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Dose Interplay Effects in Stereotactic Radiosurgery (SRS) of Multiple Brain Lesions

L Ma

L Ma1*, A Sahgal2, B Wang3, S Hossain4, S Ahmad 4,D Larson5, (1) UCSF Comprehensive Cancer Center, San Francisco, CA, (2) University of Toronto, Toronto, Ontario, (3) University Utah, Salt Lake City, UT, (4) University of Oklahoma Health Sciences Center, Oklahoma City, OK, (5) UCSF Radiation Oncology, San Francisco, CA

SU-E-T-669 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Volumetric modulated arc therapy (VMAT) has enabled rapid treatments of multiple brain tumors with a single or few isocenters. We investigated inter-lesion dose interplay effects for such a treatment and compared against standard multi-isocenteric Gamma Knife (GK) or dynamic-conformal-arc (DCA) SRS deliveries.

Methods: A patient case with 12 intracranial targets and simulated cases with 2-60 targets in the brain parenchyma were used for the study. For the patient case, all targets and organs-at-risk were contoured by a senior clinician. A subset of 3, 6, 9 and 12 targets were then planned at different institutions for GK, DCA and VMAT SRS. Identical dose-volume constraints to the targets and critical structures were applied. Each target was prescribed with 20 Gy covering at least 99% of the target volume. Relationships between the mean 4-Gy to 12-Gy isodose volumes per lesion versus increasing number of lesions were analyzed for each modality.

Results: For all the cases, 12-Gy isodose volumes per lesion exhibited negligible dependence with the increasing number of targets for GK SRS and DCA deliveries. However, for VMAT delivery, a strong statistically significant dependence with increasing number of targets was found at all levels of isodose volumes. For example, the increase in the 12-Gy volumes of the patient case was 0.068+/-0.016 cc/lesion (p=0.05) for the VMAT delivery in contrast to 0.0+/-0.0 cc/lesion (p < 0.0001) for both GK and DCA deliveries. The increase in the 4-Gy isodose volumes was 2.79+/-0.40 cc/lesion (p=0.02) for the VMAT delivery, and 2.13+/-0.15 cc/lesion (p=0.005) for the DCA delivery in contrast to 0.08+/-0.29 cc/lesion (p=0.10) for the GK delivery.

Conclusion: Significant dose interplay effects were found for single- or few-isocenter VMAT SRS of multiple lesions, somewhat for multi-isocenteric DCA SRS, but nearly negligible for GK SRS treatments.

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