ESTRO 2022 - Abstract Book

S1542

Abstract book

ESTRO 2022

Beam data acquisition for RayStation was performed at fixed SSD, and included percent depth dose (PDD), profiles at 15, 50, 100 and 300 mm depth, a single diagonal for the largest field, and output factors (OF). A microDiamond detector 60019 and a Beamscan water tank (PTW, Freiburg, Germany) were used for all measurements. Output factors (OF) were double- checked with a microSilicon detector (60023, PTW) and not adjusted by Monte Carlo (MC) correction factors. Beam modelling was performed for collapsed cone (CC) and MC algorithms for all collimators, except for the iris 5-mm aperture that cannot be modelled in RayStation. Validation tests for the quality of beam modeling were performed following the AAPM Practice Guideline 5.a. This included: a verification of the dose calculated in calibration conditions, comparisons of calculated dose distributions with commissioning data, comparisons of calculated dose distributions with 2D detector array measurements (Octavius 729 2D-Array, PTW), measurements for various MLC-shaped fields and fields at oblique incidence, and comparison of calculated dose in an heterogeneous medium with dose measurements in bone and lung inserts of a thorax phantom (CIRS, VA, USA). Results Beam data acquisition and processing for RayStation took approximately seven days ( vs . 11 days for Precision). Calculated dose in reference conditions (only available through scripting in RayStation) was below 0.3% compared to calibration. Comparison between commissioning data and clinical dose calculated in a homogeneous phantom with CC showed that more than 97% of PDD and profile points respected gamma criteria of 1%/1mm (global), except for the iris aperture of 7.5 mm, the MLC smallest field, and at the depth 300 mm. The agreement increased when modeling the beam with MC algorithm, which also improved the beam modeling in the profile tails of the largest fields. MLC fields measured with the 2D detector at perpendicular and oblique incidence all agreed with TPS calculation within gamma 1.5%/1.5mm criteria. Differences between calculated and measured doses in the heterogeneous phantom were below 2% in the lung and bone inserts, for both CC and MC algorithms. Conclusion RayStation 11A TPS for CK was successfully commissioned for clinical use in our department. Preliminary results show promising planning capabilities for stereotactic treatment planning with the CK S7.

PO-1741 A dosimetric study of dose algorithm impact and number of VMAT arcs in single fraction lung SBRT.

W.Y.C. Koh 1 , H.Q. Tan 1 , Y.Y. Ng 1 , K.W. Ang 1 , K.S. Lew 1 , Y.H. Lin 1 , G.A.C. Chua 1 , H.H.J. Yap 1 , J.H. Phua 1 , Y.M. Wong 1 , S.Y. Park 1 , J.C.L. Lee 1

1 National Cancer Centre Singapore, Radiation Oncology, Singapore, Singapore

Purpose or Objective TROG 13.01 SAFRON II have shown similar 1-year toxicity, OS, PFS and local control between single and multi fractions Lung SBRT. While preparing the planning protocol for single fraction Lung SBRT, dosimetric measurements for different dose calculation algorithms and a different number of VMAT arcs were performed to ensure plan deliverability and agreement between planning and delivered dose (28Gy/1 fraction). Materials and Methods 5 VMAT plans (AXB-6FFF-3A, AXB-10FFF-3A, AAA-10FFF-3A, AAA-10FFF-4A and AAA-10FFF-6A) were output by an experienced dosimetrist on 5 retrospective patients. 3A, 4A and 6A denote the number of radiation beam arcs and AXB or AAA refers to the dose algorithm used for optimizing the plan. 1mm and 2mm calculation grid size and dose to medium were used in dose reporting for AXB algorithm. PSQA was performed using point-dose measurement (Static QUASAR TM phantom with Cedarwood insert; both Pinpoint and Farmer Chamber from PTW were used) and PD. Global gamma passing rates at 1%/1mm, 2%/1mm, 1%/2mm and 2%/2mm were reported for the PD QA. Lastly, plan complexity comparisons were evaluated using modulation complexity score (MCS) across all the plans. Results The results show that the 4A and 6A plans are less complex ( P -value < 0.05) as compared to other plans and the respective MCS for each plan types for all patients are as shown in Figure 1.

Figure 1(A) shows the MCS for all plan types across all patients and 1(B) shows the gamma passing rate of 1mm/1% for all plan types across all patients. The point dose difference between measurement and plan calculated with two different grid sizes using two different dose calculation algorithms are shown in Figure 2. All plans are evaluated to be less than 5% dose discrepancy except AXB-6FFF- 3A. This discrepancy result between 6FFF and 10FFF is similar to that reported by Öllers MC et al. [2]. The least discrepancy occurs using the AXB dose calculation algorithm with a 1mm calculation grid size. Subsequently, PSQA using PD was done and the result follow a similar trend from point dose measurement. AXB-6FFF-3A gives a lower gamma passing rate for all criteria. Despite being less complex, 4 and 6 arcs plan does not result in a statistically significant higher gamma passing