PMID- 32196950 OWN - NLM STAT- MEDLINE DCOM- 20210621 LR - 20210621 IS - 1526-9914 (Electronic) IS - 1526-9914 (Linking) VI - 21 IP - 5 DP - 2020 May TI - The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets-Quantification and possible corrections. PG - 56-64 LID - 10.1002/acm2.12854 [doi] AB - PURPOSE: To assess the impact of isocenter shifts due to linac gantry and table rotation during cranial stereotactic radiosurgery on D(98) , target volume coverage (TVC), conformity (CI), and gradient index (GI). METHODS: Winston-Lutz (WL) checks were performed on two Elekta Synergy linacs. A stereotactic quality assurance (QA) plan was applied to the ArcCHECK phantom to assess the impact of isocenter shift corrections on Gamma pass rates. These corrections included gantry sag, distance of collimator and couch axes to the gantry axis, and distance between cone-beam computed tomography (CBCT) isocenter and treatment beam (MV) isocenter. We applied the shifts via script to the treatment plan in Pinnacle 16.2. In a planning study, isocenter and mechanical rotation axis shifts of 0.25 to 2 mm were applied to stereotactic plans of spherical planning target volumes (PTVs) of various volumes. The shifts determined via WL measurements were applied to 16 patient plans with PTV sizes between 0.22 and 10.4 cm(3) . RESULTS: ArcCHECK measurements of a stereotactic treatment showed significant increases in Gamma pass rate for all three measurements (up to 3.8 percentage points) after correction of measured isocenter deviations. For spherical targets of 1 cm(3) , CI was most severely affected by increasing the distance of the CBCT isocenter (1.22 to 1.62). Gradient index increased with an isocenter-collimator axis distance of 1.5 mm (3.84 vs 4.62). D(98) (normalized to reference) dropped to 0.85 (CBCT), 0.92 (table axis), 0.95 (collimator axis), and 0.98 (gantry sag), with similar but smaller changes for larger targets. Applying measured shifts to patient plans lead to relevant drops in D(98) and TVC (7%) for targets below 2 cm(3) treated on linac 1. CONCLUSION: Mechanical deviations during gantry, collimator, and table rotation may adversely affect the treatment of small stereotactic lesions. Adjustments of beam isocenters in the treatment planning system (TPS) can be used to both quantify their impact and for prospective correction of treatment plans. CI - (c) 2020 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine. FAU - Wack, Linda J AU - Wack LJ AD - Radiation Oncology, University of Wuerzburg, Wuerzburg, Germany. FAU - Exner, Florian AU - Exner F AD - Radiation Oncology, University of Wuerzburg, Wuerzburg, Germany. FAU - Wegener, Sonja AU - Wegener S AD - Radiation Oncology, University of Wuerzburg, Wuerzburg, Germany. FAU - Sauer, Otto A AU - Sauer OA AD - Radiation Oncology, University of Wuerzburg, Wuerzburg, Germany. LA - eng PT - Journal Article DEP - 20200320 PL - United States TA - J Appl Clin Med Phys JT - Journal of applied clinical medical physics JID - 101089176 SB - IM MH - Humans MH - *Particle Accelerators MH - Phantoms, Imaging MH - Prospective Studies MH - *Radiosurgery MH - Radiotherapy Planning, Computer-Assisted PMC - PMC7286018 OTO - NOTNLM OT - Winston-Lutz test OT - isocenter OT - quality assurance OT - stereotactic radiotherapy COIS- This publication was funded by the German Research Foundation (DFG) and the University of Wuerzburg in the funding program Open Access Publishing. EDAT- 2020/03/21 06:00 MHDA- 2021/06/22 06:00 PMCR- 2020/03/20 CRDT- 2020/03/21 06:00 PHST- 2019/09/23 00:00 [received] PHST- 2020/01/26 00:00 [revised] PHST- 2020/02/07 00:00 [accepted] PHST- 2020/03/21 06:00 [pubmed] PHST- 2021/06/22 06:00 [medline] PHST- 2020/03/21 06:00 [entrez] PHST- 2020/03/20 00:00 [pmc-release] AID - ACM212854 [pii] AID - 10.1002/acm2.12854 [doi] PST - ppublish SO - J Appl Clin Med Phys. 2020 May;21(5):56-64. doi: 10.1002/acm2.12854. Epub 2020 Mar 20.