Volumetric-based image guidance is superior to marker-based alignments for stereotactic body radiotherapy of prostate cancer.

Wen Li,Toufik Djemil,Rahul D Tendulkar,Naveen Sharma,Kevin L Stephans,Andrew Vassil,Ping Xia,Abigail Stockham,Zhilei Liu Shen,Lan Lu

doi:10.1002/acm2.12280

Abstract

PurposesThe aim of this study was to evaluate a dual marker‐based and soft‐tissue based image guidance for inter‐fractional corrections in stereotactic body radiotherapy (SBRT) of prostate cancer.Methods/MaterialsWe reviewed 18 patients treated with SBRT for prostate cancer. An endorectal balloon was inserted at simulation and each treatment. Planning margins were 3 mm/0 mm posteriorly. Prior to each treatment, a dual image guidance protocol was applied to align three makers using stereoscopic x ray images and then to the soft tissue using kilo‐voltage cone beam CT (kV‐CBCT). After treatment, prostate (CTV), rectal wall, and bladder were delineated on each kV‐CBCT, and delivered dose was recalculated. Dosimetric endpoints were analyzed, including V36.25 Gy for prostate, and D0.03 cc for bladder and rectal wall.ResultsFollowing initial marker alignment, additional translational shifts were applied to 22 of 84 fractions after kV‐CBCT. Among the 22 fractions, ten fractions exceeded 3 mm shifts in any direction, including one in the left‐right direction, four in the superior‐inferior direction, and five in the anterior‐posterior direction. With and without the additional kV‐CBCT shifts, the average V36.25 Gy of the prostate for the 22 fractions was 97.6 ± 2.6% with the kV x ray image alone, and was 98.1 ± 2.4% after applying the additional kV‐CBCT shifts. The improvement was borderline statistical significance using Wilcoxon signed‐rank test (P = 0.007). D0.03 cc was 45.8 ± 6.3 Gy vs. 45.1 ± 4.9 Gy for the rectal wall; and 49.5 ± 8.6 Gy vs. 49.3 ± 7.9 Gy for the bladder before and after applying kV‐CBCT shifts.ConclusionsMarker‐based alignment alone is not sufficient. Additional adjustments are needed for some patients based kV‐CBCT.

Highlights

Stereotactic body radiation therapy (SBRT) is a promising treatment regimen for localized prostate cancer because of the low a/b ratio of the prostate adenocarcinoma.[1]
After applying the additional kilo-voltage cone beam CT (kV-CBCT) guided shifts, the average V36.25 Gy increased to 98.1 Æ 2.4%
After marker-based kV x ray guidance and 6D corrections, additional translational adjustments based on soft-tissue alignment on kV-CBCT are needed in 26% of the fractions (22 of 84), indicating that marker-based alignment alone is not sufficient, CBCT-based soft-tissue alignment with implanted marker guidance is superior to the marker alignment alone

Summary

Introduction

Stereotactic body radiation therapy (SBRT) is a promising treatment regimen for localized prostate cancer because of the low a/b ratio of the prostate adenocarcinoma.[1]. Intra-fractional motion can be managed with use of an endorectal balloon.[4,5,6,7,8] it may introduce a large prostate inter-fraction motion, rotation, and deformation. Jones et al reported that 69% of fractions required insertion adjustments of the endorectal balloon to reduce prostate rotation and deformation.[6] They recommended to acquire two Cone-Beam Computed Tomography (CBCTs) for each patient: one after insertion and the other after adjustment. With 2-, 3-, and 5-mm Planning Target Volume (PTV) margins, Amro et al showed that only 39%, 65%, and 84% of 26 patients had adequate dose coverage to the prostate without rotation correction, respectively.[9]

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