Abstract book - ESTRO meets Asia

S31 ESTRO meets Asia 2018

occurred. Partial response and a stable disease to the second irradiation was found in 4 and 33 patients. Conclusion The cumulative doses to OAR as dose constraints were acceptable and safe, which could be used as a reference in the decision for re-irradiation with SBRT after prior SBRT for pancreatic cancer. OC-079 Comparing manual and automatic planning based on optimal parameters in Rapidplan for ESO cancer C. Ling 1 , P. zhai 1 1 Fudan university shanghai cancer center, radiation oncology, Shanghai, China Purpose or Objective Choosing a fast and better way to optimise treatment planning for oesophageal cancers based on planning target volume (PTV) can reduce planning time for the radiation physicist and provide experience for the development of automatic planning. Material and Methods In this paper, the optimal parameters given by RapidPlan were used to compare the discrepancy between manual and automatic planning, and to test whether the prediction value is better than the optimal parameters set by the physicist. We established a data model based on Rapidplan for 70 oesophageal cancer patients to predict the optimal parameters for another 20 oesophageal cancer patients. The 20 patients were prescribed the same dose with target volumes outlined by the same doctor, and the same field angles were put on. The same optimal parameters for organs at risk (OARs) were entered for manual and automatic Pinnacle plans, and the doses to OARs were compared. Results Conformity index (CI) and homogeneity index (HI) of auto planning were found to be better, especially CI, which is 10% higher than that of manual planning. The dose to heart in auto planning had no obvious difference from manual planning. This may be related to the location of the PTV and the field. The values for the percentage of the lung that received 20Gy (V20), 30Gy (V30), mean dose from auto planning are lower than that of manual planning, and the maximum dose of spinal cord from auto planning is also significantly lower than that of manual planning. Conclusion From the result, we found that the auto plan is better than the manual plan for OAR sparing, except for V5 of the lung OC-080 Monte Carlo calculation of kQ factors in modulated and unmodulated proton beams C. Gomà 1 1 KU Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective To calculate the beam quality correction factor (kQ) of the NE 2571 Farmer chamber in modulated and unmodulated proton beams using Monte Carlo simulation. Second, to investigate the effect of positioning the effective point of measurement of cylindrical chambers at the reference depth (Zref) on the reference dosimetry of unmodulated proton beams. Material and Methods The kQ factor of the NE 2571 Farmer chamber was calculated using the Monte Carlo code PENH. The new ICRU 90 recommendations on key data for radiation dosimetry (Iw = 78 eV, Ig = 81 eV and Wair = 34.44 eV for proton beams) were adopted. Two different proton fields were simulated: (i) a 10x10x10 cm3 spread-out Bragg peak (SOBP) with a range of 15.5 cm and (ii) a 10x10 cm2 mono- energetic energy-layer with the same energy (146 MeV) as the most distal Bragg peak of the SOBP. For the modulated

(i.e. SOBP) field, IAEA TRS-398 reference conditions were simulated. For the unmodulated (i.e. mono-energetic) field, two different scenarios were studied: positioning the geometrical center of the chamber (Pref) at Zref and positioning the effective point of measurement (Peff) of the chamber at Zref, with Peff = 0.75·r (where r is the radius of the air cavity). Finally, the kQ factors were calculated at three different depths: 6.5, 10 and 13.5 cm; which correspond to a residual range (Rres) of approximately 9, 5.5 and 2 cm, respectively. Results The figure shows the kQ factor of the NE 2571, as a function of the residual range, in a modulated proton beam (SOBP) and an unmodulated proton beam (mono- energetic) with both Pref at Zref and Peff at Zref. The kQ values calculated theoretically in IAEA TRS-398 using ICRU 49 key data (Iw = 75 eV, Ig = 78 eV and Wair = 34.23 eV for proton beams) are also shown. Uncertainty bars (and dashed lines) correspond to one standard uncertainty.

For modulated beams, the kQ factors agree with IAEA TRS- 398 values within 1%. For unmodulated beams, the kQ factors agree with those in modulated beams (within 1%) when Peff is positioned at Zref. When the geometrical center of the chamber is positioned at Zref, the kQ factors increase rapidly with decreasing residual range, due to dose gradient effects. Conclusion The results of this work seem to indicate that the adoption of ICRU 90 recommendations has little impact on the kQ factors in modulated proton beams. For unmodulated beams, it appears that positioning the effective point of measurement at the reference depth results in kQ factors which are consistent with those in modulated proton beams. OC-081 A multi-modality end-to-end audit for measuring conformal and IMRT/VMAT radiotherapy J. Lye 1 , A. Alves 1 , C. Davey 1 , F. Kadeer 1 , J. Kenny 1 , M. Shaw 1 , J. Supple 1 1 Australian Radiation Protection and Nuclear Safety Agen cy, Australian Clinical Dosimetry Service, Melbourne- Victoria, Australia Purpose or Objective The ACDS has commissioned a custom phantom and audit incorporating conformal, IMRT and VMAT modalities. In a single visit to a hospital, the ACDS measures a range of cases beginning with reference and large field conformal deliveries to the more complex IMRT and VMAT deliveries. Material and Methods The Level III audit is an end-to-end test using a humanoid thorax phantom (CIRS, Norfolk, VA). The custom phantom has a central insert for either conformal modality with two farmer chambers, or for IMRT and VMAT with seven CC13 ion chambers as the primary detectors. The IMRT/VMAT