Comparative toxicity of synchrotron and conventional radiation therapy based on total and partial body irradiation in a murine model

Abstract

Synchrotron radiation can facilitate novel radiation therapy modalities such as microbeam radiation therapy (MRT) and high dose-rate synchrotron broad-beam radiation therapy (SBBR). Both of these modalities have unique physical properties that could be exploited for an improved therapeutic effect. While pre-clinical studies report promising normal tissue sparing phenomena, systematic toxicity data are still required. Our objective was to characterise the toxicity of SBBR and MRT and to calculate equivalent doses of conventional radiation therapy (CRT). A dose-escalation study was performed on C57BLJ/6 mice using total body and partial body irradiations. Dose-response curves and TD₅₀ values were subsequently calculated using PROBIT analysis. For SBBR at dose-rates of 37 to 41 Gy/s, we found no evidence of a normal tissue sparing effect relative to CRT. Our findings also show that the MRT valley dose, rather than the peak dose, best correlates with CRT doses for acute toxicity. Importantly, longer-term weight tracking of irradiated animals revealed more pronounced growth impairment following MRT compared to both SBBR and CRT. Overall, this study provides the first in vivo dose-equivalence data between MRT, SBBR and CRT and presents systematic toxicity data for a range of organs that can be used as a reference point for future pre-clinical work.

Figure 1

Dose response curves following conventional radiation therapy (CRT), microbeam radiation therapy (MRT) and high dose-rate synchrotron broad-beam radiation therapy (SBBR) for (A) total body irradiation (TBI), (B) abdominal partial body irradiation (PBI) and (C) head PBI. The horizontal TD₅₀ line indicates the dose predicted to cause toxicity (>15–20% weight loss, severe diarrhoea, moribund behaviour) in 50% of the animals. For all three irradiation sites, there was no significant difference in TD₅₀ values between CRT and SBBR. The peak MRT TD₅₀ dose was an order of magnitude higher for each irradiation site. Dose response curves were generated using PROBIT analysis with N = 3–4 doses per modality and N = 4–5 mice per dose.

Figure 2

Post-irradiation weight gain for surviving mice. Weight gain is measured by a percentage change in weight compared to pre-experimental weight following (A) total body irradiation (TBI) 60 days post-irradiation, (B) abdominal partial body irradiation (PBI) 60 days post-irradiation, (C) head PBI 37 days post-irradiation and, (D) thoracic PBI 140 days following irradiation. Mice had subnormal weights compared to controls following irradiation regardless of the modality used or irradiation site. Mice in the high dose-rate synchrotron broad-beam radiation therapy (SBBR) and microbeam radiation therapy (MRT) groups had the most significant growth impairment. There was no statistically significant difference in weight gain between SBBR and conventional radiation therapy (CRT) at near equal doses, except for following TBI, with the 5.4 Gy SBBR group having significantly less weight gain compared to 5.1 Gy CRT. Differences between groups were analysed using ANOVA with N = 2–5 surviving mice per dose/modality; *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001.

Figure 3

Intestinal histopathology following abdominal partial body irradiation. (A) Hematoxylin and eosin (HE) stained sections of small intestine from surviving mice showed relatively normal crypt villus architecture 60 days following irradiation with the exception of the 249 Gy microbeam radiation therapy (MRT) group. Short arrows point to regions where the sub-epithelial space has been extended and long arrows show where the epithelial cells of the villus have completely lifted off from the underlying lamina propria. For the MRT groups, valley doses are indicated in parentheses. (B) HE stained sections of small intestine from mice that were euthanized due to acute gastrointestinal syndrome showed a loss of normal crypt-villus architecture.

Figure 4

Histopathological changes to the cerebellum following microbeam radiation therapy (MRT). Top panels depict hematoxylin and eosin stained cerebellar sections and the lower panels depict magnified images corresponding to regions of interest (boxed). Wide, evenly spaced bands of cerebellar granular cell loss (short arrows), corresponding with microbeam paths, were evident following 455 Gy MRT and was associated with neurotoxicity within two to four hours of irradiation. Narrow bands of granular layer scarring (long arrows) were evident at 38 days following 377 Gy MRT in surviving mice.

Figure 5

Pulmonary damage and fibrosis 170 to 180 days following thoracic partial body irradiation. (A) Hematoxylin and eosin stained lung sections showed severe pulmonary damage, including alveolar destruction, airspace enlargement (asterisks) and the thickening of alveolar walls (short arrows), in all dose groups for each modality. For the microbeam radiation therapy groups, valley doses are indicated in parentheses. (B) Masson’s Trichrome staining revealed the deposition of collagen fibres (blue) in sub-pleural (short arrows) and intra-parenchymal (asterisks) regions.

Figure 6

Mouse positioning for irradiation. (A) For microbeam radiation therapy at the Imaging and Medical Beamline, microbeams were orientated vertically and therefore in parallel with respect to the superior-inferior plane of mice. Microbeam width and spacing are not to scale in this beams-eye-view diagram. (B) Radiographic imaging shows mice positioned vertically in the path of radiation. Mice were gently strapped to a plastic holder, with support provided by small positioning pegs. Superior and inferior field borders for the partial body irradiations are denoted by overlayed dotted lines.