Clinical Experience Using the SRS MapCHECK for Patient Specific Plan Verification of Flattening Filter Free, Non-Coplanar Stereotactic Brain Plans Using HyperARC.

HyperArc (HA) treatment planning from Varian is a stereotactic specific planning tool enabling quick and efficient optimisation of treatment planning, and delivery. HA was commissioned and implemented at Waikato Regional Cancer Centre (WRCC) in 2019 to fulfil the demands of dose delivery for stereotactic radiosurgery (SRS), allowing for treatment of multiple targets with a single isocenter at non-coplanar angles. The extra levels of plan complexity involved in creating and verifying HA SRS plans required extensive checks and verifications using film and an ion chamber, along with a significant allocation of time and resources. The Sun Nuclear SRS MapCHECK (SRSMC) offered an alternative to the cumbersome film measurements. It is an all-encompassing tool meeting the requirements of TG 218 and ICRU 91 for complex treatment plan verification, claiming to save time and effort, without sacrificing accuracy, enabling for a smoother planning and verification process. SRSMC was initially commissioned on 6MV single target treatments using standard planning, then updated and commissioned for 6FFF multi-target non-coplanar treatments using HA. The SRSMC gamma pass rates were compared to film measurements in the same plane, and the central diode CAX reading compared to ionisation chamber measurements at the same position for a range of plans covering a range of PTV sizes and plan complexities. Pass rates on the SRSMC were comparable to measurements using film (Gamma 3%/1mm, 99.41%, 96.39% SRSMC and film respectively). The central diode is an adequate surrogate for a chamber measurement if the SRSMC is positioned in a similar position as that of the ionisation chamber would be – high dose homogenous region, avoiding steep gradients (mean dose difference Diode vs Chamber: -0.73%). Differences between exposing non-coplanar plans at couch 0 and at planned couch angles were negligible (Gamma 3%/1mm 99.28 coplanar, 99.41% non-coplanar on SRSMC). At WRCC the SRSMC has replaced film and chamber measurements for plan verifications of 6FFF HA multiple metastatic brain treatments at a single isocenter and we are currently investigating its use in other treatment sites.

Investigation of fission product isomeric ratios and angular momenta of 132Sn populated in the 241Pu(nth,f) reaction

During an experimental campaign performed at the LOHENGRIN recoil spectrometer of the Institut Laue-Langevin (ILL), a kinetic energy dependence of 132Sn fission product isomeric ratio (IR) has been measured by inducing thermal fission of 241Pu. The IRs are deduced using gamma ray spectrometry in coincidence with the ionisation chamber. To interpret these data, we use the FIFRELIN Monte-Carlo code to simulate the de-excitation of the fission fragments. Combining the IRs with the FIFRELIN calculations, the angular momentum distribution with kinetic energy of the doubly magic nucleus of 132Sn was deduced. This will be compared with the angular momentum distribution obtained for the reaction 235U(nth,f) for 132Sn.

Investigation of central electrode artefacts of ionisation chamber effect on dose calculation using advanced calculation algorithms AAA and Acuros XB

Aim: To investigate the central electrode artefact effect of different ion chambers in the verification phantom using the dose calculation algorithms Analytical Anisotropic Algorithm (AAA) and Acuros XB. Materials and methods: The dosimetric study was conducted using an in-house fabricated polymethyl methacrylate head phantom. The treatment planning system (TPS)-calculated doses in the phantom with detectors were compared against the dummy detector fillets using AAA and Acuros XB algorithm. The planned and measured doses were compared for the study. Results: The mean percentage variation in volumetric-modulated arc therapy plans using Acuros XB and the measurement in the head phantom are statistically significant (p-value = 0.001) for FC65 and CC13 chambers. In small volume chambers (A14SL and CC01), the measured and TPS-calculated dose shows a good agreement. Findings: The study confirmed the CT set of the phantom with detectors (FC65 and CC13) give more artefacts/heterogeneity caused a significant variation in dose calculation using Acuros XB. Therefore, the study suggests a method of using phantom CT set with the dummy detector for mean dose calculation for the Acuros XB algorithm.

EVALUATION OF AMBIENT DOSE EQUIVALENT SURROUNDING TRANSMISSION FULL-BODY SCANNERS

This study analyses the ambient dose equivalent around transmission full-body scanners used in Brazilian prisons and airports. In order to achieve this goal, three transmission full-body scanners of the same model assembled by a Brazilian manufacturer were evaluated. Ambient dose equivalent rates were measured at several positions around these screening devices with the help of an ionisation chamber made by Ludlum, model 9DP. These systems consist of an X-ray generator with three distinct modes of operation, each with a unique value for maximum energy (100, 150 and 160 keV) and current (0.4, 0.5 and 0.9 mA). Results show the integrated dose per scan at each of the measurement positions considered, presenting values for ambient dose equivalent with and without an individual being scanned, in order to take into account the scattering due to the presence of a human body.

Pitfalls in the beam modelling process of Monte Carlo calculations for proton pencil beam scanning

Objective: Monte Carlo (MC) simulations substantially improve the accuracy of predicted doses. This study aims to determine and quantify the uncertainties of setting up such a MC system. Methods: Doses simulated with two Geant4-based MC calculation codes, but independently tuned to the same beam data, have been compared. Different methods of MC modelling of a pre-absorber have been employed, either modifying the beam source parameters (descriptive) or adding the pre-absorber as a physical component (physical). Results: After the independent beam modelling of both systems in water (resulting in excellent range agreement) range differences of up to 3.6/4.8 mm (1.5% of total range) in bone/brain-like tissues were found, which resulted from the use of different mean water ionisation potentials during the energy tuning process. When repeating using a common definition of water, ranges in bone/brain agreed within 0.1 mm and gamma-analysis (global 1%,1mm) showed excellent agreement (>93%) for all patient fields. However, due to a lack of modelling of proton fluence loss in the descriptive pre-absorber, differences of 7% in absolute dose between the pre-absorber definitions were found. Conclusion: This study quantifies the influence of using different water ionisation potentials during the MC beam modelling process. Furthermore, when using a descriptive pre-absorber model, additional Faraday cup or ionisation chamber measurements with pre-absorber are necessary. Advances in knowledge: This is the first study quantifying the uncertainties caused by the MC beam modelling process for proton pencil beam scanning, and a more detailed beam modelling process for MC simulations is proposed to minimise the influence of critical parameters.