Improvements in treatment planning calculations motivated by tightening IMRT QA tolerances

Abstract

Implementing tighter intensity modulated radiation therapy (IMRT) quality assurance (QA) tolerances initially resulted in high numbers of marginal or failing QA results and motivated a number of improvements to our calculational processes. This work details those improvements and their effect on results. One hundred eighty IMRT plans analyzed previously were collected and new gamma criteria were applied and compared to the original results. The results were used to obtain an estimate for the number of plans that would require additional dose volume histogram (DVH)-based analysis and therefore predicted workload increase. For 2 months and 133 plans, the established criteria were continued while the new criteria were applied and tracked in parallel. Because the number of marginal or failing plans far exceeded the predicted levels, a number of calculational elements were investigated: IMRT modeling parameters, calculation grid size, and couch top modeling. After improvements to these elements, the new criteria were clinically implemented and the frequency of passing, questionable, and failing plans measured for the subsequent 15 months and 674 plans. The retrospective analysis of selected IMRT QA results demonstrated that 75% of plans should pass, while 19% of IMRT QA plans would need DVH-based analysis and an additional 6% would fail. However, after applying the tighter criteria for 2 months, the distribution of plans was significantly different from prediction with questionable or failing plans reaching 47%. After investigating and improving several elements of the IMRT calculation processes, the frequency of questionable plans was reduced to 11% and that of failing plans to less than 1%. Tighter IMRT QA tolerances revealed the need to improve several elements of our plan calculations. As a consequence, the accuracy of our plans have improved, and the frequency of finding marginal or failing IMRT QA results, remains within our practical ability to respond.

Keywords: IMRT QA; patient-specific QA; treatment planning systems.

Figure 1

Eclipse calculated 1 mm grid displayed in purple and Eclipse calculated 2.5 mm grid interpolated to 1 mm by SNC displayed in black.

Figure 2

ArcCHECK measurement results of a 360° open arc, including transmission through the couch, compared to the TPS calculation of the same arc. Cold spots occur at ~240° and ~130° which correspond to the thickest portion of the couch.

Figure 3

Setup for determining couch top model values using a rectangular solid water phantom as per TG‐176.

Figure 4

Prostate gamma passing rate results under original and new gamma criteria.

Figure 5

Head and neck gamma passing rate results under original and new gamma criteria.

Figure 6

Differences in isodose distributions between the distributed calculation framework (DCF) angular resolution settings (a) 5 cm field, dynamic conformal arc with 5° DCF angular resolution setting (b) 5 cm field, dynamic conformal arc with 0.7° DCF angular resolution setting (c) Lung IMRT plan, DCF angular resolution setting of 5 degrees (d) Lung IMRT plan, DCF angular resolution setting OFF.

Figure 7

The ArcCHECK measurement results compared to TPS calculations for the same field with (a) distributed calculation framework (DCF) angular resolution setting of 5° and (b) DCF angular resolution setting off.