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BACKGROUND Adjuvant stereotactic radiotherapy (SRT) to the surgical cavity of brain metastases (BM) improves the local control. An early post-operative irradiation is desirable to limit the risk of recurrence while delaying it might reduce the risk of radionecrosis by reducing the size of the target volume (TV) thanks to the cavity constriction. The purpose of our study was to examine cavity dynamics after resection to determine if delaying SRT influences cavity size as well as local progression (LP). MATERIAL AND METHODS We reviewed 70 BM resection cavities treated with adjuvant SRT. A preoperative, immediate postoperative and at the time of SRT, 3D T1-weigthed MRI, were available for all patients. The volumes were retrospectively calculated with the PACS software Telemis® version 4 and the iPlan RT image software. For all cavities, the TV was obtained by expanding the cavity by a median margin of 2mm to account for microscopic spread. The surgical track and edema areas were not included in the TV. The dynamic evolution of cavities and LP according to the delay of SRT were analyzed. RESULTS The median time from surgery to SRT was 27 days (3–99). The median preoperative and SRT cavity volumes were 7.2 cc (ranges: 0.5–29.5) and 8 cc (ranges: 0.8–31), respectively. After margin expansion, the TV was increased to a median of 14.8 cc. The volumetric cavity changes were an increase by 2 cc in 35 %, whereas only 22.5 % decreased by 2 cc and 42.5% cavities were stable (defined as a change ≤2 cc). We noted that 11% of volume change were attributable to the delay (r2 = 0.11). forty-seven patients were treated within a delay of 30 days and experienced early LP in 10.6 % against 20 % beyond 30 days (p= 0.6). CONCLUSION In the interval between surgery and SRT, BM cavities dynamics are inconstant and unpredictable in function of time. Furthermore, delaying SRT may compromise local control, though the difference was not statistically significant due to the small size of our population. A preoperative SRT strategy looks more promising by alleviating the problem of time and reducing the volume of irradiated brain.

P03.03 Dynamics of brain metastasis cavities between resection and adjuvant stereotactic radiotherapy and impact of the delay on local control