Effects of changing modulation and pitch parameters on tomotherapy delivery quality assurance plans

Abstract

This study was aimed at investigating delivery quality assurance (DQA) discrepancies observed for helical tomotherapy plans. A selection of tomotherapy plans that initially failed the DQA process was chosen for this investigation. These plans failed the fluence analysis as assessed using gamma criteria (3%, 3 mm) with radiographic film. Each of these plans was modified (keeping the planning constraints the same), beamlets rebatched and reoptimized. By increasing and decreasing the modulation factor, the fluence in a circumferential plane as measured with a diode array was assessed. A subset of these plans was investigated using varied pitch values. Metrics for each plan that were examined were point doses, fluences, leaf opening times, planned leaf sinograms, and uniformity indices. In order to ensure that the treatment constraints remained the same, the dose‐volume histograms (DVHs) of all the modulated plans were compared to the original plan. It was observed that a large increase in the modulation factor did not significantly improve DVH uniformity, but reduced the gamma analysis pass rate. This also increased the treatment delivery time by slowing down the gantry rotation speed which then increases the maximum to mean non‐zero leaf open time ratio. Increasing and decreasing the pitch value did not substantially change treatment time, but the delivery accuracy was adversely affected. This may be due to many other factors, such as the complexity of the treatment plan and site. Patient sites included in this study were head and neck, right breast, prostate, abdomen, adrenal, and brain. The impact of leaf timing inaccuracies on plans was greater with higher modulation factors. Point‐dose measurements were seen to be less susceptible to changes in pitch and modulation factors. The initial modulation factor used by the optimizer, such that the TPS generated ‘actual’ modulation factor within the range of 1.4 to 2.5, resulted in an improved deliverable plan.PACS number: 87.55.‐x, 87.55.Qr, 87.55.D‐

Analysis of modulation factor to shorten the delivery time in helical tomotherapy

“…If a leaf has an extremely long open time with a large setting MF, the open time of the leaf can be adopted as LOT max . In this case, even if the beamlet does not have a major impact on the dose distribution, the delivery time is idly longer because of the longer gantry rotation period …”

Technical Note: Relationships between gamma criteria and action levels: Results of a multicenter audit of gamma agreement index results

“…These results were weighted by the number of beams in each set, listed in Table I All of the QA systems used by the centers participating in this study (see Table I) have been described and examined in previous studies. Specifically, The  system and its use in obtaining QA measurements from high-resolution EPID images has been described by Nicolini et al 10 The two-dimensional planar MapCheck2 diode array has been described and investigated by Jursinic et al 6,29 The cylindrical ArcCheck diode array and its use for producing three-dimensional dose measurements and performing threedimensional gamma evaluations has been described by Li et al 7 and applied to TomoTherapy treatment QA by Binny et al 30 The  Patient Software is a more recent version of the MapCheck software. Some of the data used in this study (in sets a, c, and d) have also been used in previous IMRT and VMAT plan complexity studies.…”

Determining efficient helical IMRT modulation factor from the MLC leaf‐open time distribution on precision treatment planning system

“…A high MF value can allow the optimizer to generate beamlets with long LOTs that have minimal impact on the dose distribution . It has been suggested for complex plans that planners start with a high MF to achieve a good conformal plan and then reduce the MF until the dosimetric qualities of the plan degrade to clinically unacceptable values …”