Utilization of cone‐beam CT for reconstruction of dose distribution delivered in image‐guided radiotherapy of prostate carcinoma — bony landmark setup compared to fiducial markers setup

Petr Paluska,Jan Jansa,Lucie Rouskova,Linda Kasaova,Jiri Petera,Milan Vosmik,Jana Sefrova,Josef Hanus,Jakub Grepl,Milan Zouhar,Miroslav Hodek

doi:10.1120/jacmp.v14i3.4203

Abstract

The purpose of this study was to compare two different styles of prostate IGRT: bony landmark (BL) setup vs. fiducial markers (FM) setup. Twenty‐nine prostate patients were treated with daily BL setup and 30 patients with daily FM setup. Delivered dose distribution was reconstructed on cone‐beam CT (CBCT) acquired once a week immediately after the alignment. Target dose coverage was evaluated by the proportion of the CTV encompassed by the 95% isodose. Original plans employed 1 cm safety margin. Alternative plans assuming smaller 7 mm margin between CTV and PTV were evaluated in the same way. Rectal and bladder volumes were compared with initial ones. While the margin reduction in case of BL setup makes the prostate coverage significantly worse (p=0.0003, McNemar's test), in case of FM setup with the reduced 7 mm margin, the prostate coverage is even better compared to BL setup with 10 mm margin (p=0.049, Fisher's exact test). Moreover, partial volumes of organs at risk irradiated with a specific dose can be significantly lowered (p<0.0001, unpaired t‐test). Reducing of safety margin is not acceptable in case of BL setup, while the margin can be lowered from 10 mm to 7 mm in case of FM setup.PACS numbers: 87.55.dk, 87.55.km, 87.55.tm

Highlights

100 Paluska et al.: Utilization of cone-beam CT (CBCT) for prostate IGRT should not be used as a surrogate for prostate motion
“The limited interuser variability and the marker stability make markers an ideal surrogate for the prostate position”.(1) While the planning target volumes (PTVs) margin required for bony anatomy alignment should be within the range of 10 mm, the PTV margin reguired for marker alignment can be lowered to under 4 mm.[4]. These margins were calculated and were considered as intrafraction motion, which plays only a marginal role in determining PTV margins for bony anatomy alignment, but it does play a crucial role for marker-based alignment
While the apex is largely immobile, prostate motion is well-described by rotation, but does undergo deformation due to rectal distension.[7]. Effects of rectal motion during prostate radiotherapy with regard to rectal dose and clinical target volume (CTV) dose coverage were studied by Sripadam et al[8] This study revealed instances of insufficient CTV coverage

Summary

Introduction

100 Paluska et al.: Utilization of CBCT for prostate IGRT should not be used as a surrogate for prostate motion. Fiducial markers kV imaging versus CBCT for prostate IGRT was compared by Barney et al[5] They preferred fiducial imaging, because it requires less daily input, is less time-consuming, and is a more reliable, reproducible treatment. A comparison of CBCT automatic grey-value alignment to implanted fiducial marker alignment was made by Shi et al[6] with the conclusion that “CBCT with soft-tissue-based automatic corrections is not an accurate alignment compared with manual alignment to fiducial markers.”. A comparison of CBCT automatic grey-value alignment to implanted fiducial marker alignment was made by Shi et al[6] with the conclusion that “CBCT with soft-tissue-based automatic corrections is not an accurate alignment compared with manual alignment to fiducial markers.” a daily manual alignment to fiducials is considered to be one of the most reliable methods to maintain accuracy in prostate IGRT

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Conclusion