PET/CT shows great sensitivity, concordance with soft-tissue sarcoma

Aug 22, 2007

Two studies on soft-tissue sarcoma show that PET/CT has a high sensitivity in initial staging and recurrence assessment, and a strong concordance between the two modalities in evaluating sarcoma lesions.

A study from McGill University Health Center in Montreal evaluated the sensitivity of FDG PET/CT imaging in detecting soft-tissue sarcoma and osseous sarcoma on the basis of avidity to FDG.

"We know that FDG uptake has been demonstrated to correlate well with the histology uptake of various malignancies," said McGill's Dr. Marc Hickeson at the 2007 SNM meeting in Washington, DC. "The literature is growing rapidly for many types of carcinomas and lymphomas, but we also are aware in the U.S. that there is a reimbursement issue for sarcomas and there is not as much reference available for sarcoma of osseous and soft tissue."

In the McGill study, the cases of 162 consecutive patients, who were referred with known histories of soft-tissue sarcoma and osseous sarcoma, were reviewed from May 2004 to November 2006. The SUV maximum for FDG uptake of each primary and/or most intense metastatic lesion was measured and compared with the final pathological report. For the purpose of this study, researchers selected a threshold of 2.5 as the SUVmax to differentiate between malignant and benign lesions.

Scans were conducted on a Discovery ST PET/CT system (GE Healthcare, Chalfont St. Giles, U.K.), which features a 16-slice CT scanner. Images were acquired almost simultaneously, with additional images obtained according to the sarcoma location. Patients received a dose of FDG between 10 and 13.5 mCi, and images were acquired 60 minutes after FDG injection.

Of the 162 patients in the study, 125 cases involved soft-tissue sarcoma, while 37 patients had osseous sarcoma. The 125 soft-tissue sarcoma cases included 80 for initial staging and 45 for assessment for the recurrence of disease. Of the 37 osseous sarcoma patients, 19 were for initial staging, with the remaining 18 for recurrence assessment.

SUV factor

The majority of soft-tissue sarcoma cases had SUVs above 2.5. The same trend held true for osseous sarcoma. "In correlating with the histopathology for primary tumors before chemotherapy, we had consistently low uptake for low-grade sarcomas, and the intermediate and high-grade sarcomas had very variable SUVs," Hickeson said. "There were a few false negatives with the high grade and one false negative with the low grade."

For all sarcomas, the study found sensitivity of 93%. When divided according to soft-tissue sarcoma and osseous sarcoma, the sensitivity rates were 92% and 96%, respectively. Based on indication, sensitivity was 95% for initial staging and 84% for recurrence assessment after therapy. The sensitivities for the most common sarcoma diagnoses were: 100% for leiomyosarcomas (18 cases); 88% for liposarcomas (17 cases); 79% for synovial sarcomas (14 cases); and 92% for osteosarcomas (12 cases).

There were a total of nine false negatives; four were due to subcentimetric size and five were due to low avidity to FDG.

Based on those results, the combined metabolic and morphologic information provided by FDG PET/CT "allows for excellent sensitivity for initial staging and good sensitivity for assessment of recurrence of soft-tissue sarcoma in the study population," Hickeson said.

When the SUV was greater than 3.5, all of the sarcomas were of moderate- and high-grade histology, the researchers noted. "We are continuing to acquire data and need larger studies to confirm the observation and increase the validity of the study before it can be incorporated into clinical practice," Hickeson added.

PET/CT concordance

The second soft-tissue sarcoma study, from St. Vincent's Hospital and Medical Center in New York City, sought to uncover discrepancies between PET and CT images when evaluating treatment response and follow-up therapy with patients.

Specifically, researchers evaluated the separate and combined efficacies of FDG PET and low-dose CT in PET/CT studies performed for initial staging, postsurgery treatment response, radiotherapy or chemotherapy, as well as short- and long-range follow-up and restaging in patients with soft-tissue sarcoma.

At the SNM meeting, Dr. Elena Piperkova of St. Vincent's presented details of the retrospective study, which reviewed 58 patients with soft-tissue sarcoma from January 2004 to December 2006. There were a total of 111 PET/CT studies for the patients, who had a mean age of 51 years, ranging from 19 to 87 years.

Seven studies were performed for initial staging analysis; 43 studies were in the evaluation of treatment response up to two months; 27 studies among patients with a follow-up time of two to six months; and 34 studies with a follow-up of greater than six months.

All patients underwent scanning on a GE Discovery PET/CT system between 60 and 90 minutes after injection of 10 to 15 mCi of FDG.

Lesion detection

The analysis discovered 264 lesions, 235 (89%) of which were concordant between the PET and CT images from the PET/CT, while discrepancies occurred between PET and CT in 29 lesions (11%).

In analyzing the study segments, of the seven initial staging patients, PET and CT were in agreement in five cases (71%). PET and CT were in concordance with 35 of the 43 lesions (81%) in the group evaluated for treatment response up to two months. In the two- to six-month follow-up group, agreement between PET and CT was found with 23 of 27 lesions (85%). Among the 34 long-range (beyond six months follow-up) patients, agreement was found in 30 studies (88%).

The researchers also found that while combined PET/CT had a higher accuracy than either PET or CT alone, FDG PET was more accurate than CT alone.

"Combined PET/CT played a more important role in staging the treatment response evaluation than each of them alone, bringing maximum clinical benefit in soft-tissue sarcoma management," Piperkova said. "We can say that PET is more accurate than CT alone in evaluating treatment response in soft-tissue sarcomas for either short-term or long-term follow-up."

By Wayne Forrest
AuntMinnie.com staff writer
August 23, 2007

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