BERKELEY, CA (UroToday.com) - Several studies have been conducted to determine whether a supine or prone fixation position is superior for external beam radiotherapy (EBRT) in patients with prostate cancer. However, the optimal treatment position for prostate EBRT remains controversial and inconclusive because each position has its own advantages and disadvantages.
We have treated patients with localized prostate cancer using intensity-modulated radiotherapy (IMRT) in the prone position, fixed with a thermoplastic shell, since 2000. We adopted the prone position for two reasons. First, the rectal dose is reduced because we treat mainly locally advanced prostate cancer patients in whom the seminal vesicles are included in the clinical target volume (CTV). Second, we used bony-structure-based positioning because we had no soft tissue-based image-guided radiotherapy (IGRT) options when we first began using IMRT for prostate cancer. To compensate for the expected large amount of prostate motion in the prone position, we immobilized patients using the thermoplastic shell.
Beginning in 2007, we were able to use a dual orthogonal kilo-voltage (kV) X-ray IGRT system for patient positioning. This system also allowed the detection of calcification within the prostate. Because calcification in the prostate is a reliable marker of prostate position, using this IGRT system, we are now able to assess inter-fractional prostate motion in all fractions without the need for an invasive procedure, based on routinely acquired clinical data.
To our knowledge, no report to date has determined the adequate planning target volume (PTV) margin in the prone position while immobilized using a thermoplastic shell. The aim of this study was to evaluate inter-fractional prostate motion in patients immobilized with a thermoplastic shell in the prone position. We also sought to validate the adequacy of the PTV margin used at our institute.
Our results for inter-fractional prostate motion fall within the range of those reported previously, including seven studies in the supine position without a thermoplastic shell, one in the supine position with a thermoplastic shell, and one in the prone position without a thermoplastic shell. This demonstrates that the effect of immobilization with a thermoplastic shell in the prone position is comparable to that in the supine position. If identical PTV margins are necessary in both positions, the prone position may be preferable due to the reduced radiation dose to the rectum.
Our required PTV margin, using the van Herk formula, is similar to the PTV margin at our institute (CTV plus a 9-mm margin everywhere except posteriorly, where a 6-mm margin is used). Indeed, the CTV of 87.5% was within our PTV margins for 90% or more of all treatment sessions. However, to prevent delivery of an insufficient dose to the CTV in patients with relatively large prostate movements, it will be necessary to shift to a prostate-based IGRT approach.
In conclusion, inter-fractional prostate motion in those immobilized in the prone position using a thermoplastic shell is equivalent to that in the supine position reported elsewhere. The PTV margin at our institute is generally appropriate when aligned to the bony structure, although prostate-based positioning will be required for patients with greater prostate motion.
Written by:
Itaru Ikeda, PhD student as part of Beyond the Abstract on UroToday.com. This initiative offers a method of publishing for the professional urology community. Authors are given an opportunity to expand on the circumstances, limitations etc... of their research by referencing the published abstract.
Department of Radiation Oncology and Image-applied Therapy
Kyoto University Graduate School of Medicine
54 Shogoin-Kawaharacho, Sakyo-ku, Kyoto 606-8507, Japan
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