Evaluation of Daily CT for EPID-Based Transit In Vivo Dosimetry

Abstract

PurposeThe difference in anatomical structure and positioning between planning and treatment may lead to bias in electronic portal image device (EPID)-based in vivo dosimetry calculations. The purpose of this study was to use daily CT instead of planning CT as a reference for EPID-based in vivo dosimetry calculations and to analyze the necessity of using daily CT for EPID-based in vivo dosimetry calculations in terms of patient quality assurance.Materials and MethodsTwenty patients were enrolled in this study. The study design included eight different sites (the cervical, nasopharyngeal, and oral cavities, rectum, prostate, bladder, lung, and esophagus). All treatments were delivered with a CT-linac 506c (UIH, Shanghai) using 6 MV photon beams. This machine is equipped with diagnosis-level fan-beam CT and an amorphous silicon EPID XRD1642 (Varex Imaging Corporation, UT, USA). A Monte Carlo algorithm was developed to calculate the transmit EPID image. A pretreatment measurement was performed to assess system accuracy by delivering based on a homogeneous phantom (RW3 slab, PTW, Freiburg). During treatment, each patient underwent CT scanning before delivery either once or twice for a total of 268 fractions obtained daily CT images. Patients may have had a position correction that followed our image-guided radiation therapy (IGRT) procedure. Meanwhile, transmit EPID images were acquired for each field during delivery. After treatment, all patient CTs were reviewed to ensure that there was no large anatomical change between planning and treatment. The reference of transmit EPID images was calculated based on both planning and daily CTs, and the IGRT correction was corrected for the EPID calculation. The gamma passing rate (3 mm 3%, 2 mm 3%, and 2 mm 2%) was calculated and compared between the planning CT and daily CT. Mechanical errors [ ± 1 mm, ± 2 mm, ± 5 mm multileaf collimator (MLC) systematic shift and 3%, 5% monitor unit (MU) scaling] were also introduced in this study for comparing detectability between both types of CT.ResultThe average (standard deviation) gamma passing rate (3 mm 3%, 2 mm 3%, and 2 mm 2%) in the RW3 slab phantom was 99.6% ± 1.0%, 98.9% ± 2.1%, and 97.2% ± 3.9%. For patient measurement, the average (standard deviation) gamma passing rates were 87.8% ± 14.0%, 82.2% ± 16.9%, and 74.2% ± 18.9% for using planning CTs as reference and 93.6% ± 8.2%, 89.7% ± 11.0%, and 82.8% ± 14.7% for using daily CTs as reference. There were significant differences between the planning CT and daily CT results. All p-values (Mann–Whitney test) were less than 0.001. In terms of error simulation, nonparametric test shows that there were significant differences between practical daily results and error simulation results (p < 0.001). The receiver operating characteristic (ROC) analysis indicated that the detectability of mechanical delivery error using daily CT was better than that of planning CT. AUCDaily CT = 0.63–0.96 and AUCPlanning CT = 0.49–0.93 in MLC systematic shift and AUCDaily CT = 0.56–0.82 and AUCPlanning CT = 0.45–0.73 in MU scaling.ConclusionThis study shows the feasibility and effectiveness of using two-dimensional (2D) EPID portal image and daily CT-based in vivo dosimetry for intensity-modulated radiation therapy (IMRT) verification during treatment. The daily CT-based in vivo dosimetry has better sensitivity and specificity to identify the variation of IMRT in MLC-related and dose-related errors than planning CT-based.

Technical note: First implementation of a one‐stop solution of radiotherapy with full‐workflow automation based on CT‐linac combination

“…The effect of systematic changes in patient anatomy, such as tumor progression and weight alteration after the time of simulation, is minimized due to speeding up of treatment preparation. This is particularly true for the first fraction, which can be inferred from the fact that in vivo dosimetry of the All‐in‐One solution demonstrates excellent γ passing rates compared with the routine workflow, as the in vivo result is sensitive to both patient‐related variations and machine‐related errors 20,23 . Over the course of therapy, dosimetrical benefits from the reduced time expended in RT preparation might be limited due to the existence of inter‐fractional uncertainties (such as treatment‐induced tumor shrinkage).…”

“…In vivo dosimetry for online QA is available in this commercial system by measuring the exit dose from the patient using an amorphous silicon electronic portal imaging device (aSi‐EPID), and comparing with the TPS calculation by γ analysis based on patient anatomy using a Monte Carlo method. Prior to clinical implementation, the EPID detector response was corrected and calibrated for absolute dosimetry, and the accuracy of the transit dose calculation was validated by phantom measurement, as described in a previous study 20 …”

“…Prior to clinical implementation, the EPID detector response was corrected and calibrated for absolute dosimetry, and the accuracy of the transit dose calculation was validated by phantom measurement, as described in a previous study. 20…”

“…This is particularly true for the first fraction, which can be inferred from the fact that in vivo dosimetry of the All-in-One solution demonstrates excellent γ passing rates compared with the routine workflow, as the in vivo result is sensitive to both patient-related variations and machine-related errors. 20,23 Over the course of therapy, dosimetrical benefits from the reduced time expended in RT preparation might be limited due to the existence of inter-fractional uncertainties (such as treatment-induced tumor shrinkage). Anatomic and/or dosimetric changes should be evaluated if another All-in-One implementation is necessary.…”

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

“…EPIDs are present on modern linear accelerators, offering the possibility to perform patient position and dosimetric verification in 2D/3D, and automation [9] , [10] . Several studies have shown the sensitivity and effectiveness of EPID-based IVD (EIVD) for different treatment sites and relevant deviations encountered in clinical routine [11] , [12] , [13] , [14] .…”