ESTRO 2020 Abstract book

S798 ESTRO 2020

separation (DLS) of a multileaf collimator (MLC). Correct value of DLS is significant for dose estimation validity of intensity modulated radiotherapy, especially when applied to small target structures with the sweeping gap delivery technique. Conventional methods to determine DLS can be laborious. In this work a simple and more specifying method is presented, and in addition applied to a variety of therapy beam qualities and two MLC models. Theoretical DLS expresses an opening of a hypothetical leaf pair with flat, focused tips, which integral radiation fluency throughput equals to integral fluency under a real closed leaf pair with three-dimensional, rounded tips. However, in clinical practice the DLS includes MLC gap calibration offset, which in spite of good attempts, might differ from negligible. The demonstrated method presents the actual DLS as a combination of a semi-empirical DLS and a mechanically determined MLC gap calibration offset. Model results are compared to reference DLS results derived conventionally using dosimetrical measurements on uniform intensity fields created by dynamic sliding windows and high resolution detection of field gap widths. Material and Methods Theoretical DLS is estimated with a ray-tracing algorithm based on a known description of the MLC leaf geometry and a solved linear attenuation coefficient. The attenuation coefficient is derived from the measured MLC transmission and the known leaf thickness. MLC gap calibration error is determined by measuring a set of actual leaf gaps corresponding to gaps (widths: 6.4 mm, 3.2 mm, 1.6 mm and 0.8 mm) planned with the treatment planning software and extrapolating offset to zero leaf gap. Actual leaf gaps are measured on MLC level using a precise feeler gauge. The proposed method is used to define actual DLSs for photon beams on TrueBeam therapy system equipped with standard definition MLC, and for four matched FFF 6 MV beams of which two are collimated with standard MLCs and two with high definition MLC Results Measured transmission, gap calibration error and DLS, in addition to modelled theoretical DLS, derived actual DLS and deviation between measured and modelled actual DLS are presented in table 1: for studied five energies and in table 2: for studied four MLCs. The measured DLS data has a very close agreement, on average 0.0 mm difference, with DLS data obtained by the proposed method.

beams’ were recalculated in the RayPhysics module. As a next step the ‘Tomo couch’ needed to be modelled. Here 3 TomoDirect plans for the Tomo ‘Cheesephantom’ with gantry angels of 0°, 180° (complete couch) and 130° (only thru the upper pallet) were calculated in Precision and imported into RS. While Precision is importing the Tomo couch into the planning CT, RS is importing a structure set with an upper and a lower pallet. The densities of the pallets were adjusted, till all 3 treatment plans showed the same dose deviation from the Precision calculation. The Tomo calibration plans were imported from Precision into RS and recalculated. The differences in dose were used to tune the dose normalization and the jaw output factors of the beam model. Finally new calibration plans were generated in RS and measured in the Cheesephantom. For the first 22 patients plan QA was performed with the Octavius detector 729 and a local gamma analysis was performed with a 3%/3mm criteria and a threshold of 10% of the dose. Additionally an independent Monte Carlo dose calculation was executed on the patient CT using SciMoCa (ScientificRT). Here a local gamma criteria of 3%/1mm with a threshold of 50% was used for the whole patient and for the target volume. Results Profiles and depth dose curves in the RayPhysics module were in good agreement. A density of 0.65g/cm³ for the upper pallet and of 1.1 g/cm³ for the lower pallet of the TomoCouch was determined. The deviation in the calculation between RS and Precision was 1% for the 3 TomoDirect plans. For the Precision calibration plans deviations of 0.9%, 0.75% and 1.35% were yield for the field lengths of 1cm, 2.5cm and 5cm. The measurements of the RS calibration plans after the adjustment of the beam model were in good agreement with the calculations (-0.17% and 0.71% deviation). Plan QA passed for all patients with a mean passrate of 98.9% for the measurements and 96.9% for the MC calculations in the whole patient, respectively 99.3% for the target. Results are shown in the graph.

Conclusion The commissioning of RS for Tomo treatment planning showed no major deviations, including plan QA. PO-1412 A method for determining dosimetric leaf separation of a multileaf collimator A. Kulmala 1 , A. Rintala 2 , J. Pennanen 3 , M. Tenhunen 2 1 Clinical Research Institute HUCH, Radiotherapy, Helsinki, Finland ; 2 Helsinki University Hospital, Cancer Center, Helsinki, Finland ; 3 Varian Medical Systems Finland, Radiotherapy, Helsinki, Finland Purpose or Objective The purpose of this work is to demonstrate a robust method for determining an actual dosimetric leaf