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A sudden increase, or flare, in the size of bone metastases following radiation therapy may not be cause for concern, researchers said here on March 6 at the 2010 Genitourinary Cancers Symposium (ASCO-GU).

A retrospective analysis of a series of patients with refractory bone metastases from renal cell carcinoma (RCC) found that most bone metastases increase in size by at least 10% immediately following a single application of image-guided intensity-modulation radiation therapy (IMRT). The flare appears to be associated with the radiation treatment itself, not tumour growth.

"Most patients had some initial flare following IMRT," said Glen Kroog, MD, Memorial Sloan-Kettering Cancer Center, New York, New York. "In a very few cases, the tumours keep getting larger, but flare is usually followed by shrinkage or stabilisation. Clinicians need to be aware that flare is rarely a cause for concern."

Hypofractionated radiation has been reported as superior to conventional fractionated radiation for RCC symptoms and local control, Dr. Kroog explained. But there are little data on the radiographic response of RCC bone lesions treated with hypofractionated radiation. The Sloan-Kettering group did a retrospective analysis of 35 consecutive lytic RCC bone metastases to start filling in the blanks.

All of the lesions had clearly defined borders on computed tomography ( CT ) or magnetic resonance imaging (MRI). The lesions were treated with 22 or 25 Gray IMRT between 2005 and 2007 and evaluated for change in size on at least 2 subsequent CT or MRI scans. One of the follow-up scans had to be performed at least 16 weeks after a baseline scan and IMRT.

Of the original 35 metastases, 20 lesions were evaluable. Ten lesions were intrinsic to the bone, that is, the entire tumour mass was inside the contours of the normal bone. The other 10 lesions were soft tissue extended, with the tumour including soft-tissue areas outside the normal bone contour. A flare was defined as an increase of at least 20% in the longest lesion diameter from the baseline scan.

Of the evaluable metastases, only 15% continued to progress radiographically after IMRT. Of the 85% of lesions that did not progress, 50% flared and stabilised, 20% flared and shrunk, and 15% showed no evidence of flare. The average follow up time was 50 weeks following IMRT.

"We know that some inflammation occurs when you blast a tumour with this very high-energy radiation," he explained. "The flare may just represent an inflammatory reaction, not actual growth. We know that post-flare shrinkage or stabilisation can be maintained for at least 2 years. Flare should not be a concern for the vast majority of patients."

The cause of lesion flare is not clear, Dr. Kroog said. He suggested that it is a reaction to the single fraction radiation therapy.

Source: ASCO-GU

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