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SU‐F‐T‐637: Single‐Isocenter Versus Multiple‐Isocenter VMAT SRS for Unusual Multiple Metastasis Case with Two Widely Separated Lesions
Author(s) -
Thomas EM,
Popple RA,
Fiveash JB
Publication year - 2016
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.4956822
Subject(s) - isocenter , collimator , nuclear medicine , radiosurgery , lesion , medicine , radiation therapy , radiology , optics , physics , surgery , imaging phantom
Purpose: Single‐isocenter (SI) volumetric modulated arc therapy has been shown to be an effective and efficient approach to multiple metastasis radiosurgery. However, certain extreme cases raise the question of whether multiple‐isocenter (MI) approaches can still generate superior plans. In this study, we ask this question with respect to a clinical case with two very widely separated lesions. Methods: A patient with two widely separated (d = 12cm) tumors was treated with SI‐VMAT SRS using 10MV flattening filter free (FFF) beam with high‐definition multi‐leaf collimator (HD‐MLC, 2.5/5mm) in two non‐coplanar arcs using concentric rings to enforce steep gradient. Because of lesion positioning with respect to collimator angle selection, lesions were treated by 5mm leaves. We re‐planned the case with a congruent arc arrangement but separate isocenter for each lesion. In this manner, lesions were treated by 2.5mm leaves. Conformity index (CI), V50%, and mean brain dose were compared. Results: Neither conformity (CI_SI = 1.12, CI_MI = 1.08) nor V50% (V50%_SI =8.82cc, V50%_MI =8.81cc) were improved by utilizing a separate isocenter for each lesion. Mean brain dose was slightly reduced (dmean_SI = 118.4 cGy, dmean_MI = 88.7 cGy) by using multiple isocenters. Conclusion: For this case with a lesion at the apex of the brain and another distantly located at the base of skull, employing a separate isocenter for each target did not meaningfully improve plan quality. Single‐isocenter VMAT has been shown feasible and equivalent to multiple‐isocenter VMAT for multiple metastasis cases in general. In this extreme case, single‐ and multiple‐ isocenter VMAT were also equivalent. If rotational setup errors are appropriately corrected, the increased delivery efficiency of the single‐isocenter approach renders it preferable to the multiple isocenter approach. Dr's Thomas, Popple, and Fiveash have all received honoraria from Varian Medical Systems for discussing their experiences with stereotactic radiosurgery.