ESTRO 2020 Abstract book

S757 ESTRO 2020

dosimetric errors. Aim of this work is to initiate an independent dose verification technique using electronic portal imaging device (EPID) to verify the delivered dose to small animal during radiation therapy. Material and Methods We calibrated a camera‐based EPID on the small animal radiation research platform (SARRP, Xstrahl, Inc.) to measure transit dose at the imager plane. Transit dose was back‐projected at object’s exit surface to determine exit dose. First, we validated developed EPID dosimetry technique against film measurements through homogeneous and heterogeneous phantoms. Second, we implemented this technique in administered image‐guided radiotherapy fraction for 20 rats with prostate cancer. We compared the planned dose distributions calculated by the treatment planning system (TPS) with delivered dose measured by the EPID. Results Transit central axis dose values measured with the EPID showed close agreement with film measurements, differences were within 4.9%. For inhomogeneous phantom, the EPID and film exit dose measurements agreed within ≤2%. For animal study, average difference between TPS and EPID was 3%, with a maximum of 9.3%. Gamma analyses for exit dose verification between TPS‐ calculated and EPID‐measured dose distributions showed average 90% passing rate under global 2mm/5% gamma criteria. Conclusion We implemented a kilovoltage EPID dosimetry technique to verify accuracy of SA‐IGRT to improve fidelity of preclinical radiation research. Beyond the current scope, we expect that this framework may be applied to any SA‐ IGRT system that incorporates an EPID. PO‐1340 Optical surface tracking for non‐coplanar SRS treatments verified by film in a RandoAlderson phantom M. Oellers 1 , A. Swinnen 1 , F. Verhaegen 1 1 MAASTRO Clinic, Medical Physics, Maastricht, The Netherlands Purpose or Objective To perform a dosimetric verification with EBT‐XD film of a non‐coplanar plan for multiple brain metastases monitored by a commercial optical surface tracking system (OST). Material and Methods A 3‐camera OST system was used (Catalyst HD TM , C‐RAD, Sweden) on a Varian Truebeam STx linac with a 6DoF couch. The set‐up accuracy and agreement between the OST system and cone beam CT (CBCT) and kV‐MV imaging at respectively couch angles 0 o and 270 o were examined. 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.

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