ESTRO 36 Abstract Book

S189 ESTRO 36 2017 _______________________________________________________________________________________________

OC-0357 Treatment planning dosimetry accuracy in 192Ir HDR brachytherapy of lip carcinoma P. Papagiannis 1 , V. Peppa 1 , T. Major 2 1 National and Kapodistrian University of Athens, Medical

Physics Lab.- Medical School, Athens, Greece 2 National Institute of Oncology, Department of Radiotherapy, Budapest, Hungary

Purpose or Objective Advanced dose calculation algorithms h ave become clinically available for 192 Ir HDR brachytherapy y to account for the effects disregarded by TG43 based dosimetry algorithms (heterogeneities, applicators and patient specific scatter conditions). The aim of this work is to study the effect of improved dosimetric accuracy in HDR brachytherapy of squamous carcinoma of the lip. Material and Methods Three anonimized patient cases were studied (treatments using the 192 Ir microSelectron-HDR v2 source, 27 Gy planning aim delivered in 3 Gy fractions b.d.). The plans were imported to OncentraBrachy v4.5 and dosimetry was repeated using both the TG43 and the Advanced Collapsed Cone Engine (ACE) TPS algorithms. The same TRAK was used with both algorithms for the same patient case. ACE calculations were performed using the high accuracy option taking into account individual voxel densities and assuming the elemental composition of water, average skin and cortical bone for the PTV and soft tissue, the skin, and the mandible, respectively. The spatial resolution of TPS dosimetry results was 1 mm, isotropic. Corresponding reference data were obtained from patient specific Monte Carlo (MC) simulations using the MCNP6 code with input files prepared from the parsing of dicom RT data with the BrachyGuide software tool. The TPS HU calibration was imported to BrachyGuide to ensure identical density input to ACE and MC. Dose was approximated by collision Kerma and kerma to medium in medium was scored using the F6 tally. BrachyGuide was also used for the comparison of the three RT dose files for each patient case (TG43, ACE, and MC). Results TG43 clearly overestimates results for all cases as shown in the left side of Figure 1 for an indicative case. This cannot be attributed solely to the difference between patient scatter conditions and TG43 assumptions since large differences are also observed close to the source dwell positions. The corresponding comparison between ACE and MC (right side of figure 1) shows agreement within MC type A uncertainty up to 5 cm from the implant. While ACE improves dosimetric accuracy, considerable differences are still observed close to the source dwell positions.

Conclusion Considerable differences between TG43 and MC dosimetry indicate that plan quality of HDR brachytherapy for lip carcinoma may be compromised. The ACE algorithm was found to improve dosimetric accuracy at clinically relevant distances. TPS dosimetric accuracy close to the source dwell positions warrants further investigation. OC-0358 Evaluation of the Advanced Collapsed-cone Engine dose calculation algorithm for COMS eye plaques H. Morrison 1,2 , G. Menon 1,2 , M. Larocque 1,2 , E. Weis 3,4 , R. Sloboda 1,2 1 University of Alberta, Oncology, Edmonton, Canada 2 Cross Cancer Institute, Medical Physics, Edmonton, Canada 3 University of Alberta, Ophthalmology, Edmonton, Canada 4 University of Calgary, Surgery, Calgary, Canada Purpose or Objective The current dosimetry protocol for ocular brachytherapy involves augmenting TG-43 dose calculations with correction factors or using look-up tables to account for plaque materials, as the water-based TG-43 calculation alone overestimates the dose in front of gold eye plaques by >20%. This work investigates the accuracy with which the Advanced Collapsed-cone Engine (ACE) algorithm (Oncentra Brachy (OcB) v4.6.0, Elekta, Sweden) can account for the ophthalmic applicator materials (gold backing and Silastic insert) for three different sizes of COMS eye plaques in a water phantom. Material and Methods The 12, 16, and 20 mm COMS eye plaques were introduced into the applicator library for OcB by creating 3D CAD models of the plaques and Silastic inserts with virtual catheter lines along each seed slot. The Nucletron selectSeed 130.002 I-125 source model for ACE was created using primary-scatter separated kernel data (generated by the CLRP (Carleton Laboratory for Radiotherapy Physics) group) and AAPM consensus TG-43 dosimetry data. Treatment plans were created in OcB for a single seed in water, a single seed loaded in the central slot of the 12 and 20 mm COMS plaques (the 16 mm COMS plaque does not have a central slot), and fully loaded 12, 16, and 20 mm COMS plaques. ACE dose calculations were performed in high accuracy mode on a high resolution 0.5 mm 3 calculation grid. The resulting dose data was compared to Monte Carlo (MC) simulated data using MCNP6, replicating the OcB treatment plans. Results

Results from the comparison of median DVH parameters in the Table show large differences between TPS calculations and MC for high dose PTV volumes (V150 και V200) in accordance with the above findings. Large differences between TPS calculations and MC are also observed for OAR parameters. These differences however correspond to low dose while the low value of V85 does not raise particular concern for tissue necrosis.