklinik für Strahlentherapie- Universitätsklinikum Würzburg, Medizinische Physik, Würzburg, Germany Purpose or Objective Switching patients between linacs is a common practice in many hospitals during machine maintenance and repair. Commonly, if the linac models are identical, the same treatment plan is used without re-planning. Nonetheless, machines, even of the same type, have slightly different mechanical uncertainties which are not accounted for in the planning system. Here, we investigate the possible impact of isocenter shifts on RT delivery. Material and Methods Isocenter shifts were determined for three Elekta Synergy linacs using the ISO-CBCT module as distributed by QualiFormeD (La Roche sur Yon, France). Shifts were determined for 6 gantry angles, as well as the distance between treatment beam (MV) and cone-beam CT (CBCT) isocenters. Five head-and-neck intensity-moduled radiotherapy (IMRT) plans with two prescribed dose levels and five lung stereotactic (STX) treatment plans were calculated in Pinnacle 9.10 (Philips, The Netherlands), using the CC convolution algorithm on a 1mm dose grid. For each machine, a copy of the plan was created and a new isocenter was assigned to each beam to account for the off-set between CBCT and MV and the isocenter movements during gantry rotation. If the beam angle was not measured in QualiFormeD, the isocenter coordinates were linearly interpolated. Resulting dose volume histograms were compared to determine dose changes in planned target volume (PTV) and boost volume. For the head-and-neck IMRT cases, doses to three organs at risk (OAR; spinal canal, parotis, and larynx) were also determined. Results Distances between CBCT and MV isocenters ranged from 0.83 to 1.52 mm. Under gantry rotation, the movement of isocenter position along the sagittal and horizontal axes ranged from 0 to 0.25mm. In longitudinal direction, the isocenter position changed by up to 1. 3mm.

Conclusion For large volumes, isocenter movements have little impact on RT treatment delivery, as observed changes in PTV coverage and OAR dose are too smalll to give rise to concern. However, for small-volume PTV in STX settings, isocenter shifts may lead to inadequate PTV coverage. EP-1970 Hybrid-IMRT as an optimal class solution for whole breast planning with boost to lumpectomy area S. Cilla 1 , F. Deodato 2 , A. Ianiro 1 , M. Boccardi 2 , P. Viola 1 , M. Craus 1 , M. Romanella 2 , I. Djan 3 , M. Buwenge 4 , R. Frakulli 4 , G. Siepe 4 , A. Arcelli 4 , S. Cammelli 4 , D. Smaniotto 5 , G. Frezza 6 , V. Valentini 5 , A. Morganti 4 , G. Macchia 2 1 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy 2 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Radiation Oncology Unit, Campobasso, Italy 3 Medical Faculty- University of Novi Sad, Institute of Oncology Vojvodina, Novi Dad, Serbia 4 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S.Orsola- Malpighi Hospital, Radiation Oncology Department, Bologna, Italy 5 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Radiation Oncology Department, Roma, Italy 6 Bellaria Hospital, Radiation Oncology Unit, Bologna, Italy Purpose or Objective The aim of this study was to dosimetrically evaluate a robust semiautomatic treatment planning approach for whole breast irradiation with simultaneous integrated boost (SIB) using an hybrid IMRT (HMRT) class solution. Material and Methods Twenty-five consecutive patients with left breast cancer were included in the study. All patients received 50Gy (2 Gy/fraction) to the whole breast (PTVbr) and an additional simultaneous 10 Gy (2.4 Gy/fraction) to the tumour cavity (PTVcav) over 25 fractions. Ipsilateral lung, heart and contralateral breast were contoured as organs- at-risk. Healthy tissue was defined as whole body minus PTVbr. Only left sided patients were included in order to