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SU‐G‐BRC‐05: Conundrum for VMAT Cranial Multiple Lesions Treated with HD120 MLC
Author(s) -
Lim S,
Kuo L,
Happersett L,
Lovelock D,
Ballangrud A,
LoSasso T
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 - 10.1118/1.4956895
Subject(s) - truebeam , dosimetry , imaging phantom , nuclear medicine , radiation treatment planning , coronal plane , sagittal plane , image guided radiation therapy , medicine , linear particle accelerator , beam (structure) , medical imaging , physics , radiation therapy , radiology , optics
Purpose: To commission a custom 6MV‐SRS‐AAA Eclipse beam model for VMAT multiple lesions cranial SRS treatment on a Varian TrueBeam STx. Methods: Six clinical plans were created using a customized beam model with dosimetric‐leaf‐gap(DLG) optimized for clinical treatments. Each plan had 4–6 non‐isocentric targets with size from 0.2 to 7.1cc. All fields were measured with EBT3 film in the coronal plane in a solid water phantom and with an AS1000 EPID using gantry rotation. In addition, an end‐to‐end test was performed with coronal and sagittal films in an anthropomorphic phantom verifying dosimetry and localization accuracy. Portal dose distributions were generated with a custom portal dosimetry algorithm(PDIP). Measured dose distributions were compared with calculations using average dose difference (DD), and gamma function, γ. Using a 1.25mm grid, the γ criteria, local DD ≤ 3% and 2mm distance‐to‐agreement, were applied in regions with dose 50% of maximum. Results: The respective DD and γ for all films were <±2% and >94.2%. The portal dose γ scores for all the plans were >94.9%. However, local regions with underdose >10%, were observed when targets were treated with the 5mm leaves. The same plans re‐optimized with two isocenters such that all lesions were under the 2.5mm leaves did not show this effect. The DD and localization error of the end‐to‐end test were within 3.4% and 1.0mm respectively. Conclusion: The custom AAA beam model is capable of calculating acceptable dosimetry for targets using only the 2.5 mm leaves. This restricts lesions to within ±4cm of isocenter. The observed underdose beneath the 5mm leaves is attributed to a limitation in Eclipse that uses a single DLG representing the DLG's of both 2.5mm and 5mm leaves. If lesions are >4cm from isocenter, a multiple isocenter technique should be considered to allow the use of only the 2.5mm leaves.