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

Abstract

Postmastectomy radiation therapy is technically difficult and can be considered one of the most complex techniques concerning patient setup reproducibility. Slight patient setup variationsparticularly when high-conformal treatment techniques are usedcan 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 γ mean < 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. K E Y W O R D Sbreast radiotherapy, in vivo dosimetry, volumetric modulated arc therapy ---

Combined Use of a Transmission Detector and an EPID-Based In Vivo Dose Monitoring System in External Beam Whole Breast Irradiation: A Study with an Anthropomorphic Female Phantom

“…To recover full sensitivity to 3D displacements a gamma-index analysis was needed. This analysis, with 2%/2mm criteria, yielded alert thresholds of about 90% for γ PR and 0.4 for γ mean , in agreement with those proposed in the literature [50]. The γ mean threshold was also demonstrated to be able to detect a phantom rotation of 2.8 • .…”

“…Separate use of IQM or SoftDiso devices has been widely discussed for different irradiation modalities [43][44][45][46][47][48][49], but to our knowledge, no study exists of their combined use, in particular for breast irradiation. A clinical study of the SoftDiso-based quality assurance procedure for post-mastectomy IMRT and VMAT radiation therapy is reported in reference [50], where specific thresholds for R-value, γ PR and γ mean indexes were proposed, based on the clinical experience and validated with a phantom chest. The authors confirmed the feasibility and the importance of a QA procedure based on EPID-based IVD to detect inter-fraction setup errors to overcome the weakness of a setup control based only on weekly cone beam CT acquisitions.…”

Analyses of integrated EPID images for on‐treatment quality assurance to account for interfractional variations in volumetric modulated arc therapy

“…In those studies, the point dose was estimated based on the photon fluence using a back‐projection algorithm. In other studies, point dose and dose distributions were evaluated using integrated EPID images for various treatment sites, and gamma analyses were performed throughout VMAT. Kang et al assessed the relationship between interfractional setup error and integrated EPID images for postmastectomy radiation therapy.…”

Comparing treatment uncertainty for ultra- vs. standard-hypofractionated breast radiation therapy based on in-vivo dosimetry

“…Quality assurance (QA) programs are designed to improve the quality and the safety of radiation treatments, including machine- and patient-specific QA [1] , [2] . The latter can be performed prior to treatment (pre-treatment verification) or during beam delivery (in-vivo dosimetry, IVD) [3] , [4] . IVD is recommended by many national and international organizations as a dosimetric safety tool to avoid major errors and to enhance accuracy [5] , [6] , [7] , [8] .…”