Tumor physiological changes during hypofractionated stereotactic body radiation therapy assessed using multi-parametric magnetic resonance imaging

Abstract

Radiation therapy is a primary treatment for non-resectable lung cancer and hypoxia is thought to influence tumor response. Hypoxia is expected to be particularly relevant to the evolving new radiation treatment scheme of hypofractionated stereotactic body radiation therapy (SBRT). As such, we sought to develop non-invasive tools to assess tumor pathophysiology and response to irradiation. We applied blood oxygen level dependent (BOLD) and tissue oxygen level dependent (TOLD) MRI, together with dynamic contrast enhanced (DCE) MRI to explore the longitudinal effects of SBRT on tumor oxygenation and vascular perfusion using A549 human lung cancer xenografts in a subcutaneous rat model. Intra-tumor heterogeneity was seen on multi-parametric maps, especially in BOLD, T2* and DCE. At baseline, most tumors showed a positive BOLD signal response (%ΔSI) and increased T2* in response to oxygen breathing challenge, indicating increased vascular oxygenation. Control tumors showed similar response 24 hours and 1 week later. Twenty-four hours after a single dose of 12 Gy, the irradiated tumors showed a significantly decreased T2* (-2.9±4.2 ms) and further decrease was observed (-4.0±6.0 ms) after 1 week, suggesting impaired vascular oxygenation. DCE revealed tumor heterogeneity, but showed minimal changes following irradiation. Rats were cured of the primary tumors by 3x12 Gy, providing long term survival, though with ultimate metastatic recurrence.

Keywords: blood oxygen level dependent (bold); dynamic contrast enhanced (dce); hypofractionated stereotactic body radiation therapy (sbrt); oxygen-sensitive mri; treatment response.

Figure 1. Hypofractionated SBRT caused tumor growth delay

A549 tumor growth curves for nine rats (Blue: control, n=3; Red: treatment, n=6). Tumors initially showed expected exponential growth. Three doses of 12 Gy irradiation (green arrows) caused effective control and shrinkage in most cases. The representative tumors used in Figures 2 and 3 are marked with red and blue *, respectively.

Figure 2. Oxygen sensitive MRI of a representative A549 lung tumor with respect to irradiation

(A) ΔSI curves showing BOLD, and (B) TOLD responses for three ROIs indicated on T₂w-image (blue tumor periphery, red tumor center and green adjacent muscle). Yellow dotted line indicates transition from air to oxygen breathing. (C) T₂w-images and overlaid parametric maps showing %ΔSI of BOLD and TOLD, T₂* and T₁ maps (left to right) of tumor (0.5 cm³) before, 24 hours and one week after 1^st dose of radiation (12 Gy).

Figure 3. Oxygen sensitive MRI of a representative control A549 lung tumor

Upper graphs: BOLD (blue), TOLD (red) and T₂* (green) curves show response to oxygen breathing challenge of a control tumor at three time points: (A) baseline, (B) 24 hours, and (C) 1 week. Yellow dotted line indicates transition from air to oxygen breathing. (D) T₂w images of tumor (0.7 cm³) and overlaid parametric response maps showing %ΔSI of BOLD and TOLD, T₂* map and T₁ map (left to right) at baseline, 24 hours and one week.

Figure 4. Summary of changes of oxygen sensitive parameters for irradiated group

Graphs showing mean and standard errors of the mean pre IR, 24 hours and 1 week post radiation for tumors and adjacent muscle of the irradiated (n = 9 for preIR and 24 hours; n = 6 for 1 week) and control rats (n = 3). Irradiated tumor (blue), adjacent muscle (red) control tumor (green) and muscle (purple).* p<0.05 based on paired t-test.

Figure 5. DCE parametric maps of the representative tumor pre, 24 hours and 1 week after first dose of radiation treatment

T₂-weighted images and overlaid parametric response maps showing, K^trans, v_e, TTM, AUC and slope (left to right) of the same tumor as Figure 2 pre, 24 hours and one week after first radiation therapy. Distinct heterogeneity is observed for all the maps. No obvious changes were observed following radiation compared to baseline.

Figure 6. DCE dynamic curves and H&E staining revealed intra-tumor heterogeneity in both control and irradiated subcutaneous A549 human lung tumor xenografts

Upper panel: control tumor (A) ROIs of tumor periphery, tumor center and muscle overlaid on T₂w anatomical image of subcutaneous A549 human lung tumor xenograft. (B) DCE signal intensity curves of tumor periphery (blue region excluding green), tumor center (green) and thigh muscle (red) regions normalized to their mean baseline. Orange arrow indicates the time of Gd infusion. (C) H&E stained section of the same tumor. (D) Expanded region (black box). Arrow indicates a capillary in the connective tissue. Lower panel: tumor 24 hours after single dose of 12 Gy. (E) T₂w MRI showing ROIs. (F) corresponding DCE signal intensity curves. (G) H&E stained section.

Figure 7. IHC of control and irradiated subcutaneous A549 human lung tumor xenografts

Upper panel: Contiguous sections showing control tumor (A) vasculature based on CD31 immunohistochemistry, (B) tumor perfusion based on Hoechst 33342 distribution, and (C) hypoxia based on immunohistochemistry for perfused pimonidazole. Lower panel: (D-F) corresponding whole mount sections from tumor 24 hours after single dose of 12 Gy. Magnified sections (white box in D) are shown in (G, H) and (I).