LOW-GRADE GLIOMAS - PRELIMINARY-ANALYSIS OF FAILURE PATTERNS AMONG PATIENTS TREATED USING 3D CONFORMAL EXTERNAL-BEAM IRRADIATION

Purpose: The pattern of failure of low grade gliomas following radioth erapy is less well known than that of the high grade gliomas. Stereota ctic histologic studies have suggested that tumor cells extend beyond imaging abnormalities, and that large margins would be required for ra diotherapy target volumes to encompass all of the neoplasm. Our experi ence using computerized tomography (CT)- and magnetic resonance (MR)-p lanned irradiation of low grade gliomas was reviewed to determine the pattern of tumor recurrence, in an effort to clinically define the min imum margin required. Methods and Materials: Forty-six patients with l ow grade supratentorial gliomas were treated between April 1985 and No vember 1992 using three-dimensional (3D) conformal CT- or MR-planned e xternal beam radiotherapy. Fields were designed to encompass a target volume created by adding a margin to the tumor in three dimensions. Ge nerally, patients were treated using shrinking fields with an initial target (tumor plus a 1 to 3 cm margin) treated to a dose of 45 to 50.4 (median 50.4) Gy, and a boost (tumor plus a 0 to 2 cm margin) treated to a total of 54 to 59.4 (median 59.4) Gy. Median follow-up was 32.9 months. Results: There have been 11 failures; all of these occurred wi thin the radiographic abnormality (either T2 prolongation or CT hypode nsity) visualized at the time of treatment planning (i.e., all failure s were within the boost volume). Median time to failure was 53 months. Because all failures were local, there was no relationship between th e amount by which the tumor volumes were expanded to create target vol umes and the eventual outcome. Conclusion: Despite pathologic data sug gesting that low grade glioma cells can be found outside the MR T2-sig nal abnormality in many cases, our results demonstrate that conformal external beam radiotherapy, in which the high dose volume is limited, does not result in increased marginal or out-of-field failures. Until control of tumor within the radiographically abnormal volume can be ac hieved, the need for large fields to treat prophylactically microscopi c disease beyond the visualized tumor volume is questionable. The use of conformal fields might be associated with reduced toxicity, and the reby allow delivery of higher total doses to the central tumor.