Abstract
PurposeThere remain uncertainties due to inter‐ and intraobserver variability in soft‐tissue‐based patient positioning even with the use of image‐guided radiation therapy (IGRT). This study aimed to reveal observer uncertainties of soft‐tissue‐based patient positioning on cone‐beam computed tomography (CBCT) images for prostate cancer IGRT.MethodsTwenty‐six patients (7–8 fractions/patient, total number of 204 fractions) who underwent IGRT for prostate cancer were selected. Six radiation therapists retrospectively measured prostate cancer location errors (PCLEs) of soft‐tissue‐based patient positioning between planning CT (pCT) and pretreatment CBCT (pre‐CBCT) images after automatic bone‐based registration. Observer uncertainties were evaluated based on residual errors, which denoted the differences between soft‐tissue and reference positioning errors. Reference positioning errors were obtained as PCLEs of contour‐based patient positioning between pCT and pre‐CBCT images. Intraobserver variations were obtained from the difference between the first and second soft‐tissue‐based patient positioning repeated by the same observer for each fraction. Systematic and random errors of inter‐ and intraobserver variations were calculated in anterior–posterior (AP), superior–inferior (SI), and left–right (LR) directions. Finally, clinical target volume (CTV)‐to‐planning target volume (PTV) margins were obtained from systematic and random errors of inter‐ and intraobserver variations in AP, SI, and LR directions.ResultsInterobserver variations in AP, SI, and LR directions were 0.9, 0.9, and 0.5 mm, respectively, for the systematic error, and 1.8, 2.2, and 1.1 mm, respectively, for random error. Intraobserver variations were <0.2 mm in all directions. CTV‐to‐PTV margins in AP, SI, and LR directions were 3.5, 3.8, and 2.1 mm, respectively.ConclusionIntraobserver variability was sufficiently small and would be negligible. However, uncertainties due to interobserver variability for soft‐tissue‐based patient positioning using CBCT images should be considered in CTV‐to‐PTV margins.
Highlights
Prostate cancer is the most frequently diagnosed cancer in males
In the current radiation therapy for prostate cancer, image‐ guided radiation therapy (IGRT) with cone‐beam computed tomography (CBCT) images has been commonly used in clinical practice to increase the accuracy of patient positioning.[4]
Zelefsky et al reported that intensity modulated radiotherapy (IMRT) with image‐guided patient positioning (IGPP) improved prostate‐ specific antigen (PSA) outcomes and toxicities of organs at risk (OAR).[5]
Summary
Prostate cancer is the most frequently diagnosed cancer in males. Approximately 1.3 million new cases of prostate cancer and 359 000 associated deaths were reported worldwide in 2018.1 Common approaches for treating localized prostate cancer include active surveillance, radical prostatectomy, radiotherapy, and hormonal therapy.[2]. In the current radiation therapy for prostate cancer, image‐ guided radiation therapy (IGRT) with cone‐beam computed tomography (CBCT) images has been commonly used in clinical practice to increase the accuracy of patient positioning.[4] Zelefsky et al reported that intensity modulated radiotherapy (IMRT) with image‐guided patient positioning (IGPP) improved prostate‐ specific antigen (PSA) outcomes and toxicities of organs at risk (OAR).[5]. Soft‐tissue‐based patient positioning with CBCT images is a noninvasive approach and has the advantage of providing soft‐tissue information such as circumstances of targets and critical organs. There are uncertainties due to inter‐ and intraobserver variability,[6,7,8,9] which may influence clinical outcomes and OAR toxicities
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