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Longitudinal response monitoring in experimental prostate tumors by DCE-MRI after C-12 ion and photon radiotherapy

Alina Leandra Bendinger¹, Christian Peter Karger^2,3, Charlotte Debus⁴, Ralf Omar Floca^3,5, Jürgen Debus^3,6, Jörg Peter¹, and Christin Glowa^2,3,6

¹Department of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany, ²Department of Medical Physics in Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany, ³Heidelberg Institute for Radiation Oncology (HIRO), National Center for Radiation Research in Oncology (NCRO), Heidelberg, Germany, ⁴Translational Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany, ⁵Division of Medical Image Computing, German Cancer Research Center (DKFZ), Heidelberg, Germany, ⁶Department of Radiation Oncology, University Hospital Heidelberg, Heidelberg, Germany

Synopsis

Dynamic contrast-enhanced MRI was used for the longitudinal monitoring of two well characterized experimental prostate tumors (Dunning R3327-AT1 and -HI) after isodose and isoeffective radiotherapy. The effect of carbon (¹²C)-ion irradition compared to photons on tumor vasculature was characterized by non-compartmental analysis and pharmacokinetic modelling employing the extended Tofts model. Isodose and isoeffective irradiation experiments indicated that the beam modality has a stronger effect on tumor perfusion and permeability than the dose. While changes in perfusion were identified for the highly undifferentiated AT1-tumor, the more differentiated HI-tumor showed only minor changes in perfusion upon irradiation.

Purpose

Carbon (¹²C)-ion radiotherapy (RT) provides enhanced biological effectiveness relative to photon RT, especially for the treatment of radioresistant tumors ^1,2. In this longitudinal study, we employ dynamic contrast-enhanced MRI (DCE-MRI) to compare treatment responses of two well characterized experimental tumors after ¹²C-ion and photon RT.

Methods

The anaplastic (AT1) and the more differentiated (HI) subline of the Dunning prostate tumor R3327 ³ were transplanted subcutaneously to both thighs of male Copenhagen rats (AT1: 12 rats, HI: 16 rats). When tumors attained diameters of 10 mm, one tumor per animal was irradiated with a single dose of either photons (6 MeV) or ¹²C-ions (20 mm spread-out Bragg-peak), while the other served as untreated control. AT1-tumors received 37 Gy photons or ¹²C-ions for isodose experiments or 16 Gy ¹²C-ions or 85 Gy photons as isoeffective doses to 37 Gy photons and ¹²C-ions, respectively. The HI-tumors were irradiated with 37 Gy isodoses as well as with 18 Gy ¹²C-ions or 75 Gy photons (isoeffective treatments). DCE-MRI (TURBO-FLASH sequence, temporal resolution: 0.75s, 380s total acquisition) was performed one day before and 3, 7, 14, and 21 days after irradiation (Fig. 1). Image signals were converted to concentration by means of absolute signal enhancement. DCE-MRI data was analyzed voxel-wise using an in-house developed software ⁴ by non-compartmental analysis (normalized area-under-the-curve, AUC), mean residence time (MRT, defined as area under first moment curve) and by pharmacokinetic modelling (PKM) using the extended Tofts model (ETM) ⁵. The image based arterial input function was extracted individually for each animal from the left ventricle. Results were correlated with histology.

Results

3 days after irradiation, a clear increase in AUC-values was observed for the AT1-tumors, while the control-tumors exhibited decreasing AUC-values (Fig. 2a). MRT elongated distinctly for control-AT1-tumors and slightly also for photon-irradiated AT1-tumors (Fig. 2c). Low AUC-tumor-areas corresponded with high MRT-areas (Fig. 3). 21 days after irradiation both, photon- and ¹²C-ion-treated groups, exhibited dose-independently similar values for AUC and MRT, respectively. For HI-tumors AUC-values remained constant while the MRT of treated tumors elongated slightly upon 7 days after treatment (Fig. 2b,d). Pharmacokinetic analysis with ETM showed a decrease in the transfer constant K^trans for AT1-control-tumors while ¹²C-ion-treated tumors exhibited increasing K^trans (Fig. 4a). Photon-treated AT1-tumors only exhibited small changes in K^trans. HI-tumors showed less response: K^trans-values decreased for control- and photon-treated tumors while they remained constant for ¹²C-ion-treated tumors (Fig. 4b). The plasma volume fraction v_p, was elevated for ¹²C-ion-treated AT1-tumors at day 21. The interstitial volume v_e, remained constant for AT1-control-tumors and showed an increase for photon and ¹²C-ion-treated tumors. For the HI-tumors an overall increase in v_p was observed 3 days after treatment which increased further for ¹²C-ion-treated tumors and returned to initial values for the control and photon-treated tumors. Comparable course of parameters was observed for v_e. 21 days after treatment all fit parameters exhibited maxima for ¹²C-ion-treated tumors. Stainings for CD31 and smooth muscle actin (SMA) revealed that irradiated AT1-tumors exhibited less but CD31⁺/SMA⁺ co-expressing vessels compared to control-tumors with a majority of small C31⁺/SMA^- capillaries 21 days after treatment (Fig 5) ⁶. At the end point, HI-tumors exhibited overall larger vessels with less SMA and decreased number of vessels compared to controls.

Discussion and conclusion

Low AUC- and high MRT-values in AT1-control-tumors indicate necrotic parts in the centers of the fast growing tumors induced by immature capillaries. At the same time, PKM reveals increased permeability/flow of remaining vessels in the ¹²C-ion-treated AT1-tumors indicated by enhanced K^trans-values. HI-tumors showed overall fewer differences with respect to irradiation modality and dose during the 21 days observation period. Constant AUC-values and a mild elongation of MRT indicate only minor changes in tumor vasculature after treatment. For isodose experiments, ¹²C-ion-treated tumors exhibited larger changes in ETM-fit parameters indicating a stronger effect of ¹²C-ion-RT on tumor vasculature ⁷. For both tumor sublines, increased v_e-values reflect the increasing stromal part of the tumors due to irradiation induced cell death.

Due to the AT1-tumor’s poor vascularization the ETM was chosen for pharmacokinetic analysis ⁸. One drawback of the ETM is the ambiguity of the parameter K^trans which reflects both plasma flow and the capillary permeability. Due to different morphology in HI-tumors a different PK model might be more suitable (e.g. two compartment exchange model) to reveal radiation-induced changes in tissue perfusion.

DCE-MRI allows characterization of permeability changes in tumors after irradiation. Isodose and isoeffective treatments further indicate that beam modality has a stronger impact on perfusion and permeability for AT1- than for HI-tumors.

Acknowledgements

This work was supported by the German Research Foundation (DFG, KFO 214, GL 893/1-1 and KA2679/3-1). The authors are grateful to the staff of the Heidelberg Ion Therapy Center (HIT) for providing excellent working conditions.

References

1. Uhl M, Herfarth K, Debus J, Comparing the use of protons and carbon ions for treatment. The Cancer Journal, 2014;20(6): 433 - 439.

2. Glowa C, Karger C, Brons S, et al., Carbon ion radiotherapy decreases the impact of tumor heterogeneity on radiation response in experimental prostate tumors. Cancer Lett, 2016;378(2): 97-103.

3. Isaacs J, Heston W, Weissman R, Coffey D, Animal model of the hormone-sensitive and -insensitive prostatic adenocarcinomas, Dunning R-3327-H, R3327-HI, and R3327-AT1. American Association for Cancer Research, 1978;38: 4353 - 4359.

4. Nolden M, Zelzer S, Seitel A, et al., The Medical Imaging Interaction Toolkit: challenges and advances. International Journal of Computer Assisted Radiology and Surgery, 2013;8(4): 607-620.

5. Sourbron S and Buckley D, Classic models for dynamic contrast-enhanced MRI. NMR Biomed, 2013;26(8): 1004-27.

6. Glowa C, In vivo Untersuchungen zur Wirksamkeit von Bestrahlungen mit Kohlenstoffionen am syngenen Prostatakarzinom Modell R3327, PhD Thesis 2013, University of Heidelberg.

7. Glowa C, Peschke P, Brons S, et al., Carbon ion radiotherapy: impact of tumor differentiation on local control in experimental prostate carcinomas. Radiation Oncology, 2017; accepted for publication DOI:10.1186/s13014-017-0914-9.

8. Sourbron S and Buckley D, On the scope and interpretation of the Tofts models for DCE-MRI. Magn Reson Med, 2011;66(3): 735-45.

Figures

Schematic illustration of the experiment: tumors were transplanted onto both thighs of male Copenhagen rats. The right tumor was treated, the left one served as an untreated control. Animals were imaged by DCE-MRI one day before and 3, 7, 14, and 21 days after either isodose or isoeffective irradiation by photons or ¹²C-ions (indicated by yellow flash).

(a)-(b) Normalized AUC-values and (c)-(d) mean residence time (MRT) for AT1-tumors (left column) and HI-tumors (right column) for control and treated tumors. AUC-values were normalized to the AUC-value of a prominent abdominal vessel to eliminate dependencies on contrast agent delivery. Results are depicted as medians and 25/75 percentiles.

Exemplary parameter maps for one AT1- and one HI-tumor from the 37 Gy ¹²C-ion treated group displaying AUC, MRT, K^trans, v_e, and v_p for first and last time points of the experiments. Tumors are indicated by white arrows: upper tumor is treated, lower tumor is untreated control.

Fit Parameters from ETM for (a) two AT1-tumors and (b) two HI-tumors treated with 37 Gy photons (red) or 37 Gy ¹²C-ions (green). Results are depicted as medians and 25/75 percentiles.

Immunohistochemistry stainings of AT1-tumors for CD31 and SMA 21 days after irradiation. Endothelial cells were stained for CD31 cell surface marker (red) and SMA marks pericytes of mature vessels (green). Cell nuclei were counterstained with DAPI (blue). In the control-tumor only large vessels are CD31⁺/SMA⁺ while many small capillaries are only CD31⁺. In irradiated tumors, also small vessels exhibit CD31⁺/SMA⁺ co-expression. Scale bar: 50 μm. ⁶

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)

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