MR-Linac（Elekta Unity）におけるテーブル下水の蓄積による説明不能な線量減衰の特定と緩和

To report and investigate a case of significant dose attenuation observed during quality assurance (QA) of an Elekta Unity MR-Linac system, and to elucidate its root cause and solution.

During a period of abnormal environmental conditions (elevated temperature and humidity), a >20% dose output reduction was detected at a gantry angle of 180°. A comprehensive investigation was undertaken, including dose output measurements at multiple gantry angles, beam quality assessment, mechanical accuracy checks, EPID image analysis, and gamma pass rate analysis using ArcCheck-MR. The source of the attenuation was localized through a process of elimination.

The investigation revealed severe dose attenuation specifically at angles traversing beneath the treatment couch, with a gamma pass rate (3%/3 mm) dropping to 39.1% at 180°. All other system parameters, including MRI quality and mechanical alignments, were within tolerance. Visual inspection after disassembly confirmed the presence of a substantial pool of water beneath the table, estimated to be at least 3.5 cm deep, which acted as an unintended attenuator. The water was attributed to condensation and drainage issues exacerbated by the failed air-conditioning system. Mitigation strategies, including hardware sealing, enhanced drainage, revised maintenance protocols, and the addition of a 180° dose check to daily QA, successfully resolved the issue.

This case highlights a previously unreported and critical failure mode for MR-Linac systems. It underscores the profound impact of environmental control on machine performance and patient safety. Robust HVAC maintenance, proactive monitoring of sub-table areas, and the inclusion of oblique-angle dosimetry in QA routines are essential for all Elekta Unity and similar high-precision radiotherapy systems.