On the sensitivity of patient‐specific IMRT QA to MLC positioning errors

Guanghua Yan,Chihray Liu,Lee‐Cheng Peng,Christopher Fox,Jonathan G Li,Thomas A Simon

doi:10.1120/jacmp.v10i1.2915

Abstract

Accurate multileaf collimator (MLC) leaf positioning plays an essential role in the effective implementation of intensity modulated radiation therapy (IMRT). This work evaluates the sensitivity of current patient‐specific IMRT quality assurance (QA) procedures to minor MLC leaf positioning errors. Random errors of up to 2 mm and systematic errors of ±1mm and ±2mm in MLC leaf positions were introduced into 8 clinical IMRT patient plans (totaling 53 fields). Planar dose distributions calculated with modified plans were compared to dose distributions measured with both radiochromic films and a diode matrix. The agreement between calculation and measurement was evaluated using both absolute distance‐to‐agreement (DTA) analysis and γ index with 2%/2mm and 3%/3mm criteria. It was found that both the radiochromic film and the diode matrix could only detect systematic errors on the order of 2 mm or above. The diode array had larger sensitivity than film due to its excellent detector response (such as small variation, linear response, etc.). No difference was found between DTA analysis and γ index in terms of the sensitivity to MLC positioning errors. Higher sensitivity was observed with 2%/2mm than with 3%/3mm in general. When using the diode array and 2%/2mm criterion, the IMRT QA procedure showed strongest sensitivity to MLC position errors and, at the same time, achieved clinically acceptable passing rates. More accurate dose calculation and measurement would further enhance the sensitivity of patient‐specific IMRT QA to MLC positioning errors. However, considering the significant dosimetric effect such MLC errors could cause, patient‐specific IMRT QA should be combined with a periodic MLC QA program in order to guarantee the accuracy of IMRT delivery.PACS numbers: 87.50.Gi, 87.52.Df, 87.52.Px, 87.53.Dq, 87.53.Tf, 87.53.Kn, 87.56.Fc

Highlights

Intensity modulated radiation therapy (IMRT) has become the treatment technique of choice for many types of cancers receiving radiation therapy
We conclude that only the -2 mm systematic multileaf collimator (MLC) positioning error could be identified by patient-specific IMRT quality assurance (QA) procedure when using EBT film and “DTA, 3%/3 mm”
Our results indicated that the studied IMRT QA procedures could detect systematic MLC positioning errors of 2 mm

Summary

Introduction

Intensity modulated radiation therapy (IMRT) has become the treatment technique of choice for many types of cancers receiving radiation therapy. LoSasso et al[4] reported that a 0.2 mm gap variation leads to 1% dose variation with an average gap width of 2 cm with dynamic beam delivery.[8] Mu et al[5] studied the dosimetric effect of leaf position errors on head and neck patients by deliberately introducing random (uniformly sampled from 0 mm, ±1 mm and ±2 mm) and systematic (±0.5 mm or ±1 mm) leaf positioning errors into the plan. Zygmanski et al[6] studied the dosimetric effect of truncated Gaussian (with 0.1 cm standard deviation) shaped random leaf position errors They found that the average composite dose to the target of a nine field IMRT plan was changed only by 3%, fluence change resulting from each single field was commonly >10%. All these studies emphasized the importance of the MLC positioning accuracy and reproducibility

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