Report from ASCO: FDG-PET predicts early metastatic colorectal cancer response to chemotherapy

FDG-PET continues to gain ground as a potential go-to modality for assessing postchemotherapy response, Belgian researchers reported in a poster at the 2009 American Society of Clinical Oncology meeting.

Their clinical experience suggests serial FDG-PET can find with near 100% certainty after a single course of chemotherapy if a patient with metastatic colorectal cancer is not responding positively to treatment, said lead author Dr. Alain Hendlisz.

"Our results show that if tumor metabolism does not respond after 14 days, the patient is not likely to experience tumor shrinkage two or three months later on. Finding this out early on in treatment can help us avoid unnecessary side effects and also allows us to try another type of therapy sooner, if possible, to optimize results for our patients," Hendlisz said.

Hedlisz is head of the gastroenterology unit at the Medical Oncology Clinic at Institut Jules Bordet at the Free University of Brussels in Belgium. Hendlisz and colleagues undertook the study because they were dissatisfied with current monitoring methods.

"We wanted to determine sooner than with the classic CT scan or MRI whether the patient would be likely to benefit or not from the chemotherapy," he said.

His group hypothesized that metabolic changes seen by comparing quantitative FDG-PET scans acquired before and two weeks after the first dose of chemotherapy would be predictive of standard morphological response for patients with advanced colorectal cancer. The prospective study compared early metabolic changes as measured by serial FDG-PET with morphological changes assessed by standard RECIST criteria using multislice CT.

Hendlisz presented interim results in 28 patients (median age, 65.9 years) with a total of 88 lesions, which were available for comparative metabolic and morphologic analysis. The mean number of lesions per patient was three, with a range of one to eight. The lesions were all visible and individualized on both analyses and measured at least 15 mm diameter on baseline CT.

The chemotherapy regimens consisted of FOLFOX in 18 patients, FOLFIRI in nine patients, and capecitabine in one patient. Nineteen patients were receiving first-line chemotherapy, and nine were receiving second-line treatment.

In accordance with European Organisation for Research and Treatment of Cancer recommendations, an early metabolic response was defined as a 15% or greater decrease of the standardized FDG uptake (SUV_max) on day 14 after the first chemotherapy dose. A patient was deemed to have overall metabolic responsive disease if most or all of the lesions observed on the baseline PET showed a metabolic response, without any progressive lesion (new or ≥25% increase of _max).

The metabolic assessment showed 49 (56%) responding and 39 (44%) nonresponding lesions, according to the results. The morphological response rate in the metabolically nonresponding lesions (5/39, 13%) was lower than in responding lesions (22/49, 45%). A heterogenic response, combining responding and nonresponding lesions in the same patient, was observed in 21 of 28 (75%) patients.

The authors noted that early metabolic response after one course of chemotherapy correlated with standard RECIST evaluation from CT performed six to eight weeks after the initiation of chemotherapy. A positive RECIST response was observed in six of 14 (43%) patients experiencing a metabolic response based on PET scans and in zero of 14 (0%) metabolically nonresponding patients.

"FDG-PET seems able to identify advanced colorectal cancer that is unlikely to show a morphological response to chemotherapy, and it does so with a predictive value of 100%," Hendlisz said.

This ongoing study will ultimately include 45 patients with advanced colorectal cancer undergoing either first- or second-line chemotherapy.