CONTEXT: The way cancer cells metabolize amino acids is associated with numerous catabolic processes favoring tumor growth. As a result, amino acid L-methionine (MET) plays a central role in the altered metabolism of cancer cells. Senior medical scientist Dr. Hong Zhang and colleagues the National Institute of Radiological Sciences in Chiba, Japan, hypothesized that MET uptake rates measured with carbon-11 PET decrease in cancer patients who respond positively to carbon ion radiotherapy (CIRT).
RESULTS: Sixty-two patients with unresectable bone or soft-tissue sarcoma underwent baseline whole-body C-11 MET PET before CIRT and again in the three weeks after radiotherapy. The median time delay between treatment and follow-up imaging was 14 days. Median patient survival was 20 months.
A change in MET uptake of greater than 30% in a comparison of post-versus-baseline scans was the strongest predictor of survival, Zhang said. About 75% of patients at or above the threshold lived for at least two years (p = 0.049). Only about 42% of patients below the threshold lived that long.
Nearly 70% of patients with a baseline MET tumor-to-normal tissue (T/N) ratio equal to or less than six survived for more than two years, while only about 33% of the patients with MET T/N ratios above that threshold survived more than two years (p = 0.01)
About 64% of patients with a T/N ratio less than or equal to 4.4 in the post-therapy MET PET studies lived for at least two years following therapy, while 41% of patients with T/N ratios above that threshold survived at least two years (p = 0.01).
Even stronger correlations may have been observed if more time between radiotherapy and follow-up imaging had been allowed, Zhang said.
IMAGE: A: CT reveals a large mass in the left pelvis of a 28-year-old woman who received CIRT aimed at left iliac osteosarcoma. B: Baseline MET PET before CIRT confirms presence of the mass. C: MRI after CIRT still depicts a large mass. D: MET uptake observed with PET decreases significantly after CIRT, suggesting a positive therapeutic response (Provided by H. Zhang).
IMPLICATIONS: Findings presented at the 2003 RSNA meeting indicate that C-11 MET uptake dependably predicts survival among patients with bone or soft-tissue sarcoma after carbon ion radiotherapy.