microCT Imaging and analysis for radiation-mediated microvascular damage in orthotopic glioblastoma multiforme

Objectives To evaluate the impact of radiation treatment on tumor microvessel integrity using contrast-enhanced microCT in a xenograft model of Glioblastoma Multiforme (GBM).

Methods Orthotopic U87-MG GBM xenografts were established in female Nu-Fox1nu mice and treated daily with 2 Gy X-irradiation (RT) for 7 consecutive days to a total of 14 Gy using two treatment regimens - a single 200 cGy fraction or 10 pulses of 20 cGy over 45 minutes (PLDRT). Tumor response was volumetrically determined using contrast-enhanced microCT (Omnipaque 350) using a GE FLEX Triumph combined PET-SPECT-CT system with CT energy set to either 50kVp/800μA or 80kVp/250μA. Raw microCT data were reconstructed to enhance tumor contrast and density values were converted to Houndsfeld Units (HU) for densitometric analysis.

Results Control animals and those treated with conventional RT displayed a pronounced increase in both mean and maximum HU tumor contrast enhancement post-treatment all the way to sacrifice. The pulsed RT regimen resulted in a substantial reduction in contrast enhancement as compared to controls and conventional RT (p=0.022-0.025, respectively). These observations were confirmed with H&E histology and antibody specific immunohistochemistry staining of blood vessel density and angiogenesis using CD31.

Conclusions This pre-clinical study demonstrated that PLDRT radiotherapy has a protective effect with respect to microvessel integrity, as compared to controls and conventional RT. Moreover, contrast-enhanced microCT has proven to be a reliable surrogate for vascular integrity since increases in contrast-enhacement is only seen where there is damaged vasculature. Such an imaging protocol could be integrated with evolving technologies that permit proper adaptive radiation treatment planning using a collimated beam for small animal work, based on microvessel integrity.

Research Support Adaptive Oncology Imaging Suite, Department of Radiation Oncology, William Beaumont Health Syste