Risk-Based Evaluation of IMRT-Focused Linac Quality Control Protocols Using Failure Mode and Effects Analysis Patrick Marcelino Wongso (a*), Endang Nuryadi (b), Wahyu Edy Wibowo (b), Supriyanto Ardjo Pawiro (a)
a) Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Indonesia, Depok, West Java, 16424, Indonesia
*patrickjilc[at]yahoo.co.id
b) Department of Radiation Oncology, Faculty of Medicine, Universitas Indonesia, Dr. Cipto Mangunkusumo National Central Public Hospital, Jakarta, 10430, Indonesia
Abstract
This study uses Failure Mode and Effects Analysis (FMEA) to assess and enhance quality control (QC) systems for application on linear accelerators (Linacs) Intensity Modulated Radiotherapy (IMRT). Complex dose modulation, high monitor unit delivery, and tight geometric accuracy define IMRT- hence, it is necessary to have quality assurance (QA) processes reflecting its specific clinical requirements. In accordance with the AAPM TG-100 guideline, each quality control test is considered a failure mode (FM). A semi-quantitative method was used to assess the occurrence (O), severity (S), and detectability (D) parameters for each FM. For O assessment, data were extracted from the QC logs of two Linacs over an estimated three-year period. For S assessment, failure mode simulations were carried out in the treatment planning system. Additionally, a questionnaire was also distributed to medical physicists to seek expert opinion for the assessment of O, S, and D. The Risk Priority Number (RPN) was calculated as the product of these three parameters. The highest RPN was recorded for the backup monitor chamber constancy test (RPN = 111) and underscores its absolute necessity in maintaining dose safety. Among the most highly ranked failure modes were imaging-based procedures such as imaging and treatment coordinate coincidence (RPN = 82, 81) and positioning/repositioning by CBCT and planar imaging (RPN = 79, 54), indicating their significant contribution toward providing spatial accuracy in IMRT delivery. Dosimetric verification such as photon output constancy (RPN = 49) proved high occurrence and severity, showing evidence of being sensitive to high frequency of deviation and impact on patient dose. Meanwhile, mechanical tests such as distance indicator, laser position, treatment couch position indicators, and collimator size indicator though not the greatest RPN, showed consistently high severity. This supports their standard inclusion in daily QA routines where early detection is critical to prevent treatment error. These findings indicate that although RPN is a critical indicator, test priority in IMRT also needs to be informed by clinical tolerance limits in addition to modality-specific risk. Risk-based but treatment-sensitive QC practice ensures that frequency and effect are aligned with the demands of accuracy in IMRT.
Keywords: FMEA- Linear accelerator- Quality control- TG-100- IMRT
