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. 2026 Jan 19.
doi: 10.1007/s13246-025-01693-0. Online ahead of print.

Evaluation of ThinkQA (v2.0.1.11) as an online secondary dose check for MR guided radiation therapy with the Elekta Unity MR-Linac

Ariadne S Brodmann  1   2 , John A Baines  3   4
Affiliations

Evaluation of ThinkQA (v2.0.1.11) as an online secondary dose check for MR guided radiation therapy with the Elekta Unity MR-Linac

Ariadne S Brodmann et al. Phys Eng Sci Med. .

Abstract

To evaluate ThinkQA (TQA), a collapsed cone convolution-based secondary dose check, as an alternative to MU2net (Clarkson-based, point-dose) for online adaptive planning on the Elekta Unity 1.5 T MR-Linac at Townsville University Hospital. Commissioning followed MPPG 5.b tests. The reference-dose agreement, magnetic-field modelling, directional dependence, output factors, off-axis points, heterogeneous slab geometries and calculation properties were assessed. Nine step-and-shoot IMRT plans (courtesy of Elekta) and 226 retrospectively analysed adapted fractions (prostate and pelvic nodes; planning target volumes 1.9-170.0 cm3) were compared between TQA and Monaco by gamma analysis (global 10.0% threshold; 2.0%/2.0 mm, 3.0%/2.0 mm). At 10.0 cm depth under TQA reference conditions, the mean absolute point-dose difference versus Monaco was 0.4%. TQA reproduced models the magnetic-field-induced cross-plane asymmetry with close agreement to Monaco. Directional dependence differences were ≤ ± 1.2% except when traversing the couch (± 1.8%). Output factors agreed within ≤ 1.0% (SSD 133.5 cm) and ≤ 2.0% (SSD 138.5 cm). In 226 clinical fractions, 3.0%/2.0 mm (global) yielded 93.0% passes in the high-dose region and 100.0% in other regions; 2.0%/2.0 mm yielded 25% high-dose passes. TQA results were available within about 1 min post Monaco export. TQA provides accurate, rapid, volumetric secondary dose verification for Unity, improves agreement with Monaco, and reduces console time by eliminating dose point re-selection. A 3.0%/2.0 mm global gamma criterion is a clinical acceptance level, with tighter criteria reserved for targeted investigations.

Keywords: Adaptive planning; MR-Linac; Radiotherapy; Secondary dose calculation.

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Conflict of interest statement

Declarations. Conflict of interest: The authors declare no conflict interest. Ethical approval: This article does not contain any studies with human participants or animals performed by any of the authors. Therefore, no ethics approval was required (THHSAudit 202512-899).

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