Session Item

Tomotherapy recommended that leaf open time (LOT) below 100 ms should be less than 30 % because the risk of multileaf collimator errors was increased and higher DQA failures. This study aims to investigate leaf open time (LOT) values by analyzing the impact of LOT on delivery quality assurance (DQA) results for HT cases. Moreover, we evaluate the DQA failure pattern according to the LOT, modulation factor (MF), pitch, and gantry period (GP).

A total of 71 patients who passed and failed for DQAs results in the first measurement randomly were retrospectively selected to evaluate the LOT pattern for the patient''s plan. There were 49 patients and 22 patients who passed and failed in DQAs, respectively. These included brain tumor (n=11), head and neck (H & N) (n=13), spine (n=7), prostate (n=15), rectal (n=9), pelvis (n=5) and CSI and large field (, n=11) patients. LOT and DQA results (dose difference and gamma passing rate) in patients passed, and failed cases were analyzed. The criteria of point dose difference and gamma passing rate with 3%/3 mm below ±5 % and 90 %, respectively. Seven new DQA plans for each patient have generated its plan qualities and LOT values were analyzed. Moreover, then, DQA measurements performed with ArcCheck. To evaluate the correlation among the DQA results (absolute point dose difference and gamma passing rate) of each studied case and the plan parameters of the four DQA measurements. Pearson correlation coefficients (r) with the corresponding p values were calculated.

The average percentage of LOTs below 100 ms in passed and failed cases for DQA results was 18.14 and 35.60%, respectively. The average value of pitch in passed and failed cases for DQA results was 0.43 and 0.32, respectively. The average value of GP in passed and failed cases for DQA results was 19.07 and 16.20, respectively. In case of the proportion of LOT below 100 ms is more than 25%, DQA fail was accounted for about 95.5 % of the total failures. In case of the GP below 13 sec/rotation, all DQAs were failed and DQA fail was accounted for about 36.4% of the total failures. In case of the pitch below 0.25, all DQAs were failed. A weak correlation was observed between the point dose difference and modulation factor (r = -0.383, p = 0.001) and No correlations were observed in the correlation between the gamma passing rate and field width and LOT for all the measurements. However, p-values were less than 0.05. For failing DQA case, plan quality after re-planning with changed pitch, and modulation factor does not change the uniformity of target and organ at risks dose constraints.

We confirmed that plans having a proportion of LOT below 100 ms is more than 25% could affect DQA failures. In case of pitch < 0.251, actual MF > 1.9 and GP < 13, the probability of DQA failure is increased. Therefore, these parameters should consider when making a plan.