Commissioning a State-of-Art Small Animal Irradiator and Novel Mini-GRID Treatment Technique

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2022

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Purpose: To validate commissioning results associated with the Xstrahl Small Animal Radiation Research Platform (SARRP) installed at Duke University in October 2021, verify the accuracy of the Xstrahl Point Dose Calculator (PDC) and MuriPlan dose calculation in simple geometries, and design and characterize a novel in-house kV mini-GRID capability on the SARRP.Methods: After installation at Duke University, Xstrahl SARRP TG-61 output was measured for independent verification using a Farmer ion chamber at reference conditions (33 cm SSD, 2 cm depth, open field, 220 kVp, 13 mA). Half-value layer was measured using the same ion chamber, with copper sheets to vary thickness. The accuracy of the PDC was investigated in simple water and bolus stack phantoms using EBT3 film. A range of field sizes (10x10, 20x20, 30x30, 40x40, 10x20, 20x10, 15x40, 10x40, 30x70 mm2) and depths (1 cm, 2 cm) were spot-checked. MuriPlan simulations of treatment delivery to the bolus phantom and water phantom were compared to results of EBT3 film measurements. Metal Oxide Semiconductor Field Effect Transistor (MOSFET) detectors were also used for independent verification, with detectors being embedded within a tissue-equivalent mouse phantom at 1 cm depth. GRID irradiations were performed with the SARRP, using a 220 kVp beam, 13 mA, and a 40 mm x 40 mm field size. Pencil and bar GRIDs with beamlet spacings of 1 mm and 1.25 mm were characterized by first inserting GRID into a 3D-printed mount and positioning the mount on top of a PLA plastic block, surrounded by distilled water. EBT3 films were infixed to the top level of the PLA block and positioned at isocenter. PDC was utilized to determine irradiation time. The beam was turned on for 102 seconds—sufficient time to administer 6 Gy with a 40 mm x 40 mm field size to the surface film at isocenter with no GRID blocking the beam. EBT3 film results were analyzed to determine the output factors, peak-to-valley ratios, integral dose relative to open field, relative dose maps, as well as to produce dose volume histograms for each GRID. Results were compared to GRID characterizations in Johnson et al [18]. GRID characterizations were used to inform experimental plan for pre-clinical trial evaluating treatment efficacy of GRID therapy with PD-L1 checkpoint inhibition compared to conventional radiation therapy. Results: TG-61 dose rate and half-value layer measured during on-site commissioning showed excellent agreement with Xstrahl factory commissioning results (≈1% difference). The PDC and MuriPlan dose calculation predicted results for field sizes and depths demonstrated acceptable agreement with actual results measured by EBT3 film (.2% to 12%), with exception of several outliers. Using EBT3 film dosimetry for verification (tissue-equivalent bolus medium), MuriPlan simulations were within 2% and 12% difference from the film measured dose for 5/7 field sizes in the bolus phantom and within 3% and 13% for 4/5 field sizes in the water phantom. MOSFET detector measurements using the mouse phantom demonstrated improved agreement with the PDC-predicted dose, with percent errors ranging from .12% to 5.97% (with a single outlier at 18.3%). Measured output factors using the SARRP for the 20 mm x 20 mm pencil GRIDs were .77 ± .03 and .74 ± .02 (1 mm and 1.25 mm beamlet spacing, respectively). For the bar GRIDs, these values were evaluated to be .83 ± .03 and .80 ± .03 (1 mm and 1.25 mm beamlet spacing, respectively). Peak:valley ratios for the 1 mm and 1.25 mm beamlet spacing pencil GRIDs were determined to be 24.5 ± 0.6 and 25.1 ± 1.3, respectively. Peak:valley ratios for the 1 mm and 1.25 mm beamlet spacing bar GRIDs were found to be lower than for pencil GRIDs with equivalent beamlet spacing, being evaluated to be 13.2 ± 1.1 and 18.5 ± 1.2, respectively. Output factors, peak:valley ratios, integral dose relative to open field, and dose volume histograms for the pencil and bar GRIDs using the SARRP largely corroborated the results of Johnson et al in terms of experimental trends (peak:valley ratios being higher for pencil GRIDs and increasing with increasing beamlet spacing, output factors decreasing with increasing beamlet spacing for both GRID types, and decreasing integral dose with increasing beamlet spacing for pencil GRIDs and increasing integral dose with increasing beamlet spacing for bar GRIDs). 4.67% to 30.5% difference was observed for experimentally measured peak:valley ratios relative to the results for the same GRIDs in Johnson et al.. Better agreement was demonstrated in GRID output factor measurements (≈0% to 14%). Integral dose experimental measurements demonstrated exceptional agreement with Johnson et al.., with percent differences ranging from 1% to 2.1%. These measured differences are likely a result of using the SARRP versus the XRAD 225Cx used in Johnson et al, but lend significant credence to reproducibility of results found using the XRAD 225Cx. Conclusions: The PDC and MuriPlan computations provide an effective estimate of the exposure time necessary to deliver dose for corresponding MVC field sizes and depths (within 6% error using the MOSFET for verification). EBT3 film was determined to be an unreliable measure of SARRP dose delivery; MOSFET detectors demonstrated more consistency and effectiveness for treatment planning verification. Xstrahl’s SARRP was able to replicate the kV mini-GRID capabilities of the XRAD 225Cx used in Johnson et al. and can be used for mini-GRID characterizations and preclinical mouse trials.

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Brundage, Simon A (2022). Commissioning a State-of-Art Small Animal Irradiator and Novel Mini-GRID Treatment Technique. Master's thesis, Duke University. Retrieved from https://hdl.handle.net/10161/26886.

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