Evaluation of interfraction setup variations for postmastectomy radiation therapy using EPID-based in vivo dosimetry.

Shengwei Kang,Xiongfei Liao,Bin Tang,Hou Qing,Jie Li,Wei Zhang,Angelo Piermattei,Lucia Clara Orlandini,Xin Xin,Tingqiang Tan,Jiabao Ma

doi:10.1002/acm2.12712

Abstract

Postmastectomy radiation therapy is technically difficult and can be considered one of the most complex techniques concerning patient setup reproducibility. Slight patient setup variations — particularly when high‐conformal treatment techniques are used — can adversely affect the accuracy of the delivered dose and the patient outcome. This research aims to investigate the inter‐fraction setup variations occurring in two different scenarios of clinical practice: at the reference and at the current patient setups, when an image‐guided system is used or not used, respectively. The results were used with the secondary aim of assessing the robustness of the patient setup procedure in use. Forty eight patients treated with volumetric modulated arc and intensity modulated therapies were included in this study. EPID‐based in vivo dosimetry (IVD) was performed at the reference setup concomitantly with the weekly cone beam computed tomography acquisition and during the daily current setup. Three indices were analyzed: the ratio R between the reconstructed and planned isocenter doses, γ% and the mean value of γ from a transit dosimetry based on a two‐dimensional γ‐analysis of the electronic portal images using 5% and 5 mm as dose difference and distance to agreement gamma criteria; they were considered in tolerance if R was within 5%, γ% > 90% and γmean < 0.4. One thousand and sixteen EPID‐based IVD were analyzed and 6.3% resulted out of the tolerance level. Setup errors represented the main cause of this off tolerance with an occurrence rate of 72.2%. The percentage of results out of tolerance obtained at the current setup was three times greater (9.5% vs 3.1%) than the one obtained at the reference setup, indicating weaknesses in the setup procedure. This study highlights an EPID‐based IVD system's utility in the radiotherapy routine as part of the patient’s treatment quality controls and to optimize (or confirm) the performed setup procedures’ accuracy.

Highlights

Postmastectomy radiation therapy (PMRT) is technically difficult, given the complexity of the target volume and its proximity to critical structures, including the heart, lung, brachial plexus, and contralateral breast.[1,2,3] More advanced techniques like intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) can achieve highly conformal dose distributions with improved target volume coverage and sparing of normal tissues compared to conventional techniques
This study aims to investigate the robustness of the patient setup procedure in use for PMRT when the Cone beam computed tomography (CBCT) considered the gold standard for the patient setup, is not used daily
The percentage of OTL registered during the current setup was three times greater (9.5% vs 3.1%) than the one registered at the reference setup

Summary

Introduction

Postmastectomy radiation therapy (PMRT) is technically difficult, given the complexity of the target volume and its proximity to critical structures, including the heart, lung, brachial plexus, and contralateral breast.[1,2,3] More advanced techniques like intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) can achieve highly conformal dose distributions with improved target volume coverage and sparing of normal tissues compared to conventional techniques. These errors cannot be detected by pretreatment verification or through accurate quality control of the connected machines and medical devices.[11,12] The notion has gained ground that these techniques only benefit patients when a good imaging and patient positioning technique is available, suggesting a combined IMRT and image‐guided radiation therapy approach.[13,14] Cone beam computed tomography (CBCT) scans can be considered a gold standard to assess interfraction uncertainties for many radiotherapy treatments[15,16] including treatments in the breast area[17]; Jain et al.[18] registered interfractional systematic (random) setup errors of 5.7 (3.9), 2.8 (3.5), 2.3 (3.2) mm in the lateral, vertical and longitudinal directions significantly affecting the target dose homogeneity (1.8% target received > 105% of the planned mean dose)

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