Functional imaging proves sensitive in measuring treatment response in sarcomas

26 Feb 2008

Positron Emission Tomography (PET) - a type of scanning that assesses the functional activity of cells in the body - is much more sensitive and more accurate than conventional imaging methods in detecting responses to treatment in patients with sarcoma, according to one of the first studies to look at this issue.

The study compared PET scanning with computed tomography (CT) in 42 patients with high-grade soft tissue sarcomas, which are cancers affecting connective tissues such as tendons and muscle. Scans were taken before and after the patients were treated with chemotherapy prior to surgery to remove their tumours (neoadjuvant treatment).

The researchers measured the metabolic or chemical activity of the tumour cells using a specific PET probe that assesses glucose metabolism. This allowed them to determine whether the cancer cells were still alive and dividing after treatment. After removing the tumours during surgery, they analysed the cells directly to assess whether chemotherapy had affected their activity.

Assessing the effects of chemotherapy in people with sarcomas has previously been difficult because the standard measure for response to cancer treatment - Response Evaluation Criteria in Solid Tumors (RECIST) - has proved unreliable in these cancers. Using this method, patients are scanned by conventional imaging with CT or magnetic resonance imaging (MRI) to assess whether a tumour has shrunk in response to treatment.

Previous research has shown that treatment may change the activity of sarcoma tumour cells in a way that improves a patient's survival even though a change in tumour size is not apparent using RECIST criteria. This has important implications for patients, because they may be taken off a treatment that is potentially improving their prognosis because their tumour is not shrinking in size.

"We knew from our experience with neoadjuvant therapy in sarcoma patients that measuring tumour size correlated poorly with response," explained Fritz Eilber, director of the Sarcoma Program at the Jonsson Comprehensive Cancer Centre at the University of California at Los Angeles, USA, and one of the authors of the new study. He said: "We have removed many tumours that have not changed in size with treatment, or have even grown, but are completely dead on pathologic analysis. Just because the tumour doesn't shrink doesn't mean the treatment didn't work."

Results from the study showed that PET scanning was much more accurate in detecting response to chemotherapy in sarcomas than conventional scanning. PET scanning identified all (100%) of the patients whose tumour cells responded to treatment. In contrast, using standard tumour size based criteria (RECIST) identified only one in four patients (25%) whose tumour cells had responded.

"PET was much more sensitive in picking up response than size-based RECIST," reported Dr Eilber. "RECIST missed a large percentage of patients who actually had a response to treatment. PET picked up all of the responders."

The researchers acknowledged some limitations in their study. The group of patients they studied had a variety of different types of sarcomas and were treated with different cancer drugs. However, they pointed out that because sarcomas are quite rare and very varied it would be impractical to look at only one type of sarcoma in a study. There is currently no standard chemotherapy for soft tissue sarcomas so it would also be difficult to look at patients treated with only one regimen.

The aim of the study was to compare two techniques for measuring tumour response to chemotherapy rather than to determine the effect of a specific treatment. "For such a comparison, including different treatment regimens is advantageous because a diagnostic test that allows assessment of tumour response for only on