Session Item

Pre-treatment patient specific quality assurance is a necessary task to ensure accurate dose delivery. When a new machine became operational all the clinically approved plans must undergo a dosimetric verification. The new Accuray platform for helical tomotherapy, Radixact, delivery has been introduced at our department in june 2018. The objective of the present work is to analyze the results obtained from a large dataset of pretreatment measurements for this new machine, to determine which parameters has to be optimized to obtain a more robust delivery. At the date of the submission this is the first study on performances evaluation of the Radixact platform and Precision TPS.

More than 350 clinical plans, delivered between June 2018 and September 2019, were measured with ArcCHECK (SunNuclear, Melbourne, FL) diode array. All the plans were optimized and calculated with the new TPS Accuray Precision v 1.1.1. All the plans were delivered on a Radixact Helical Tomotherapy unit. According to the AAPM''s report TG-119 (updated by TG-218 report), the gamma index passing rate (GP%) chosen for the analysis is: 3% dose difference, 2 mm Distance to Agreement (DTA), Van Dyk criteria of 10% threshold, absolute and global dose comparison. The plans were considered deliverable if the universal tolerance limit, GP% passing rate  ≥ 95%, is met. In all the other cases we replanned the case in order to produce a less complex plan. The values of the actual modulation factor (MF), pitch, field width (FW), gantry period (Trot), dose per fraction (D/fr), dose per fraction on pitch (D/p), maximum, median, mean and standard deviation of Leaf Open Time (Max-LOT, m-LOT, M-LOT and SD-LOT respectively) and site of the disease were collected. A multivariate analysis of variance (mANOVA) was performed to test the influence of these parameters on GP%. A post-hoc test was carried out to assess the significance of the quantities identified as predictors.

An average GP% (±SD) of 97.5 (±2.5) was found. All the results are reported in figure 1. Only 2% of the plans fails to meet the action level defined by TG-218. They were replanned to meet the GP%>90% criteria. Max-LOT, M-LOT and pitch resulted significant predictors of dosimetric accuracy of the plans, defined as GP% (3%/2mm). No correlation with the treatment site nor with other parameters was found.


Figure 1: Box Plot of GP% 3%/2mm. Horizontal lines indicates the TG-218 tolerance and action limits.

The results here presented suggest that the calculation accuracy of the new Precision TPS and the delivery accuracy of Radixact unit is adequate, with respect to the international guidelines and reports. Despite the general and positive conclusion on the performances of the presented pre-treatment QA, the predictive value of the analyzed parameters on QA failures is still under investigation.