Rotational output and beam quality evaluations for helical tomotherapy with use of a third-party quality assurance tool.

Our aim was to determine whether a third-party quality assurance (QA) tool was suitable for the measurement of rotational output and beam quality in place of on-board detector signals. A Rotational Therapy Phantom 507 (507 Phantom) was used as a QA tool. The rotational output constancy (ROC507) and the beam quality index ([Formula: see text]) were evaluated by analysis of signals from an ion chamber inserted into the 507 Phantom. On-board detector signals were obtained for comparisons with the data from the 507 Phantom. The rotational output (ROC(detector)) and beam quality (corrected cone ratio; CCR) were determined by analysis of on-board detector signals that were generated by irradiation. The tissue phantom ratio at depth 20 and 10 cm (TPR20, 10) was measured with a Farmer-type ionization chamber inserted in a plastic-slab phantom. For rotational output measurement, the correlation coefficient between ROC507 and ROC(detector) values was 0.68 (p < 0.001). ROC507 and ROC(detector) values showed a reduced coefficient of variation after magnetron replacement, which was done during the measurement period. In addition, ROC507 values were reduced significantly along with ROC(detector) values after target replacement (p < 0.001). Regarding the beam quality index, [Formula: see text] showed a change similar to CCR and an increase similar to TPR20, 10 after magnetron/target replacement. This QA tool could check for daily rotational output and detect changes in rotational output and beam quality caused by magnetron or target failure as well as when on-board detector signals were used. Without needing a tomotherapy quality assurance license, we could effectively and quantitatively estimate the rotational output and beam quality at a low cost.