Statistical process control analysis for patient-specific IMRT and VMAT QA

Abstract

This work applied statistical process control to establish the control limits of the % gamma pass of patient-specific intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) quality assurance (QA), and to evaluate the efficiency of the QA process by using the process capability index (Cpml). A total of 278 IMRT QA plans in nasopharyngeal carcinoma were measured with MapCHECK, while 159 VMAT QA plans were undertaken with ArcCHECK. Six megavolts with nine fields were used for the IMRT plan and 2.5 arcs were used to generate the VMAT plans. The gamma (3%/3 mm) criteria were used to evaluate the QA plans. The % gamma passes were plotted on a control chart. The first 50 data points were employed to calculate the control limits. The Cpml was calculated to evaluate the capability of the IMRT/VMAT QA process. The results showed higher systematic errors in IMRT QA than VMAT QA due to the more complicated setup used in IMRT QA. The variation of random errors was also larger in IMRT QA than VMAT QA because the VMAT plan has more continuity of dose distribution. The average % gamma pass was 93.7% ± 3.7% for IMRT and 96.7% ± 2.2% for VMAT. The Cpml value of IMRT QA was 1.60 and VMAT QA was 1.99, which implied that the VMAT QA process was more accurate than the IMRT QA process. Our lower control limit for % gamma pass of IMRT is 85.0%, while the limit for VMAT is 90%. Both the IMRT and VMAT QA processes are good quality because Cpml values are higher than 1.0.

IMRT QA: Selecting gamma criteria based on error detection sensitivity

“…Differences between QA results for IMRT deliveries and VMAT deliveries are less straightforward. Sanghangthum et al 31 reported less variation of random errors and fewer systematic errors in VMAT results compared to IMRT results. However, different measurement devices were used for the two delivery techniques.…”

Because of the advantages of rotational techniques, conventional IMRT will soon become obsolete

“…21 Finally, the proof is in the pudding: VMAT plans are no less likely to fail QA testing than conventional IMRT plans. 22,23 With regard to reliability, there is no reported evidence to support the supposition that VMAT increases MLC failures.…”

What can statistical process control show us about ionization chamber stability?

“…In recent years several groups have published work which illustrates how SPC can be applied to the different systems controlled by medical physicists in the various radiotherapy processes related to linear particle accelerator (linac) monitoring (Pawlicki et al, 2005;Able et al, 2011;de la Vega et al, 2012;Sanghangthum et al, 2013b;Létourneau et al, 2014;López-Tarjuelo et al, 2015), image guided radiation therapy (IGRT; Ung and Wee, 2011), monitor-unit verifications (Nordström et al, 2012), high-dose rate (HDR) brachytherapy treatments (Able et al, 2013), proton therapy beams (Rah et al, 2014), and different aspects of intensity-modulated treatment quality assurance (Breen et al, 2008;Pawlicki et al, 2008a;Pawlicki et al, 2008b;Gérard et al, 2009;Sanghangthum et al, 2013c;Sanghangthum et al, 2013a;Gagneur and Ezzell, 2014).…”