Interfraction Anatomical Variability Can Lead to Significantly Increased Rectal Dose for Patients Undergoing Stereotactic Body Radiotherapy for Prostate Cancer - PubMed (original) (raw)

Interfraction Anatomical Variability Can Lead to Significantly Increased Rectal Dose for Patients Undergoing Stereotactic Body Radiotherapy for Prostate Cancer

Michael Wahl et al. Technol Cancer Res Treat. 2017 Apr.

Abstract

Stereotactic body radiotherapy for prostate cancer is rapidly growing in popularity. Stereotactic body radiotherapy plans mimic those of high-dose rate brachytherapy, with tight margins and inhomogeneous dose distributions. The impact of interfraction anatomical changes on the dose received by organs at risk under these conditions has not been well documented. To estimate anatomical variation during stereotactic body radiotherapy, 10 patients were identified who received a prostate boost using robotic stereotactic body radiotherapy after completing 25 fractions of pelvic radiotherapy with daily megavoltage computed tomography. Rectal and bladder volumes were delineated on each megavoltage computed tomography, and the stereotactic body radiotherapy boost plan was registered to each megavoltage computed tomography image using a point-based rigid registration with 3 fiducial markers placed in the prostate. The volume of rectum and bladder receiving 75% of the prescription dose (V75%) was measured for each megavoltage computed tomography. The rectal V75% from the daily megavoltage computed tomographies was significantly greater than the planned V75% (median increase of 0.93 cm3, P < .001), whereas the bladder V75% on megavoltage computed tomography was not significantly changed (median decrease of -0.12 cm3, P = .57). Although daily prostate rotation was significantly correlated with bladder V75% (Spearman ρ = .21, P = .023), there was no association between rotation and rectal V75% or between prostate deformation and either rectal or bladder V75%. Planning organ-at-risk volume-based replanning techniques using either a 6-mm isotropic expansion of the plan rectal contour or a 1-cm expansion from the planning target volume in the superior and posterior directions demonstrated significantly improved rectal V75% on daily megavoltage computed tomographies compared to the original stereotactic body radiotherapy plan, without compromising plan quality. Thus, despite tight margins and full translational and rotational corrections provided by robotic stereotactic body radiotherapy, we find that interfraction anatomical variations can lead to a substantial increase in delivered rectal doses during prostate stereotactic body radiotherapy. A planning organ-at-risk volume-based approach to treatment planning may help mitigate the impact of daily organ motion and reduce the risk of rectal toxicity.

Keywords: CyberKnife; IGRT; SABR; SBRT; dosimetry.

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Conflict of interest statement

Declaration of Conflicting Interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Figures

Figure 1.

Figure 1.

Rectal and bladder contours on planning computed tomographies and daily megavoltage computed tomographies (MVCTs). A, Rectal contour (brown) delineated on CK planning computed tomography (CT) for patient 5. B, Rectal contour delineated on an MVCT acquired during external beam radiotherapy, shown on same slice as (A). C, Bladder contour (yellow) delineated on CK planning CT for patient 1. D, Bladder contour delineated on MVCT acquired during external beam radiotherapy, shown on same slice as (C). For (A) to (D), prescription isodose line (19 Gy) is shown in purple, whereas the 75% isodose line (14.25 Gy) is shown in teal. Note: The color version of the figure is available at

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Figure 2.

Figure 2.

Rectal and bladder dosimetry. A, Distribution of changes in rectal V75% for all 8 patients analyzed. B, Distribution of changes in bladder V75% for all 6 patients analyzed. C, Boxplot depicting median and distribution of change in rectal V75% for each individual patient. D, Boxplot depicting median and distribution of change in bladder V75% for each individual patient. For boxplots, the lower and upper border of the box indicates the 25th and 75th percentile, respectively, the dashed whiskers indicate the 1.5 interquartile range above the upper quartile and the lower quartile, and the circles indicate outliers.

Figure 3.

Figure 3.

Example patient (#5) demonstrating variable rectal position and received dose without prostate rotation or deformation. Parasagittal view of the prostate, rectum, and bladder shown on (A) stereotactic body radiotherapy (SBRT) planning computed tomography (CT) and (B) daily megavoltage computed tomography (MVCT) during external beam radiotherapy. For both images, the rectal contour is shown in brown, the bladder contour is shown in yellow, the prescription isodose line (19 Gy) is shown in magenta, and the 75% isodose line (14.25 Gy) is shown in teal. The yellow arrow denotes a region of gas-filled rectum projecting superior to the prostate seen on MVCT, which yields a significant portion of the rectum receiving over 75% of prescription dose. This region was not contoured as part of the rectum on planning CT. For this MVCT, the prostate rotation (pitch) was 4.3°, maximum rigid body error (RBEmax) was 2.28 mm, and rectal and bladder V75% were 14.97 and 7.64 cm3, respectively. Note: The color version of the figure is available at

journals.sagepub.com/home/tct

Figure 4.

Figure 4.

Planning organ-at-risk volume (PRV)-based planning technique with simulated replanning performed on patients 3 and 5. A, Dose–volume histogram (DVH) comparing original plan (solid) with PTVexp plan (dashed) for planning target volume (PTV), urethra, sum of daily rectal contours (RectumSUM), and bladder contours (BladderSUM) for patient 3. B, Distribution of the change in daily rectal V75% using the original planning technique and 3 PRV-based planning techniques (PRVempiric, PRVexpand, and PRVshell). All PRV-based planning approaches significantly reduced daily rectal V75% compared to the original stereotactic body radiotherapy (SBRT) plan.

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