G. Cranmer-Sargison

Medical linear accelerator mounted mini-beam collimator: design, fabrication and dosimetric characterization.

The goal of this work was to design, build and experimentally characterize a linear accelerator mounted mini-beam collimator for use at a nominal 6 MV beam energy. Monte Carlo simulation was used in the design and dosimetric characterization of a compact mini-beam collimator assembly mounted to a medical linear accelerator. After fabrication, experimental mini-beam dose profiles and central axis relative output were measured and the results used to validate the simulation data. The simulation data was then used to establish traceability back to an established dosimetric code of practice. The Monte Carlo simulation work revealed that changes in collimator blade width have a greater influence on the valley-to-peak dose ratio than do changes in blade height. There was good agreement between the modeled and measured profile data, with the exception of small differences on either side of the central peak dose. These differences were found to be systematic across all depths and result from limitations associated with the collimator fabrication. Experimental mini-beam relative output and simulation data agreed to better than ± 2.0%, which is well within the level of uncertainty required for dosimetric traceability of non-standard field geometries. A mini-beam collimator has now been designed, built and experimentally characterized for use with a commercial linear accelerator operated at a nominal 6 MV beam energy.

Sci-Thur PM – Colourful Interactions: Highlights 01: Design to delivery of spatially fractionated mini-beam canine radiotherapy

Spatial fractionation of radiation using arrays of narrow parallel micro-planar beams (less than 1 mm), is a relatively new concept with many unknowns specifically within the underlying biology of cell death. A tungsten collimator has been designed to produce mini-beams with a Varian linear accelerator for translational animal research into the effectiveness of spatial fractionation mini-beam radiotherapy (MBRT). This work presents the treatment planning process and workflow for the application of MBRT treatments within a clinical study. For patient dose calculations, the MBRT collimator was incorporated into a Monte Carlo based treatment planning system called MMCTP. Treatment planning was split between Eclipse and MMCTP, as the field apertures were determined within Eclipse prior to being sent to MMCTP for dose calculations. The calculated plan was transferred back into Aria with updated MUs per field for patient treatment. Patients were positioned within a vac-lock bag lying prone with a bite block and a thermoplastic mask to immobilize the head. Prior to treatment, a delivery verification plan was created within MMCTP. DQA output measurements of the treatment fields agreed with the calculated dose to within 1.5%. We have presented a workflow for MBRT treatments that include the planning technique, dose calculation method, DQA process and data integration into a record and verify system. The clinical study following this workflow represent the first series of linac based MBRT patients and depending on the clinical outcome of the study, our technique could be applied to human MBRT treatments.

Poster - 23: Dosimetric Characterization and Transferability of an Accessory Mounted Mini-Beam Collimator

Objective: The dosimetric characterization of an accessory-mounted mini-beam collimator across three beam matched linear accelerators. Materials and Methods: Percent depth dose and profiles were measured for the open and mini-beam collimated fields. The average beam quality and peak-to-valley dose ratio (PVDR), the ratio of average peak dose to average valley dose, were obtained from these measurements. The open field relative output and the mini-beam collimator factor, the ratio of the mini-beam dose to open field dose at the beam center, were measured for square fields of side 2, 3, 4, and 5 cm. Mini-beam output as a function of collimator inclination angle relative to the central axis was also investigated. Results and Discussion: Beam quality for both the open and mini-beam collimated fields agreed across all linacs to within ±1.0%. The PVDR was found to vary by up to ±6.6% from the mean. For the 2, 3, and 4 cm fields the average open field relative output with respect to the 5 cm field was 0.874±0.4%, 0.921±0.3%, and 0.962±0.1%. The average collimator factors were 0.450±3.9%, 0.443±3.9%, 0.438±3.9%, and 0.434±3.9%. A decrease in collimator factor greater than 7% was found for an inclination angle change of 0.09°. Conclusion: The mini-beam collimator has revealed a difference between the three linacs not apparent in the open field data, yet transferability can still be attained through thorough dosimetric characterization.

SU‐E‐T‐292: CBCT Imaging and the Assessment of PTV Margin Size for Rectal Cancer Patients Treated Prone on Belly Board

Purpose: The goal of this work was to use CBCTimages taken at the time of treatment to derive PTV margin sizes that would account for inter and intra‐fractional systematic and random errors associated with CTV position for patients treated prone on a new couch top belly board (CDR Systems Inc.). Methods: Twenty‐four patients (12 male and 12 female) were included in this study. CBCT data was acquired once every 5 fractions for a total of 5 images per patient. A 3D‐3D bony anatomy auto‐match was performed offline and the residual difference used as a surrogate for inter‐fractional positional errors of the CTV. Systematic and random variations in CTV position were evaluated in a manner consistent with that of Stroom et al and used in PTVmargin = 1.96Σ + 0.7σ. The influence of hypothetical intra‐fractional motion was included in the margin evaluation by introducing the following values: 1.0, 2.0 and 3.0mm.Results: PTVmargin required to account for inter‐fraction positional errors was found to be (AP, SI, LR) = (5.2 mm, 3.1 mm, 2.8 mm). If we assume any intra‐fractional motion to be similar to that presented by Xu et al, then the required PTVmargin increases to (AP, SI, LR) = (7.0 mm, 5.0 mm, 5.0 mm). A 7.0 mm AP expansion is consistent with that quoted in the “Elective Clinical Target Volumes in Anorectal Cancer: An RTOG Consensus Contouring Atlas“, which recommends a margin between 7.0 and 10.0 mm. However, 7.0 mm is 2.0 mm greater than the 5.0 mm margin specified in the RTOG 0822 trial. Conclusions: A PTVmargin expansion of (AP, SI, LR) = (7.0 mm, 5.0 mm, 5.0 mm) will account for inter and intra‐fractional systematic and random errors associated with CTV position for patients treated prone on a new couch top belly board.

Poster — Thur Eve — 20: Fine Tuning the Source Parameters of a Varian 6 MV BEAMnrc Model through the Simulation of Small Jaw Collimated Photon Fields

Purpose: The goal of this work was to finalize the electron source parameters of a Varian 6 MV BEAMnrc model through the simulation of jaw collimated small fields. Methods and Materials: For nominal field sizes of 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 cm profile data was acquired in water using a stereotactic field diode (SFD) and EBT2 film. To achieve a positional error below ± 0.01 cm a stepper motor driven linear actuator device was incorporated into the Wellhofer water tank. At electron energies of 6.0, 6.1 and 6.2 MeV the spatial distribution of the electrons incident on the target was modelled as a Gaussian. The FWHM was decreased in steps of 0.010 cm from 0.150 to 0.100 cm. DOSXYZnrc simulations were run and profile data extracted for comparison. Results: The SFD and EBT2 profile data were found to be in good agreement with small differences believed to be the result of non‐uniformities inherent to the film. In all cases the measured field sizes were found to be smaller than the nominal and the simulated field sizes adjusted accordingly. At a FWHM = 0.150 cm the penumbral width was much too broad and the smallest field widths wider than experiment. As the FWHM was decreased from 0.150 to 0.110 cm the fit became progressively better, yet worsened at a FWHM = 0.100 cm for all but the smallest field size.Conclusion: Small jaw collimated fields can be used to fine tune the electron source parameters used in a BEAMnrc linac model.