METHODS: A planning computed tomography (CT) scan and 2 subsequent CT scans were prospectively obtained for 20 prostate cancer patients with intraprostatic fiducial markers. After CT registration, SV displacement relative to the prostate was quantified as a function of margin size for both the proximal (1 cm) SV (PSV) and the full SV (FSV). Two IMRT plans were simulated for each patient (prostate + PSV and prostate + FSV) both with a uniform 5-mm PTV margin. Minimum clinical target volume (CTV) dose (Dmin) and the volume of SV receiving 95% of the prescription dose (V95%) were assessed during treatment and compared with the initial plan.
RESULTS: In all cases, SV displacement with respect to the prostate was greater for the FSV compared with the PSV. To ensure at least 95% geometrical coverage of the CTV for 90% of patients, margins of 5 and 8 mm were required for the PSV and FSV, respectively. Dosimetrically, residual SV displacement had minimal impact on PSV coverage compared with FSV coverage. For the PSV Dmin was ≥95% of the prescribed dose in 90% of patients with an overall mean V95% of 99.6 ± 0.8%; for the FSV Dmin was ≥95% of the prescribed dose in only 45% of patients with a mean V95% of 97.9 ± 2.4%.
CONCLUSIONS: The SVs move differentially from the prostate and exhibit greater variation with increasing distance from the prostate. For plans targeting just the prostate and PSVs, 5-mm PTV expansions are adequate. However, despite daily localization of the prostate, larger PTV margins are required for cases where the intent is to completely cover the FSV.
Writtenby:
Stenmark MH, Vineberg K, Ten Haken RK, Hamstra DA, Feng M. Are you the author?
Department of Radiation Oncology, University of Michigan, Ann Arbor, MI.
Reference: Med Dosim. 2011 Dec 19. Epub ahead of print.
doi: 10.1016/j.meddos.2011.09.002
PubMed Abstract
PMID: 22189029
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