MRI response of obturator internus muscle to carbon-ion dose in prostate cancer treatment.

It is important to confirm the dose distribution and its biophysiological response in patients subjected to carbon-ion radiotherapy (CIRT) by using medical imaging methods. In this study, the correlation between the signal intensity changes of muscles observed in magnetic resonance imaging (MRI) after CIRT and planned dose distribution was evaluated. Seven patients were arbitrarily selected from among localized prostate cancer patients on whom CIRT was performed in our facilities in 2010. All subjects received the same dose of CIRT, namely, 57.6 Gy relative biological effectiveness (RBE) in 16 fractions. The following two types of images were acquired for each subject: planning computed tomography (CT) images overlaying the dose distribution of CIRT and MRI T2-weighted images (T2WI) taken 1 year after CIRT. The fusion image of the planning CT and MRI images was registered by using a treatment-planning system, and the CIRT dose distribution was compared with changes observed in the MRI of the obturator internus muscles located near the prostate. The signal changes in the axial image passing through the isocenter of the planning target volume were digitized, and a scatter diagram was created showing the relationship between the radiation dose and digitized signal changes. A strong correlation between the radiation dose and the MRI signal intensity changes was observed, and a quadratic function was found to have the best fit. However, estimating the dose distribution from the normalized MRI signal intensity is difficult at this point, owing to the wide variation. Therefore, further investigation is required.