The purpose of this study was to characterize changes in tumor vascular parameters hours after a single radiation exposure in an orthotopic brain tumor model. U-251 human brain tumors were established intracerebrally in rat brains, and tumor blood flow, forward volume transfer constant (K(trans)) and interstitial volume fraction (v(e)) were measured using magnetic resonance imaging (MRI). Tumors were exposure to a single stereotactic radiation treatment of 20 Gy. Vascular parameters were assessed one additional time between 2 and 24 h after irradiation. After the second MRI session, brain tissue histology was examined for gross changes and apoptosis. In separate studies, cerebral blood flow was measured in nonimplanted controls before radiation exposure and 2 and 24 h after 20 Gy irradiation, and in implanted rats before radiation exposure and at 2 and 24 h after 6 Gy irradiation. Significant changes were observed in tumor-bearing rat brains in the hours after 20 Gy irradiation. Two hours after 20 Gy irradiation, tumor blood flow decreased nearly 80% and ve decreased by 30%. At 4 h, the K(trans) increased by 30% over preirradiation values. Extensive vacuolization and an increase in apoptosis were evident histologically in rats imaged 2 h after irradiation. Between 8 and 12 h after irradiation, all vascular parameters including blood flow returned to near preirradiation values. One day after irradiation, tumor blood flow was elevated 40% over preirradiation values, and other vascular parameters, including K(trans) and ve, were 20-40% below preirradiation values. In contrast, changes in vascular parameters observed in the normal brain 2 or 24 h after 20 Gy irradiation were not significantly different from preirradiation values. Also, tumor blood flow appeared to be unchanged at 2 h after 6 Gy irradiation, with a small increase observed at 24 h, unlike the tumor blood flow changes after 20 Gy irradiation. Large and significant changes in vascular parameters were observed hours after 20 Gy irradiation using noninvasive MRI techniques. It is hypothesized that cellular swelling hours after a high dose of radiation, coinciding with vacuolization, led to a decrease in tumor blood flow and v(e). Four hours after radiation exposure, K(trans) increased in concert with an increase in tumor blood flow. Vascular permeability normalized, 24 h after 20 Gy irradiation, as characterized by a decrease in K(trans). Vascular parameters did not change significantly in the normal brain after 20 Gy irradiation or in the tumor-bearing brain after 6 Gy irradiation.

This research suggests that MRI dynamic studies contain enough information to noninvasively estimate TIFP in this, and possibly other, tumor models, and thus might be used to assess tumor aggressiveness and response to therapy.

MRI estimates of extracellular volume and tumor exudate flux in peritumoral tissue are demonstrated in an experimental model of cerebral tumor. Peritumoral extracellular volume predicted the tumor exudate flux. Eighteen RNU athymic rats were inoculated intracerebrally with U251MG tumor cells and studied with dynamic contrast-enhanced MRI (DCE-MRI) approximately 18 days post-implantation. Using a model selection paradigm and a novel application of Patlak and Logan plots to DCE-MRI data, the distribution volume (i.e., tissue porosity) in the leaky rim of the tumor, and in the tissue external to the rim (the outer rim), was estimated, as was the tumor exudate flow from the inner rim of the tumor through the outer rim. Distribution volume in the outer rim was approximately half that of the inner adjacent region (p < 1×10−4). The distribution volume of the outer ring was significantly correlated (R2 = 0.9) with tumor exudate flow from the inner rim. Thus, peritumoral extracellular volume predicted the rate of tumor exudate flux. One explanation for these data is that perfusion, i.e., the delivery of blood to the tumor, was regulated by the compression of the mostly normal tissue of the tumor rim, and that the tumor exudate flow was limited by tumor perfusion.

In this study we used magnetic resonance imaging (MRI) biomarkers to monitor the acute temporal changes in tumor vascular physiology with the aim of identifying the vascular signatures that predict response to combined anti-angiogenic and radiation treatments. Forty-three athymic rats implanted with orthotopic U-251 glioma cells were studied for approximately 21 days after implantation. Two MRI studies were performed on each animal, pre- and post-treatment, to measure tumor vascular parameters. Two animal groups received treatment comprised of Cilengitide, an anti-angiogenic agent and radiation. The first group received a subcurative regimen of Cilengitide 1 h before irradiation, while the second group received a curative regimen of Cilengitide 8 h before irradiation. Cilengitide was given as a single dose (4 mg/kg; intraperitoneal) after the pretreatment MRI study and before receiving a 20 Gy radiation dose. After irradiation, the post-treatment MRI study was performed at selected time points: 2, 4, 8 and 12 h (n = ≥5 per time point). Significant changes in vascular parameters were observed at early time points after combined treatments in both treatment groups (1 and 8 h). The temporal changes in vascular parameters in the first group (treated 1 h before exposure) resembled a previously reported pattern associated with radiation exposure alone. Conversely, in the second group (treated 8 h before exposure), all vascular parameters showed an initial response at 2–4 h postirradiation, followed by an apparent lack of response at later time points. The signature time point to define the “synergy” of Cilengitide and radiation was 4 h postirradiation. For example, 4 h after combined treatments using a 1 h separation (which followed the subcurative regimen), tumor blood flow was significantly decreased, nearly 50% below baseline (P = 0.007), whereas 4 h after combined treatments using an 8 h separation (which followed the curative regimen), tumor blood flow was only 10% less than baseline. Comparison between the first and second groups further revealed that most other vascular parameters were maximally different 4 h after combined treatments. In conclusion, the data are consistent with the assertion that the delivery of radiation at the vascular normalization time window of Cilengitide improves radiation treatment outcome. The different vascular responses after the different delivery times of combined treatments in light of the known tumor responses under similar conditions would indicate that timing has a crucial influence on treatment outcome and long-term survival. Tracking acute changes in tumor physiology after monotherapy or combined treatments appears to aid in identifying the beneficial timing for administration, and perhaps has predictive value. Therefore, judicial timing of treatments may result in optimal treatment response.