ESTRO 2022 - Abstract Book

S1541

Abstract book

ESTRO 2022

A planning study was performed in Pinnacle 3 v16.4.3 using the CIRS lung phantom, investigating the effect of PTV volume, PTV oblate/prolateness, and lung density on the resulting R50%. VMAT plans were produced to local departmental practice, with Personalised Planning producing dual 210° arcs calculated at 2mm resolution. GTVs were defined using density overrides and were expanded by 5mm to form PTVs which ranged from 5 to 50cc, GTV oblateness was defined by major/minor axis ratio (a/b) from 0.5 to 2 (see figure), and lung density ranged from 0.15 to 0.35gcm -3 .

Results In line with [1] and [2], a relationship was found between PTV volume and R50%, but was acquired with finer increments than the UK national guidance. Prolate targets (with respect to the cranio-caudal axis) had a significantly higher predicted R50% compared to spherical or oblate geometries, for the same volume. For an 18cc PTV, prolate (a/b =2) geometries had an average 10% larger R50% compared to oblate (a/b = 0.5). This relationship is more pronounced for larger PTV volumes as the PTV oblateness more closely matches that of the GTV. Lung density has a significant impact on R50% due to wider scatter in low density media; a lung density of 0.2g/cm ³ resulted in an 11% increase in R50% compared to the same 18cc PTV in a 0.3g/cm ³ lung. Very low densities resulted in much larger R50%. Conclusion Our simple study demonstrates R50% is required at a finer volume resolution compared to the UK SABR Consortium guidance, but also that additional considerations are needed when assessing plan quality to account for PTV geometry and lung density. This helps planners assess whether SABR plans are within expected ranges for given target volume, geometry and surrounding tissue, and reduce the likelihood of over-optimising the plan to meet the R50% target, or under-optimising a plan which could be improved. References [1] UK SABR Consortium, Stereotactic ablative body radiotherapy (SABR): a resource, v6.1, January 2019, available https://www.sabr.org.uk/wp-content/uploads/2019/04/SABRconsortium-guidelines-2019-v6.1.0.pdf [2] Yaparpalvi R, Garg MK, Shen J, Bodner WR, Mynampati DK, Gafar A, et al. Evaluating which plan quality metrics are appropriate for use in lung SBRT. Br J Radiol 2018; 91: 20170393

PO-1740 Commissioning of a new treatment planning system for CyberKnife robotic stereotactic radiotherapy

M. Jaccard 1 , N. Perichon 2 , M. Fargier-Voiron 1 , O. Matzinger 1 , S. Bulling 1

1 Swiss Medical Network, Radiation Therapy, Genolier, Switzerland; 2 Swiss medical Network, Radiation Therapy, Genolier, Switzerland Purpose or Objective Treatment planning with RayStation (RaySearch Laboratories AB, Stockholm, Sweden) for the CyberKnife (CK, Accuray Incorporated, Sunnyvale, CA, USA) has been available since July 2021 (RayStation 11A). We present the beam data measurements, and describe the commissioning and validation process for this new vendor-independent TPS for the fixed, iris and MLC collimators of CK S7. The CK TPS (Precision, Accuray) was commissioned in parallel. Our commissioning data will be among the first to be part of the reference standardized CK S7 beam data set.

Materials and Methods