Abstract Book

S1240

ESTRO 37

90 patients (8 fractions x 4 Gy: 53 patients; 7 fractions x 4.3 Gy: 37 patients) treated with accelerated partial breast irradiation (APBI) using high dose rate (HDR) interstitial brachytherapy were evaluated. EQD2g was calculated using the values α/β = 4 Gy, mean reparation time T 1/2 = 1.5 h, and time interval between fractions delivered the same day x = 6 hours. Results The highest excluded doses are (median ± standard deviation) 19 ± 2 Gy (range: 13.4 - 26.1 Gy). EQD2g obtained with the current model was (median ± standard deviation): 87 ± 8 Gy (range: 67 - 106 Gy). Conclusion The model presented in this study took into account the “hot spot” dose regions in the CTV in a robust way. Calculated EQD2g reveals large inter-patient dosimetry heterogeneity. However, it exhibits several limitations. As we are considering hot spot regions in brachytherapy (more than four times the PD), the LQ model is used in extreme circumstances. Furthermore, the highest doses excluded from the DVHs are supposed to be inside catheters, while optimisation can make the radioactive source stay much longer in selected catheters. Despite these limitations, the conclusions on the high variability in APBI dosimetry in brachytherapy are still effective. EP-2243 Fast automated multi-criteria planning for HDR brachytherapy explored for prostate cancer S. Breedveld 1 , A. Bennan 1 , S. Al-Uwini 1 , D. Schaart 2 , I.K. Kolkman-Deurloo 1 , B. Heijmen 1 1 Erasmus Medical Center Rotterdam Daniel den Hoed Cancer Center, Radiation Oncology, Rotterdam, The Netherlands 2 Delft University of Technology, Radiation Science & Technology, Delft, The Netherlands Purpose or Objective To develop an automated treatment planning workflow for high dose rate (HDR) brachytherapy, compatible with our clinical treatment planning system (TPS). In this workflow, the patient-CT with catheter reconstructions and dwell positions are imported from the clinical TPS into our in-house developed system for automated multi- criteria dwell time optimisation. The optimised dwell times are then imported in the clinical TPS. The aims of automation are planner-independent, enhanced plan quality and short optimisation times. Material and Methods Our in-house developed system for fully automated, multi-criteria EBRT treatment planning was extended with an option for optimisation of HDR dose distributions. The test cohort consisted of 22 planning CTs with catheter reconstructions and delineations for low- intermediate risk prostate cancer patients who were previously treated with 4 x 9.5 Gy HDR brachytherapy. Data of 5 patients was used to configure the autoplanning system. The fixed configuration was then used to also automatically generate treatment plans for the other 17 patients. Automatically generated plans were compared to the corresponding clinical plans. All evaluations were performed in the clinical TPS. Clinically applied hard constraints were used for automated planning: D 1cc < 80% of the prescribed dose (PD) for rectum and bladder, and D 1% < 120% of PD for the urethra. The objectives in order of priority were to 1) prostate V 100% = 95%, 2) minimise urethra D 1% , 3) improve conformality. For comparison, both the clinical and automated plans were rescaled to match 95% coverage. Results Automated plan optimisation took on average 19.5 seconds (range: 5.5 – 43.2 seconds), including computation of the dose kernels.

The autoplans of 21/22 patients showed 95% tumour coverage within the imposed hard constraints. Coverage of patient 4 was lower than desired (94.8%), though slightly higher than in the clinical plan (94.5%), both due to the urethra constraint. For 21/22 patients the autoplan showed a reduction in the most important OAR objective, urethra D 1% (see figure). The absolute mean and maximum reductions were 1.8 Gy and 4.1 Gy, respectively.

Conclusion Fast, automated multi-criteria treatment planning for prostate HDR brachytherapy is feasible. For 21/22 patients, the autoplan met the coverage condition for the prostate with consistent reduction in near-maximum urethra dose (most important clinical objective). Plan optimisation took less than 20 seconds on average. EP-2244 Accuracy Verification of Dose Calculation Algorithm Based-on TG-43 and AcurosBV on HDR Brachytherapy P. Nakkrasae 1 1 Faculty of Medicine Siriraj Hospital- Bangkok- Thailand, Department of Radiology- Division of Radiation Oncology, Bangkok, Thailand Purpose or Objective To verify dosimetric accuracy of dose calculation algorithm based-on TG-43 and Varian Acuros TM BV on high dose rate (HDR) brachytherapy in in-house heterogeneous medium phantom using TLD-100 and radiochromic EBT3 film. Material and Methods In-house wax phantom was constructed and drilled carefully to hold titanium tandem and ring applicator. To mimic the cervix brachytherapy, various materials which density equivalent to bladder, rectum, spongy bone and cortex bone were inserted. Moreover, inside the phantom was placed a cap for contained TLD to measure absorbed dose. Total of eleven TLD-100 points were selected, seven interested points which deposition in homogeneous region are represent the point dose for front of bladder, rectum, cortex bone as well as spongy bone, 1 cm from tip end of the tandem, right and left point A. Heterogeneous region contained four TLD measuring points as backside of bladder, rectum, cortex bone and spongy bone. After that the phantom was underwent CT scan with pelvic exposure protocol, and generated the pear-shape dose distribution with 10 Gy prescription dose at point A on BrachyVision TM treatment planning system(TPS) version 13.6 using either of TG-43 and Acuros TM BV algorithms. Then, the absorbed doses at all chosen points were measured by the TLDs, and radiochormic film EBT3 model was used to measure dose distribution.