Abstract Book

S957

ESTRO 37

Conclusion The proposed pencil beam algorithm can accurately describe dose distributions effects induced by external magnetic fields. The successful integration into a treatment planning system make possible next benchmarking stages using in depth Monte Carlo simulations with complex phantoms as well pave the way for experimental measurements and validation. EP-1782 Pre-treatment Patent Specific QA on the Varian Halcyon Linear Accelerator T. Jarema 1 , T. Aland 2 , A. Walsh 2 , R. Murry 1 1 Radiation Oncology Centres, ROC, Toowoomba, Australia 2 Radiation Oncology Centres, ROC, Brisbane, Australia Purpose or Objective The purpose of this study was to compare the results of several different pre-treatment patient specific QA devices as used on the Varian Halcyon Linear Accelerator – including Varian portal dosimetry, point dose measurement in a homogenous slab phantom, and PTW Octavius4D with PTW 1500 detector. Material and Methods Treatment plans for prostate, prostate and nodes, prostate bed, rectum, and head & neck were planned using varying numbers of arcs ranging from 2-6 for VMAT plans, and also with 7 and 9 field IMRT. Verification plans were then created within the Eclipse treatment planning system on phantoms ready for QA. The plans were delivered to the various phantoms in ‘QA mode’ and adjusted for any change in linac output on the day of delivery. All phantoms were imaged by the Halcyon prior to delivery to ensure correct positioning. For the array devices and portal dosimetry, delivered results were then compared with exported results from the planning system using a 2%/2mm and 3%/3mm gamma evaluation (3D for PTW Octavius4D and 2D for Varian portal dosimetry) with a 10% low dose threshold referenced to the global dose maximum. For the point dose measurements (using a PTW 3D pin point chamber), delivered results were compared using local dose difference expressed as a percentage. Total plan dose was assessed, rather than individual fields doses. Results Point dose measurements were in agreement with the treatment planning system to within 3%. Varian Portal dosimetry results were all greater than 90% and 97% for 2%/2mm and 3%/3mm respectively. PTW Octavius4D results were greater than 74% and 92% for 2%/2mm and 3%/3mm with a trend of decreasing gamma result as the number of VMAT arcs increased and also when the gantry speed exceeded the limits of the devices rotation speed. SNC ArcCheck is currently under investigation. Conclusion Point dose measurements, Varian portal dosimetry, and PTW Octavius4D were all used successfully to conduct pre-treatment patient specific QA on a variety of plans delivered using a Varian Halcyon linear accelerator. EP-1783 TPS validation for the Varian Halcyon Linear Accelerator T. Jarema 1 , A. Walsh 2 , T. Aland 2 , R. Murry 1 1 Radiation Oncology Centres, ROC, Toowoomba, Australia 2 Radiation Oncology Centres, ROC, Brisbane, Australia Purpose or Objective The purpose of this study was to validate the preconfigured Varian Eclipse AAA data for the Varian Halcyon Linear Accelerator, as the pre-configured data cannot be edited or manipulated in any way. PDDs, profiles and output factors were used for the initial validation work.

Material and Methods PDDs, profiles and output factors were measured in a PTW BEAMSCAN water tank, using a variety of chambers and were completed from a 28 cm x 28 cm field size, the maximum on the Halcyon system, down to an off axis 1 cm x 1 cm field. Output factors were also measured off axis to ensure the accuracy of the Eclipse calculations. All measurements were compared to Varian’s published data, along with the data generated by Eclipse, which was calculated on a homogenous water phantom. Results All measured PDDs agreed with Eclipse values to within 1.2%, output factors within 2%. There was a trend in Eclipse of underestimating the dose in the build-up region, with agreement improving with depth. Eclipse was also found to overestimate small field output factors compared to those measured, a trend which disappeared with increasing field size. Conclusion Point dose measurements, Varian portal dosimetry, and PTW Octavius4D were all used successfully to conduct pre-treatment patient specific QA on a variety of plans delivered using a Varian Halcyon linear accelerator. EP-1784 Real-time dosimetry for IORT procedures P. Stevens 1 , L. Verbraeken 2 , A. Plesu 2 , E. D'Agostino 2 , D. Verellen 1 , R. Weytjens 1 1 GZA- Ziekenhuizen - St. Augustinus, Radiation Physics, Wilrijk, Belgium 2 DoseVue, Radiation Physics, Turnhout, Belgium Purpose or Objective Intraoperative Radiation Therapy (IORT) is a boosting technique where the tumor site is irradiated during surgery. IORT is demonstrated to be an effective boost strategy in breast cancer with excellent local control rates and low healthy tissue toxicity. It also improves the quality of life of the patient limiting the time spent for radiation treatments and the treatment costs. The main limitation of IORT lies in its uncertainties. A large source of uncertainty is the deposited dose, with possible consequences on the outcome of the treatment. In-vivo dosimetry offers the possibility to solve the issue in dose uncertainty by measuring the dose during or immediately after the treatment. Unfortunately, the currently used dosimeters are not designed for IORT, where aspects as minimal invasiveness, capacity to cope with large instantaneous dose rates and real-time capability are fundamental. We are testing a device based on optical fibers that will serve to monitor the amount of radiation dose deposited at given positions in the patient, in real-time. Optical fiber dosimetry is in general hampered by the so-called stem effect: when an optical fiber is exposed to radiation, a light signal is produced along its length. This spurious signal overlaps with the signal generated in the sensitive part of the fiber (dosimetric sensor) and needs to be effectively suppressed, to obtain reliable dosimetric results. Material and Methods A rare-earth doped material was used to build a sensor on top of a PMMA optical fiber. The fiber has an external diameter of 1.3mm and the sensor has a length below 1mm. Preliminary clinical measurements were obtained in a water phantom, using a 6MeV Mobetron electrons accelerator, with a dose rate of 1000MU/min. The dose measuring device was optimized in a way that the stem signal originating from the fiber itself was below 2% of the dosimetric signal. Results Our optical fiber based system, tested under clinical conditions, showed a satisfactory sensitivity, as well as a robust suppression of the stem effect.