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SU‐F‐T‐638: Is There A Need For Immobilization in SRS?
Author(s) -
Masterova K,
Sethi A,
Anderson D,
Prabhu V,
Rusu I,
Gros S,
Melian E
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 - 10.1118/1.4956823
Subject(s) - nuclear medicine , radiosurgery , cone beam computed tomography , medicine , residual , image registration , quality assurance , biomedical engineering , radiation therapy , computed tomography , computer science , radiology , artificial intelligence , algorithm , external quality assessment , pathology , image (mathematics)
Purpose: Frameless Stereotactic radiosurgery (SRS) is increasingly used in the clinic. Cone‐Beam CT (CBCT) to simulation‐CT match has replaced the 3‐dimensional coordinate based set up using a stereotactic localizing frame. The SRS frame however served as both a localizing and immobilizing device. We seek to measure the quality of frameless (mask based) and frame based immobilization and evaluate its impact on target dose. Methods: Each SRS patient was set up by kV on‐board imaging (OBI) and then fine‐tuned with CBCT. A second CBCT was done at treatment‐end to ascertain intrafraction motion. We compared pre‐ vs post‐treatment CBCT shifts for both frameless and frame based SRS patients. CBCT to sim‐CT fusion was repeated for each patient off‐line to assess systematic residual image registration error. Each patient was re‐planned with measured shifts to assess effects on target dose. Results: We analyzed 11 patients (12 lesions) treated with frameless SRS and 6 patients (11 lesions) with a fixed frame system. Average intra‐fraction iso‐center positioning errors for frameless and frame‐based treatments were 1.24 ± 0.57 mm and 0.28 ± 0.08 mm (mean ± s.d.) respectively. Residual error in CBCT registration was 0.24 mm. The frameless positioning uncertainties led to target dose errors in Dmin and D95 of 15.5 ± 18.4% and 6.6 ± 9.1% respectively. The corresponding errors in fixed frame SRS were much lower with Dmin and D95 reduced by 4.2 ± 6.5% and D95 2.5 ± 3.8% respectively. Conclusion: Frameless mask provides good immobilization with average patient motion of 1.2 mm during treatment. This exceeds MRI voxel dimensions (∼0.43mm) used for target delineation. Frame‐based SRS provides superior patient immobilization with measureable movement no greater than the background noise of the CBCT registration. Small lesions requiring submm precision are better served with a frame based SRS.