Detection of Treatment Success after Photodynamic Therapy Using Dynamic Contrast-Enhanced Magnetic Resonance Imaging

Abstract

Early evaluation of response to therapy is crucial for selecting the optimal therapeutic follow-up strategy for cancer patients. PDT is a photochemistry-based treatment modality that induces tumor tissue damage by cytotoxic oxygen radicals, generated by a pre-injected photosensitive drug upon light irradiation of tumor tissue. Vascular shutdown is an important mechanism of tumor destruction for most PDT protocols. In this study, we assessed the suitability of Dynamic Contrast-Enhanced Magnetic Resonance Imaging (DCE-MRI) to evaluate treatment efficacy within a day after photodynamic therapy (PDT), using the tumor vascular response as a biomarker for treatment success. Methods: DCE-MRI at 7 T was used to measure the micro-vascular status of subcutaneous colon carcinoma tumors before, right after, and 24 h after PDT in mice. Maps of the area under the curve (AUC) of the contrast agent concentration were calculated from the DCE-MRI data. Besides, tracer kinetic parameters including K^trans were calculated using the standard Tofts-Kermode model. Viability of tumor tissue at 24 h after PDT was assessed by histological analysis. Results: PDT led to drastic decreases in AUC and K^trans or complete loss of enhancement immediately after treatment, indicating a vascular shutdown in treated tumor regions. Histological analysis demonstrated that the treatment induced extensive necrosis in the tumors. For PDT-treated tumors, the viable tumor fraction showed a strong correlation (ρ ≥ 0.85) with the tumor fraction with K^trans > 0.05 min^-1 right after PDT. The viable tumor fraction also correlated strongly with the enhanced fraction, the average K^trans , and the fraction with K^trans > 0.05 min^-1 at 24 h after PDT. Images of the viability stained tumor sections were registered to the DCE-MRI data, demonstrating a good spatial agreement between regions with K^trans > 0.05 min^-1 and viable tissue regions. Finally, 3D post-treatment viability detection maps were constructed for the tumors of three mice by applying a threshold (0.05 min^-1) to K^trans at 24 h after PDT. As a proof of principle, these maps were compared to actual tumor progression after one week. Complete tumor response was correctly assessed in one animal, while residual viable tumor tissue was detected in the other two at the locations where residual tumor tissue was observed after one week. Conclusion: This study demonstrates that DCE-MRI is an effective tool for early evaluation of PDT tumor treatment.

Keywords: Dynamic contrast-enhanced magnetic resonance imaging; mouse tumor model; photodynamic therapy; treatment response evaluation; tumor perfusion.

Figure 1

Representative examples of DCE-MRI-derived quantitative parameter maps of a fully and a partially treated tumor, at all three time points. For each time point, a T₂-weighted anatomical reference image, a T₁-weighted DCE-MRI frame at 1 min after contrast injection, an AUC map, a K^trans map, and a v_e map are shown for a central axial tumor slice. The v_e maps are only color-coded in pixels that were enhanced and satisfied the quality criteria for the tracer kinetic model fit. The tumor is outlined by a red contour. The color bar applies to all quantitative maps and represents the range of values for each parameter displayed in the text labels on the left.

Figure 2

Average tumor values of (a-c) DCE-MRI-derived parameters and (d) histology-derived viable fractions for the untreated, fully treated, and partially treated group. The legend next to (b) applies to all subfigures. Error bars represent one standard deviation. Asterisks within or above bars indicate statistically significant differences with respect to baseline values within a treatment group, based on a mixed ANOVA with Bonferroni post-hoc tests (* p < 0.05, ** p < 0.005). Similarly, lines with asterisks above the bar pairs indicate differences between treatment groups at the corresponding time point.

Figure 3

Examples of histologically stained tumor sections. The upper row shows NADH-diaphorase stained tumor sections of an untreated tumor, a fully treated tumor, and a partially treated tumor. Treated tumors were excised at 24 h after PDT. The approximate angle of light incidence during PDT is indicated by the arrows. For the partially treated tumor, the black cross roughly indicates the part of the light beam that was blocked by black paper covering the skin. The middle row shows magnifications of H&E, NADH-diaphorase, and CD31 stained sections of an untreated tumor. The fluorescence signal of the CD31 detection is shown in red, while the Hoechst 33342 dye injected 5 min before animal sacrifice is shown in blue. The bottom row shows the same types of images for a treated tumor.

Figure 4

Scatter plots of the histology-derived viable tumor fractions versus 4 DCE-MRI parameters: average AUC (a & e), enhanced tumor fraction (b & f), average K^trans (c & g), and tumor fraction with a K^trans value larger than 0.05 min^-1 (d & h). The DCE-MRI parameters obtained right after PDT (a-d) and 24 h after PDT (e-h) were both compared to the viable fractions determined at 24 h after PDT. Red, yellow, and blue points represent fully treated tumors, partially treated tumors, and controls, respectively. For each comparison, two linear regressions were calculated, either based on all tumors (solid lines), or on treated tumors only (dashed lines). Moreover, the Pearson's correlation coefficients are displayed for all tumors (ρ_all) and for the treated tumors only (ρ_PDT), together with the coefficient of determination (R²) of the corresponding regression lines.

Figure 5

NADH-diaphorase stained tumor sections, and AUC and K^trans maps of an entire tumor of (a) a fully treated animal and (b) a partially treated animal. Both histological sections and AUC map slices are ordered from distal to proximal, and rotated in the same orientation. The AUC and K^trans maps are only displayed within the tumor, as an overlay over the T₂-weighted images. For K^trans maps, the pixels without enhancement or low data quality are displayed in white. The color bar applies to both AUC and K^trans maps, for the value range displayed left to the maps. For the fully treated tumor, red arrows indicate viable tumor tissue and tissue with non-zero AUC and K^trans at the same location. For the partially treated tumor, red contours were drawn around the viable regions in 2 central slices. The contours were overlaid on the AUC and K^transmaps to emphasize the spatial correspondence. A similar correspondence can be observed in the other slices.

Figure 6

Data processing workflow for spatial comparison of DCE-MRI data and histology-derived viability data. For both datasets, a stack of sections or slices covering the entire tumor was available. The histology sections were segmented into viable (dark blue) and non-viable (light blue) tumor tissue by drawing manual ROIs. A 2D rigid transformation was then applied to each section, to register with the MRI-based tumor ROIs. Next, the registered tumor data was divided into 8 octants, defined by the 3 green planes through the tumor center. Within each octant, the DCE-MRI data were averaged and compared to the histology-derived fraction of the corresponding octant.

Figure 7

Comparison of four DCE-derived parameters to viable fractions in spatial tumor subsections. The following DCE-MRI parameters measured right after and 24 h after PDT are plotted against the viable fraction at 24 h: average AUC (a & e), enhanced tumor fraction (b & f), average K^trans (c & g), and tumor fraction with a K^trans value larger than 0.05 min^-1 (d & h). Each data point represents the average parameter value or the volume fraction within each tumor octant. Pooled data of all octants of all treated tumors are shown. Linear regression lines are also shown. The Pearson correlation coefficients and the R² of the regression lines are displayed above each plot.

Figure 8

Post-treatment viability detection maps calculated by applying a threshold of 0.05 min^-1 to K^trans at 24 h after PDT (a-c) and tumor photographs obtained at 1 week after PDT (d-f). The 3D visualized detection maps show the upper part of the hind limb in light gray, with the tumor in green and red. Red volumes indicate suspected viable tissue, while green volumes represent successfully treated tissue. The black arrows indicate the positions of tumor recurrence. Apart from these recurrences, the tumors consisted of dry necrotic tissue.