Better ways to interpret results have been gleaned from one of the first large human trials of fluorine-18 choline PET/CT for prostate cancer staging and management.
F-18 choline PET/CT has been the focus of intense research interest because of its unique ability to report information about tumor metabolism contained in the prostate gland and among metastases that have spread to pelvic nodes and bone, according to lead author Dr. Werner Langsteger, director of nuclear medicine and endocrinology at St. Vincent's Hospital in Linz, Austria.
Problems with false positives affect the technique, however. In a presentation at the Academy of Molecular Imaging meeting in March, Langsteger and colleagues pinpointed several factors that appear to create opportunities for false positives:
- variations in standard uptake value readings during the eight-minute dynamic scan;
- late-enhancement imaging performed two hours after F-18 choline injection; and
- whole-body imaging.
Lymph nodes that produced false-positive readings exhibited rapid initial choline uptake before a rapid wash-out seen in whole-body and late enhancement images. True positives were characterized by uptake in the whole body and late images, Langsteger said.
Eleven of 14 false negatives related to metastases of less than 5 mm in diameter. They were negative on F-18 choline scans and appeared unsuspicious on corresponding CT imaging.
In the two-year study of 219 patients, PET/CT identified 93 bone metastases, compared with 53 bone lesions for CT. PET/CT also provided diagnostically relevant metabolic information about bone and lymph node metastases that was not measured during morphologic CT evaluation.
F-18 choline PET/CT resulted in six of the 84 biopsy-confirmed cases being upstaged because of observed metastatic lymph nodes or bone metastases. Fifteen cases were referred to hormonal or radiation therapy, and 63 underwent surgery. The sensitivity and specificity of F-18 choline PET/CT for preoperative staging were 57% and 98%, respectively, with positive and negative predictive values of 90% and 95%, respectively.
"For preoperative staging, the results were disappointing," he said.
He referred to micrometastases less than 2 mm and lymph node metastases less than 5 mm.
Yet the choline PET studies revealed important additional diagnostic information for several high-risk patients.
PET identified multiple bone metastases in two patients with PSA levels less than 10 and Gleason scores of 7 and 9. Imaging revealed generalized bone metastases in one patient with a PSA of about 15 and a Gleason score of 8.
Among patients with a Gleason score of 8 or higher, three false-negative lymph nodes larger than 5 mm escaped detection. Five false negatives for lymph nodes and micrometastases appeared among patients with a Gleason score above 7. Two patients who had a PSA score above 50 had histologically proven negative lymph nodes.
For patients undergoing hormone therapy, all of them with bone metastases showed increases in the standard uptake values, but no SUV increase was observed for patients with lymph node metastases. This indicates that choline uptake in primary and lymph node metastases declines during hormone therapy, Langsteger said.
