ESTRO 36 Abstract Book

S191 ESTRO 36 _______________________________________________________________________________________________

to low dose while the low value of V85 does not raise particular concern for tissue necrosis.

compared to Monte Carlo (MC) simulated data using MCNP6, replicating the OcB treatment plans. Results The ACE doses for the single seed in water agree with MC simulations on average within 4.4 ± 2.1% in a 60x60x60 mm 3 cube centered on the seed, with the largest differences near the end-welds of the seed. Percent differences between ACE and MC doses along the plaque central axes (CAX) for all eye plaque plans are shown in Figure 1. The agreement improves beyond ~3 mm from the outer scleral surface, and is generally better for the fully loaded plaques than the single seed plaques, due to more overlapping dose from each seed washing out ray effects caused by the ACE calculation. Compared to using the previous minimum calculation grid size of 1 mm 3 , the smaller 0.5 mm 3 grid size results in less voxel averaging, and therefore more accurate doses immediately adjacent to the plaques, though both agree well with MC in the eye region (Figure 2).

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