PET/CT Bolsters Sarcoma Armamentarium

Independent of one another, CT and PET have played important roles in the identification and staging of sarcomas. CT’s anatomic capabilities have helped physicians locate sarcomas in patients, while PET’s functional capabilities have assisted in monitoring treatment, grading sarcoma, separating benign from malignant masses, selecting biopsy sites, and assessing the extent of sarcomas.

The fusion of PET and CT has shown to be greater than the sum of their constituent technologies, particularly for surgical oncologists such as Frederick (Fritz) Eilber, MD, an assistant professor of surgery at the University of California at Los Angeles Medical Center, who has found PET/CT a great benefit to his practice.

“You need the CT for anatomic relationships for surgical purposes,” he says. “Adding the PET gives you an idea of how aggressive that tumor is and helps you operatively. What we’ve found is that malignant sarcomas tend to have a high metabolic activity, and benign or less malignant tumors tend to have low metabolic activity. That correlation has yet to be totally worked out, but it’s pretty consistent.”

Although imaging assists in surgical planning, sarcomas are diagnosed by pathology, Eilber noted.

“Anatomic Imaging can tell you there’s a mass, but it can’t tell you whether it’s malignant or benign or what type it is,” he says. “Molecular PET imaging determined with a very high likelihood whether a mass is malignant or benign.

There are more than 50 types of soft-tissue sarcomas and they can present in muscles, connective tissues, vessels, joints, and fat. Osteosarcoma can present itself in any of the bones of the body, but it is diagnosed more often in the long bones of the arms and legs. With this many variants of the disease, pathological identification of the cancer is the gold standard.

The main focus of PET/CT in sarcoma treatment has been for monitoring response to therapy, Eilber says.

“We don’t use PET for diagnosis or screening,” he says. “These tumors tend to be really obvious masses on CT, unlike carcinoma. Sarcomas tend to be relatively large masses and their metastatic lesions are relatively obvious.”

The hybrid modality has provided surgical oncologists with a tool for assessing neoadjuvant therapy such as chemotherapy or chemoradiation therapies, which are delivered first in an effort to make the next treatment step go more smoothly.

“We have a lot of experience with giving neoadjuvant therapy to patients with large, high-risk tumors,” Eilber says. “We’ve found that a lot of tumors don’t shrink or change in size at all, yet we take them out and they’re dead.

“If I was to use CT [only] as an assessment of response, it’s not very accurate because the tumor size is not changing. Unfortunately, that’s the current standard of care to assess for response to therapy in sarcomas and all solid tumors and it’s notoriously inaccurate. It’s nice if it shrinks. But just because it didn’t shrink doesn’t mean that neoadjuvant therapy didn’t work. That’s where PET comes in,” he says.

Eilber and his colleagues are currently conducting a prospective study on the use of PET/CT to assess response to patients with sarcoma who are undergoing neoadjuvant therapy prior to surgery. The cohort undergoes a baseline PET/CT study prior to treatment and another PET/CT study post-treatment and prior to surgery. The exams are compared, both on the basis of tumor size in the CT images as well as the standardized uptake value (SUV) on the PET images.

“What we’ve found, based on our results to date, is that the change in metabolic activity is much more sensitive in picking up whether or not the tumor’s dead, as compared to size,” he observes.

Eilber foresees the possibility of an even greater role for the future of PET/CT in the continuum of care for sarcoma patients.

“The real goal would be to get a baseline PET/CT, give the patient a course of drugs, and get another PET/CT,” he says. “Based on the comparison of SUVs, you could make a decision to switch treatment or just go to surgery. That would be the ideal goal.”