ESTRO 38 Abstract book

S569 ESTRO 38

PTV vertebra and spinal cord, these measurements were compared with expected values from the TPS. Point doses were measured using alanine and the microdiamond detector. Detector output corrections were used for each centre based on measurements at standard output conditions. Centres also recalculated plans using all commissioned planning algorithms available on their TPS.

Portugal ; 3 A.O."G.Brotzu", Radiation Oncology, Cagliari, Italy Purpose or Objective Deep inspiration breath hold (DIBH) for left breast cancer radiotherapy has shown beneficial dosimetric results on cardiac dose. Recently, DIBH was clinically implemented in our department for these cancer patients without lymph node (LN) involvement. This study reports on: a) validation of the optical surface tracking system used; b) aspects of treatment delivery with IMRT in DIBH. Material and Methods We evaluated the static and dynamic performances of optical surface monitoring system (OSMS, Varian), which monitors patient surface, using a thorax phantom that can move periodically. To test system stability the phantom was continuously monitored during 90 minutes. Ten OSMS readings of the phantom position were done to test reproducibility. To evaluate the static accuracy, translational shifts from 1 to 20mm were manually applied to the couch and compared to OSMS shift readings. Dynamic tracking was analyzed, by comparing phantom surface markers position with pre-defined thresholds. Coincidence between linac beam hold signals, measured with an oscilloscope, marker positions and OSMS hold status, was analyzed. Nineteen left breast patients were planned with IMRT (15 fractions, 2.7Gy to the whole breast with an integrated boost of 3.2Gy). CT scans were performed in free breathing (FB) and breath hold (BH) for each patient (fig1a) and contours were delineated in both by the radiation oncologist (RO). Similar plans were performed in Eclipse (Varian), with the same dose tolerances (fig1b). Doses to breast PTV, PTV and CTV boost, heart, left descending artery (LAD), right breast, right and left lung were analysed. The final delivery plan was chosen by RO. Before delivery a CBCT in FB was first acquired to verify patient position and a partial CBCT to evaluate BH. A beam hold threshold of 3mm was used for the OSMS (fig1c), given our CTV-PTV margin of 3mm. After delivery, a CBCT in BH was acquired.

Results The output of the microdiamond detector had a mean deviation from the PTW 0.125cc chamber of 0.13% (S.D. 0.48%, P = 0.301) at reference conditions for the 16 centres. The mean difference between outputs measured at the start and end of each audit visit was 0.07% (S.D. 0.32%, P = 0.929). P values were calculated using a Wilcoxon sign rank test. Results of the phantom measurement are given in Table 1. No significant difference was noted between the different detectors using a Wilcoxon sign rank test (P=0.926). All linac treatments utilised coplanar VMAT delivery, while Cyberknife treatments utilised non-coplanar delivery.

Conclusion The results of the output measurements have shown that the microdiamond detector is consistent with the PTW 0.125cc chamber when measuring absolute dose. No drift was seen over the duration of any individual audits or the entire audit programme for the microdiamond detector. When measuring point doses during treatment delivery, the microdiamond detector was able to accurately measure dose independent of treatment equipment for both coplanar and non-coplanar delivery. The results were comparable with measurements with alanine, which has been widely used in national audit programmes. The results of this audit show that the microdiamond detector is suitable for future use in complex dosimetry audit, although further work is required to assess the long term stability of the detector and frequency of recalibration required. PO-1027 IMRT in breath hold for left breast cancer patients: surface tracking and clinical implementation M. Cardoso 1,2 , S. Vieira 1 , A. Martins 1 , A. Soares 1,2 , M. Possanzini 1,3 , J. Morales 1 , C. Greco 1 1 Champalimaud Foundation, Radiotherapy, Lisboa, Portugal ; 2 Mercurius Health, Radiotherapy, Lisboa,

Results Concerning stability, a drift of 0.09mm was noted. The static reproducibility was found to be 0.04±0.01mm and