ESTRO 2020 Abstract Book

S709 ESTRO 2020

GmbH/ University of Heidelberg, Dept. of Radiation Oncology, Munich, Germany

Then dosimetric film measurements at 3 depths in the Rando-Alderson phantom were performed based on a single isocenter non-coplanar VMAT plan containing 4 brain lesions. Initial set-up of the phantom was performed with CBCT at couch 0° and subsequently monitored by Catalyst HD TM at all other couch angles. The absolute measured film doses were compared to the calculated TPS doses. Results For repeated tests with the Rando-Alderson phantom, the deviations between rotational and translational isocenter corrections for CBCT and OST system are always within 0.2° (pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0° and 270°, respectively. Dose deviations between the absolute measured and calculated doses in the center of the 4 PTVs irradiated with a non-coplanar plan monitored by the OST system are -1.2%, -0.1%, -0.0% and -1.9% for PTV-1, PTV-2, PTV-3 and PTV-4, respectively. The dosimetric agreement between film and TPS presented by a local gamma evaluation criterion of 2%/2mm and 3%/1mm (both with cut-off dose value of 20%) yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for PTV-1, PTV-2, PTV-3, and PTV-4 respectively. Conclusion Patient positioning for a non-coplanar single isocenter VMAT treatment of multiple brain metastases can be monitored with an OST system with submillimeter and subdegree accuracy. Figure: A five arc single isocenter non-coplanar VMAT plan with 4 PTVs, verified using film dosimetry with EBT-XD films in the first 3 slices of the Rando-Alderson phantom head.

Purpose or Objective The purposes of this study were 1) to assess the accuracy of 3D dose calculation with Monte Carlo (MC) algorithm of the automated treatment plan verification software SciMoCa v1.4.2 (Radialogica, USA) and 2) to establish action limits for plan QA based on the gamma criteria taking into account the sensitivity of SciMoCa to specific plan errors. Material and Methods The MC model for 6MV VersaHD linac was commissioned by comparisons between measured data and calculations done with SciMoCa and a MC-based TPS (Monaco 5.11, Elekta). In case of square fields the verification with water phantom measurements was performed, whereas for complex fields and 20 IMRT/VMAT (head, lung, breast) cases SciMoCa was compared to TPS and measurements with 2D-array. The same plans were used to determine the sensitivity of the γ-analysis in SciMoCa to treatment plan errors, such as incorrect HU-ED curve, density overrides or a wrong MLC offset in TPS model. Results were correlated with plan complexity indices and used to derive action limits for plan QA with SciMoCa. Results The study shows good agreement between SciMoCa, TPS and measurements for PDDs, profiles and point doses. A maximum difference of 0.4% and 1.5% in output factors was found for 2x2cm 2 field for SciMoCa vs. measurement and TPS, respectively. For clinical cases 3D γ-analysis (global, 2%-1mm) vs TPS showed mean values of 99,2% in phantom and 97,5% in patient geometry. Analysis of measured 2D doses with 3%-3mm criteria for all cases showed mean global γ-values of 97.2% and 97.3% and local γ of 94.2% and 94.7% for TPS and SciMoCa doses, respectively. The γ-analysis for SciMoCa vs. TPS with 2%-2mm and 2%- 1mm criteria showed moderate correlation with gamma pass rates of the TPS calculation vs. measurement QA (r=0.54, p<0.05). Moderate correlation was present between SciMoCa γ-pass rates (2%-2mm and 2%-1mm) and few plan complexity indices: Field Irregularity (FI) (r= - 0.66, p<0.05) and product of the FI and Small Segment Contribution index (r=-0.48, p<0.05). SciMoCa plan QA results applying different γ-criteria for all plans with two exemplary induced errors are shown in the Fig 1.

Table:The dosimetric agreement between film and TPS doses is presented by the deviations between measured and calculated doses in the center of the 4 PTVs and by a local gamma evaluation criterion of 2%/2mm as well as 3%/1mm for the 4 PTVs irradiated with single isocenter non-coplanar VMAT and monitored using Catalyst HD TM . To put these numbers into perspective, also the results for PTV-1 irradiated in a co-planar set-up (couch 0˚) are given (highlighted in bold).

Agreement score (2%21mm)

Agreement score (3%/1mm)

D_film (Gy)

D_TPS (gy)

Δ(D_film- D_TPS) %

PTV

1 9.71 9.82 -1.2%

99.2% 99.8% 99.2% 98.6% 89.9%

98.8% 99.9% 97.5% 81.7% 78.1%

1

9.15

9.30 -1.6%

2 9.86 9.87 -0.1% 3 9.82 9.82 -0.0% 4 9.11 9.29 -1.9%

PO-1341 Monte Carlo secondary plan check: validation and definition of the action limits for patient QA I. Fotina 1 , S. Siamkousky 2 , A. Zverava 2 , M. Alber 3 1 IBA Dosimetry GmbH, Physics Dept., Nuremberg, Germany ; 2 Minsk City Clinical Oncology Center, Radiation Therapy Dept., Minsk, Belarus ; 3 ScientificRT