Tumor R2 is a prognostic indicator of acute radiotherapeutic response in rodent tumors

• Published in: Journal of magnetic resonance imaging Vol. 19; no. 4; pp. 482 - 488
• Main Authors: Rodrigues, Loreta M., Howe, Franklyn A., Griffiths, John R., Robinson, Simon P.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc., A Wiley Company 01-04-2004
• Subjects: Animals; BOLD MRI; carbogen; Carbon Dioxide - administration & dosage; Female; Fibrosarcoma - metabolism; Fibrosarcoma - pathology; Fibrosarcoma - radiotherapy; Image Processing, Computer-Assisted; Magnetic Resonance Imaging - methods; Mice; Mice, Inbred C3H; Neoplasm Transplantation; Oxygen - administration & dosage; Oxygen - metabolism; oxygenation; Prognosis; Prolactinoma - metabolism; Prolactinoma - pathology; Prolactinoma - radiotherapy; Radiation-Sensitizing Agents - administration & dosage; radiotherapy; Radiotherapy Dosage; Rats; Rats, Inbred WF; tumor
• ISSN: 1053-1807; 1522-2586
• DOI: 10.1002/jmri.20024

Purpose
To test the prognostic potential of tumor R2* with respect to radiotherapeutic outcome. Blood oxygenation level dependent (BOLD) MRI images are sensitive to changes in deoxyhemoglobin concentration through the transverse MRI relaxation rate R2* of tissue water, hence the quantitative measurement of tumor R2* may be related to tissue oxygenation.
Methods and Materials
Tumor growth inhibition in response to radiation was established for both GH3 prolactinomas and RIF‐1 fibrosarcomas with animals breathing either air or carbogen during radiation. In a separate cohort, the baseline R2* and carbogen (95% O2, 5% CO2)‐induced ΔR2* of rat GH3 prolactinomas and murine RIF‐1 fibrosarcomas were quantified using multigradient echo (MGRE) MRI prior to radiotherapy, and correlated with subsequent tumor growth inhibition in response to ionizing radiation, while the animals breathed air.
Results
A radiation dose of 15 Gy caused pronounced growth delay in both tumor models and transient regression of the GH3 prolactinomas. When the animals breathed carbogen during radiation, the growth delay/regression was enhanced only in the GH3 prolactinomas. The GH3 prolactinomas, which exhibit a relatively fast baseline R2* and large ΔR2* in response to carbogen breathing prior to radiotherapy, showed a substantial reduction in normalized tumor volume to 66 ± 3% with air breathing and 36 ± 5% with carbogen seven days after 15 Gy irradiation. In contrast, the effect of 15 Gy on the RIF‐1 fibrosarcomas, which give a relatively slow baseline R2* and negligible ΔR2* response to carbogen prior to treatment, showed a much smaller growth inhibition (143 ± 3% with air, 133 ± 12% with carbogen).
Conclusion
Quantitation of tumor R2* and carbogen‐induced ΔR2* by MGRE MRI provides completely noninvasive prognostic indicators of a potential acute radiotherapeutic response. J. Magn. Reson. Imaging 2004;19:482–488. © 2004 Wiley‐Liss, Inc.