11C-methionine-PET for evaluation of carbon ion radiotherapy in patients with pelvic recurrence of rectal cancer

Purpose: Progress of the novel carbon ion radiotherapy (CIRT) in the treatment of cancers has created the need for a method to accurately evaluate the response. We investigated whether L-[(11)C]methyl-methionine ((11)C-methionine) uptake at pre- and post-CIRT could be an early response predictor in patients with pelvic recurrence of rectal cancer. Procedures: (11)C-Methionine-positron emission tomography (PET) was performed prospectively in 53 patients with pelvic recurrence of rectal cancer before CIRT, and 48 patients were performed (11)C-methionine PET at 1 month after CIRT. (11)C-Methionine tumor uptake was measured by the tumor to muscle ratio (T/M ratio). The T/M ratios were evaluated in relation to clinical outcomes such as local re-recurrence, distant metastasis, and survival. The response to CIRT was also judged by computed tomography (CT) and magnetic resonance imaging (MRI). (11)C-Methionine PET judgment was compared with CT/MRI judgment regarding the relevance to clinical outcome. Results: Baseline T/M ratio was 5.27 +/- 1.90 (mean +/- SD) in patients without developing local re-recurrence and 7.66 +/- 3.17 in patients with local re-recurrence (p=0.023, Mann-Whitney U test). Post-CIRT T/M ratios were 3.10 +/- 1.28 in patients without local re-recurrence and 6.15 +/- 2.98 in patients with local re-recurrence (p=0.006, Mann-Whitney U test). By Kaplan-Meier analysis with log-rank test, patients with a baseline T/M ratio of <= 7.6 or a post-CIRT T/M ratio of <= 5.0 had significant lower pelvic re-recurrence rate. However, the percent change (reduction rate) from baseline to post-CIRT T/M ratio did not have significant relation to pelvic re-recurrence. There were no significant differences between (11)C-methionine results (baseline T/M ratio, post-CIRT T/M ratio and percent change) and other clinical parameters (distant metastasis and survival). Conclusion: (11)C-methionine-PET can be used for early prediction of local re-recurrence after CIRT. Because CIRT is local therapy, (11)C-methionine-PET cannot predict distant metastasis or survival after CIRT.