Monte Carlo optimization of a microbeam collimator design for use on the small animal radiation research platform (SARRP)

Abstract

Microbeam radiation therapy (MRT) is a pre-clinical, spatially-fractionated treatment modality noted for its ability to achieve a large differential response between normal and tumoral tissues. In the present study, TOPAS Monte Carlo (MC) simulations were used to optimize the design of a compact, affordable multi-slit collimator (MSC) suitable for use with the small animal radiation research platform (SARRP). MRT dose distributions in a (1 × 1 × 3)cm water phantom were simulated for a tungsten MSC using different focal spot sizes (0.4, 3 mm), beam energies (40, 80, 220 kVp), slit widths (100, 125, 150, 175, 200 µm), collimator thicknesses (1.5, 2.5, 3 cm) and collimator-to-surface distances (CSD of 1 and 3 cm). Key MRT figures of merit, namely the peak-to-valley dose ratio (PVDR), full-width at half-maximum and peak dose rate were determined. Use of the small focal spot maximized the PVDR (~40 at surface) and reduced the system's sensitivity to changes in CSD, but decreased the collimated beam output to 55.2 cGy min. The large focal spot was ill-suited for large CSD irradiations, but increased the beam output by a factor of 2.8, to 153.0 cGy min, and decreased the sensitivity to changes in slit width. A modular MSC, using divergent plastic spacer materials in place of excavated slits, was also investigated. Polypropylene and polyethylene terephthalate material spacers were considered and while neither reduced the PVDR compared to air slits, the dose rate was reduced by 37% and 47%, respectively. Lastly, a steel parallel-slit MSC was used in a preliminary test of MRT delivery using the SARRP. Discrepancies between the results of film dosimetry and the corresponding MC simulations highlight the need to fabricate a more well-defined collimator for use in future validation and radiobiological work. The simulated results of this study are being used to inform the design of such a collimator, which will additionally boast a high degree of modularity at reasonable cost.

Establishment of Microbeam Radiation Therapy at a Small-Animal Irradiator

“…Compared with a former microbeam setup 22 and former setups with small-animal radiation systems, 20,24 the field homogeneity in terms of peak dose rates was substantially improved to standard deviations of 5% to 8% and is similarly low compared with reported values for synchrotron generated microbeams. 36 Whereas peak entrance dose rates at the synchrotron for MRT are up to 10 kGy/s, the presented setup achieves only up to 8.0 Gy/min for microbeams, depending on the choice of the focal spot and beam width.…”

“…Such PVDRs are similar to those achieved with synchrotron-generated microbeams 30,35 and higher than for most other x-ray tubeebased minibeam or microbeam systems. 20,24 In contrast to synchrotrons, the PVDR at the presented setup strongly decreased with depth, but nevertheless, the decrease was sufficiently shallow to treat mice when positioned close to the collimator surface.…”

“…Small-animal irradiators have been developed that provide image guidance and adaptive treatment planning. 23 Prezado et al 20 and Esplen et al 24 developed minibeam setups for such systems. Prezado et al made major modifications to the small-animal platform by introducing multiple stages.…”

Preclinical dose verification using a 3D printed mouse phantom for radiobiology experiments

“…The film batch was cross‐calibrated with respect to a PTW Farmer ® TN30013 (PTW, Freiburg, Germany) ion chamber to measure dose‐to‐water for the SARRP 220 kVp beam using the AAPM TG‐61 protocol, as outlined in our previous work . Specifically, 12 calibration films were irradiated to doses ranging from 0 to 8 Gy while preserving the setup of the corresponding ion chamber measurements.…”

Clinical microbeam radiation therapy with a compact source: specifications of the line-focus X-ray tube

“…For a clinical MRT, compact sources are required. Various concepts have been suggested, including inverse Compton scattering sources [12] , carbon nanotube field emission technology [13] , and conventional X-ray tube-based sources [14] , [15] . However, none of the suggested alternatives to synchrotrons achieve dose rates adequate to patient treatments.…”