Establishment of Microbeam Radiation Therapy at a Small-Animal Irradiator

Dosimetric study for breathing-induced motion effects in an abdominal pancreas phantom for carbon ion mini-beam radiotherapy

BACKGROUND

Particle mini-beam therapy exhibits promise in sparing healthy tissue through spatial fractionation, particularly notable for heavy ions, further enhancing the already favorable differential biological effectiveness at both target and entrance regions. However, breathing-induced organ motion affects particle mini-beam irradiation schemes since the organ displacements exceed the mini-beam structure dimensions, decreasing the advantages of spatial fractionation.

PURPOSE

In this study, the impact of breathing-induced organ motion on the dose distribution was examined at the target and organs at risk(OARs) during carbon ion mini-beam irradiation for pancreatic cancer.

METHODS

As a first step, the carbon ion mini-beam pattern was characterized with Monte Carlo simulations. To analyze the impact of breathing-induced organ motion on the dose distribution of a virtual pancreas tumor as target and related OARs, the anthropomorphic Pancreas Phantom for Ion beam Therapy (PPIeT) was irradiated with carbon ions. A mini-beam collimator was used to deliver a spatially fractionated dose distribution. During irradiation, varying breathing motion amplitudes were induced, ranging from 5 to 15 mm. Post-irradiation, the 2D dose pattern was analyzed, focusing on the full width at half maximum (FWHM), center-to-center distance (ctc), and the peak-to-valley dose ratio (PVDR).

RESULTS

The mini-beam pattern was visible within OARs, while in the virtual pancreas tumor a more homogeneous dose distribution was achieved. Applied motion affected the mini-beam pattern within the kidney, one of the OARs, reducing the PVDR from 3.78  ± $\pm$  0.12 to 1.478  ± $\pm$  0.070 for the 15 mm motion amplitude. In the immobile OARs including the spine and the skin at the back, the PVDR did not change within 3.4% comparing reference and motion conditions.

CONCLUSIONS

This study provides an initial understanding of how breathing-induced organ motion affects spatial fractionation during carbon ion irradiation, using an anthropomorphic phantom. A decrease in the PVDR was observed in the right kidney when breathing-induced motion was applied, potentially increasing the risk of damage to OARs. Therefore, further studies are needed to explore the clinical viability of mini-beam radiotherapy with carbon ions when irradiating abdominal regions.

Preclinical photon minibeam radiotherapy using a custom collimator: Dosimetry characterization and preliminary in-vivo results on a glioma model

PURPOSE

The purpose of this study is to investigate the dosimetric characteristics of a collimator for minibeam radiotherapy (MBRT) with film dosimetry and Monte Carlo (MC) simulations. The outcome of MBRT with respect to conventional RT using a glioma preclinical model was also evaluated.

METHODS

A multi-slit collimator was designed to be used with commercial small animal irradiator. The collimator was built by aligning 0.6 mm wide and 5 mm thick parallel lead leaves at 0.4 mm intervals. Dosimetry characteristics were evaluated by Gafchromic (CG) films and TOPAS Monte Carlo (MC) code. An in vivo experiment was performed using a glioma preclinical model by injecting two million GL261cells subcutaneously and treating with 25 Gy, single fraction, with MBRT and conventional RT. Survival curves and acute radiation damage were measured to compare both treatments.

RESULTS

A satisfactory agreement between experimental results and MC simulations were obtained, the measured FWHM and distance between the peaks were respectively 0.431 and 1.098 mm. In vivo results show that MBRT can provide local tumor control for three weeks after RT treatment and a similar survival fraction of open beam radiotherapy. No severe acute effects were seen for the MBRT group.

CONCLUSIONS

We developed a minibeam collimator and presented its dosimetric features. Satisfactory agreement between MC and GC films was found with differences consistent with uncertainties due to fabrication and set-up errors. The survival curves of MBRT and open field RT are similar while atoxicity is dramatically lower with MBRT, preliminarily confirming the expected effect.

Dosimetry of microbeam radiotherapy by flexible hydrogenated amorphous silicon detectors

Objective.Detectors that can provide accurate dosimetry for microbeam radiation therapy (MRT) must possess intrinsic radiation hardness, a high dynamic range, and a micron-scale spatial resolution. In this work we characterize hydrogenated amorphous silicon detectors for MRT dosimetry, presenting a novel combination of flexible, ultra-thin and radiation-hard features.Approach.Two detectors are explored: an n-type/intrinsic/p-type planar diode (NIP) and an NIP with an additional charge selective layer (NIP + CSC).Results.The sensitivity of the NIP + CSC detector was greater than the NIP detector for all measurement conditions. At 1 V and 0 kGy under the 3T Cu-Cu synchrotron broadbeam, the NIP + CSC detector sensitivity of (7.76 ± 0.01) pC cGy-1outperformed the NIP detector sensitivity of (3.55 ± 0.23) pC cGy-1by 219%. The energy dependence of both detectors matches closely to the attenuation coefficient ratio of silicon against water. Radiation damage measurements of both detectors out to 40 kGy revealed a higher radiation tolerance in the NIP detector compared to the NIP + CSC (17.2% and 33.5% degradations, respectively). Percentage depth dose profiles matched the PTW microDiamond detector's performance to within ±6% for all beam filtrations except in 3T Al-Al due to energy dependence. The 3T Cu-Cu microbeam field profile was reconstructed and returned microbeam width and peak-to-peak values of (51 ± 1)μm and (405 ± 5)μm, respectively. The peak-to-valley dose ratio was measured as a function of depth and agrees within error to the values obtained with the PTW microDiamond. X-ray beam induced charge mapping of the detector revealed minimal dose perturbations from extra-cameral materials.Significance.The detectors are comparable to commercially available dosimeters for quality assurance in MRT. With added benefits of being micron-sized and possessing a flexible water-equivalent substrate, these detectors are attractive candidates for quality assurance,in-vivodosimetry and in-line beam monitoring for MRT and FLASH therapy.

In Vivo Microbeam Radiation Therapy at a Conventional Small Animal Irradiator

Microbeam radiation therapy (MRT) is a still pre-clinical form of spatially fractionated radiotherapy, which uses an array of micrometer-wide, planar beams of X-ray radiation. The dose modulation in MRT has proven effective in the treatment of tumors while being well tolerated by normal tissue. Research on understanding the underlying biological mechanisms mostly requires large third-generation synchrotrons. In this study, we aimed to develop a preclinical treatment environment that would allow MRT independent of synchrotrons. We built a compact microbeam setup for pre-clinical experiments within a small animal irradiator and present in vivo MRT application, including treatment planning, dosimetry, and animal positioning. The brain of an immobilized mouse was treated with MRT, excised, and immunohistochemically stained against γH2AX for DNA double-strand breaks. We developed a comprehensive treatment planning system by adjusting an existing dose calculation algorithm to our setup and attaching it to the open-source software 3D-Slicer. Predicted doses in treatment planning agreed within 10% with film dosimetry readings. We demonstrated the feasibility of MRT exposures in vivo at a compact source and showed that the microbeam pattern is observable in histological sections of a mouse brain. The platform developed in this study will be used for pre-clinical research of MRT.

Small field proton irradiation for in vivo studies: Potential and limitations when adapting clinical infrastructure

PURPOSE

To evaluate the dosimetric accuracy for small field proton irradiation relevant for pre-clinical in vivo studies using clinical infrastructure and technology. In this context additional beam collimation and range reduction was implemented.

METHODS AND MATERIALS

The clinical proton beam line employing pencil beam scanning (PBS) was adapted for the irradiation of small fields at shallow depths. Cylindrical collimators with apertures of 15, 12, 7 and 5mm as well as two different range shifter types, placed at different distances relative to the target, were tested: a bolus range shifter (BRS) attached to the collimator and a clinical nozzle mounted range shifter (CRS) placed at a distance of 72cm from the collimator. The Monte Carlo (MC) based dose calculation engine implemented in the clinical treatment planning system (TPS) was commissioned for these two additional hardware components. The study was conducted with a phantom and cylindrical target sizes between 2 and 25mm in diameter following a dosimetric end-to-end test concept.

RESULTS

The setup with the CRS provided a uniform dose distribution across the target. An agreement of better than5% between the planned dose and the measurements was obtained for a target with 3mm diameter (collimator 5mm). A 2mm difference between the collimator and the target diameter (target being 2 mm smaller than the collimator) sufficed to cover the whole target with the planned dose in the setup with CRS. Using the BRS setup (target 8mm, collimator 12mm) resulted in non-homogeneous dose distributions, with a dose discrepancy of up to 10% between the planned and measured doses.

CONCLUSION

The clinical proton infrastructure with adequate beam line adaptations and a state-of-the-art TPS based on MC dose calculations enables small animal irradiations with a high dosimetric precision and accuracy for target sizes down to 3mm.

Development and characterization of a versatile mini-beam collimator for pre-clinical photon beam irradiation

BACKGROUND

Interest in spatial fractionation radiotherapy has exponentially increased over the last decade as a significant reduction of healthy tissue toxicity was observed by mini-beam irradiation. Published studies, however, mostly use rigid mini-beam collimators dedicated to their exact experimental arrangement such that changing the setup or testing new mini-beam collimator configurations becomes challenging and expensive.

PURPOSE

In this work, a versatile, low-cost mini-beam collimator was designed and manufactured for pre-clinical applications with X-ray beams. The mini-beam collimator enables variability of the full width at half maximum (FWHM), the center-to-center distance (ctc), the peak-to-valley dose ratio (PVDR), and the source-to-collimator distance (SCD).

METHODS

The mini-beam collimator is an in-house development, which was constructed of 10 ×  40 mm2 tungsten or brass plates. These metal plates were combined with 3D-printed plastic plates that can be stacked together in the desired order. A standard X-ray source was used for the dosimetric characterization of four different configurations of the collimator, including a combination of plastic plates of 0.5, 1, or 2 mm width, assembled with 1 or 2 mm thick metal plates. Irradiations were done at three different SCDs for characterizing the performance of the collimator. For the SCDs closer to the radiation source, the plastic plates were 3D-printed with a dedicated angle to compensate for the X-ray beam divergence, making it possible to study ultra-high dose rates of around 40 Gy/s. All dosimetric quantifications were performed using EBT-XD films. Additionally, in vitro studies with H460 cells were carried out.

RESULTS

Characteristic mini-beam dose distributions were obtained with the developed collimator using a conventional X-ray source. With the exchangeable 3D-printed plates, FWHM and ctc from 0.52 to 2.11 mm, and from 1.77 to 4.61 mm were achieved, with uncertainties ranging from 0.01% to 8.98%, respectively. The FWHM and ctc obtained with the EBT-XD films are in agreement with the design of each mini-beam collimator configuration. For dose rates in the order of several Gy/min, the highest PVDR of 10.09 ± 1.08 was achieved with a collimator configuration of 0.5 mm thick plastic plates and 2 mm thick metal plates. Exchanging the tungsten plates with the lower-density metal brass reduced the PVDR by approximately 50%. Also, increasing the dose rate to ultra-high dose rates was feasible with the mini-beam collimator, where a PVDR of 24.26 ± 2.10 was achieved. Finally, it was possible to deliver and quantify mini-beam dose distribution patterns in vitro.

CONCLUSIONS

With the developed collimator, we achieved various mini-beam dose distributions that can be adjusted according to the needs of the user in regards to FWHM, ctc, PVDR and SCD, while accounting for beam divergence. Therefore, the designed mini-beam collimator may enable low-cost and versatile pre-clinical research on mini-beam irradiation.

An investigation of kV mini-GRID spatially fractionated radiation therapy: dosimetry and preclinical trial

Objective. To develop and characterize novel methods of extreme spatially fractionated kV radiation therapy (including mini-GRID therapy) and to evaluate efficacy in the context of a pre-clinical mouse study.Approach. Spatially fractionated GRIDs were precision-milled from 3 mm thick lead sheets compatible with mounting on a 225 kVp small animal irradiator (X-Rad). Three pencil-beam GRIDs created arrays of 1 mm diameter beams, and three 'bar' GRIDs created 1 × 20 mm rectangular fields. GRIDs projected 20 × 20 mm2fields at isocenter, and beamlets were spaced at 1, 1.25, and 1.5 mm, respectively. Peak-to-valley ratios and dose distributions were evaluated with Gafchromic film. Syngeneic transplant tumors were induced by intramuscular injection of a soft tissue sarcoma cell line into the gastrocnemius muscle of C57BL/6 mice. Tumor-bearing mice were randomized to four groups: unirradiated control, conventional irradiation of entire tumor, GRID therapy, and hemi-irradiation (half-beam block, 50% tumor volume treated). All irradiated mice received a single fraction of 15 Gy.Results. High peak-to-valley ratios were achieved (bar GRIDs: 11.9 ± 0.9, 13.6 ± 0.4, 13.8 ± 0.5; pencil-beam GRIDs: 18.7 ± 0.6, 26.3 ± 1.5, 31.0 ± 3.3). Pencil-beam GRIDs could theoretically spare more intra-tumor immune cells than bar GRIDs, but they treat less tumor tissue (3%-4% versus 19%-23% area receiving 90% prescription, respectively). Bar GRID and hemi-irradiation treatments significantly delayed tumor growth (P < 0.05), but not as much as a conventional treatment (P < 0.001). No significant difference was found in tumor growth delay between GRID and hemi-irradiation.Significance. High peak-to-valley ratios were achieved with kV grids: two-to-five times higher than values reported in literature for MV grids. GRID irradiation and hemi-irradiation delayed tumor growth, but neither was as effective as conventional whole tumor uniform dose treatment. Single fraction GRID therapy could not initiate an anti-cancer immune response strong enough to match conventional RT outcomes, but follow-up studies will evaluate the combination of mini-GRID with immune checkpoint blockade.

Immune-Mediated Effects of Microplanar Radiotherapy with a Small Animal Irradiator

Spatially fractionated radiotherapy has been shown to have effects on the immune system that differ from conventional radiotherapy (CRT). We compared several aspects of the immune response to CRT relative to a model of spatially fractionated radiotherapy (RT), termed microplanar radiotherapy (MRT). MRT delivers hundreds of grays of radiation in submillimeter beams (peak), separated by non-radiated volumes (valley). We have developed a preclinical method to apply MRT by a commercial small animal irradiator. Using a B16-F10 murine melanoma model, we first evaluated the in vitro and in vivo effect of MRT, which demonstrated significant treatment superiority relative to CRT. Interestingly, we observed insignificant treatment responses when MRT was applied to Rag-/- and CD8-depleted mice. An immuno-histological analysis showed that MRT recruited cytotoxic lymphocytes (CD8), while suppressing the number of regulatory T cells (Tregs). Using RT-qPCR, we observed that, compared to CRT, MRT, up to the dose that we applied, significantly increased and did not saturate CXCL9 expression, a cytokine that plays a crucial role in the attraction of activated T cells. Finally, MRT combined with anti-CTLA-4 ablated the tumor in half of the cases, and induced prolonged systemic antitumor immunity.