Effect of accounting for interfractional CTV shape variations in PTV margins on prostate cancer radiation treatment plans

Taka-Aki Hirose; Hidetaka Arimura; Yusuke Shibayama; Jun-Ichi Fukunaga; Saiji Ohga

doi:10.1016/j.ejmp.2018.09.008

Effect of accounting for interfractional CTV shape variations in PTV margins on prostate cancer radiation treatment plans

Phys Med. 2018 Oct:54:66-76. doi: 10.1016/j.ejmp.2018.09.008. Epub 2018 Sep 29.

Authors

Taka-Aki Hirose¹, Hidetaka Arimura², Yusuke Shibayama³, Jun-Ichi Fukunaga³, Saiji Ohga⁴

Affiliations

¹ Graduate School of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.
² Faculty of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka 812-8582, Japan. Electronic address: arimurah@med.kyushu-u.ac.jp.
³ Kyushu University Hospital, 3-1-1, Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.
⁴ Faculty of Medical Sciences, Kyushu University, 3-1-1, Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.

PMID: 30337012
DOI: 10.1016/j.ejmp.2018.09.008

Abstract

Purpose: The aim of this study was to account for interfractional clinical target volume (CTV) shape variation and apply this to the planning target volume (PTV) margin for prostate cancer radiation treatment plans.

Methods: Interfractional CTV shape variations were estimated from weekly cone-beam computed tomography (CBCT) images using statistical point distribution models. The interfractional CTV shape variation was taken into account in the van Herk's margin formula. The PTV margins without and with the CTV shape variation, i.e., standard (PTV_ori) and new (PTV_shape) margins, were applied to 10 clinical cases that had weekly CBCT images acquired during their treatment sessions. Each patient was replanned for low-, intermediate-, and high-risk CTVs, using both margins. The dose indices (D98 and V70) of treatment plans with the two margins were compared on weekly pseudo-planning computed tomography (PCT) images, which were defined as PCT images registered using a deformable image registration technique with weekly CBCT images, including contours of the CTV, rectum, and bladder.

Results: The percentage of treatment fractions of patients who received CTV D98 greater than 95% of a prescribed dose increased from 80.3 (PTV_ori) to 81.8% (PTV_shape) for low-risk CTVs, 78.8 (PTV_ori) to 87.9% (PTV_shape) for intermediate-risk CTVs, and 80.3 (PTV_ori) to 87.9% (PTV_shape) for high-risk CTVs. In most cases, the dose indices of the rectum and bladder were acceptable in clinical practice.

Conclusion: The results of this study suggest that interfractional CTV shape variations should be taken into account when determining PTV margins to increase CTV coverages.

Keywords: PTV margin; Prostate cancer radiation therapy; Shape variation; Treatment planning.

MeSH terms

Cone-Beam Computed Tomography
Dose Fractionation, Radiation*
Humans
Male
Organs at Risk / radiation effects
Prostatic Neoplasms / diagnostic imaging
Prostatic Neoplasms / radiotherapy*
Radiotherapy Planning, Computer-Assisted / methods*
Radiotherapy, Image-Guided / adverse effects