Coverage-based treatment planning to accommodate delineation uncertainties in prostate cancer treatment

Abstract

Purpose: To compare two coverage-based planning (CP) techniques with fixed margin-based (FM) planning for high-risk prostate cancer treatments, with the exclusive consideration of the dosimetric impact of delineation uncertainties of target structures and normal tissues.

Methods: In this work, 19-patient data sets were involved. To estimate structure dose for each delineated contour under the influence of interobserver contour variability and CT image quality limitations, 1000 alternative structures were simulated by an average-surface-of-standard-deviation model, which utilized the patient-specific information of delineated structure and CT image contrast. An IMRT plan with zero planning-target-volume (PTV) margin on the delineated prostate and seminal vesicles [clinical-target-volume (CTV prostate) and CTVSV] was created and dose degradation due to contour variability was quantified by the dosimetric consequences of 1000 alternative structures. When D98 failed to achieve a 95% coverage probability objective D98,95 ≥ 78 Gy (CTV prostate) or D98,95 ≥ 66 Gy (CTVSV), replanning was performed using three planning techniques: (1) FM (PTV prostate margin = 4,5,6 mm and PTVSV margin = 4,5,7 mm for RL, PA, and SI directions, respectively), (2) CPOM which optimized uniform PTV margins for CTV prostate and CTVSV to meet the D98,95 objectives, and (3) CPCOP which directly optimized coverage-based objectives for all the structures. These plans were intercompared by computing percentile dose-volume histograms and tumor-control probability/normal tissue complication probability (TCP/NTCP) distributions.

Results: Inherent contour variability resulted in unacceptable CTV coverage for the zero-PTV-margin plans for all patients. For plans designed to accommodate contour variability, 18/19 CP plans were most favored by achieving desirable D98,95 and TCP/NTCP values. The average improvement of probability of complication free control was 9.3% for CPCOP plans and 3.4% for CPOM plans.

Conclusions: When the delineation uncertainties need to be considered for prostate patients, CP techniques can produce more desirable plans than FM plans for most patients. The relative advantages between CPCOP and CPOM techniques are patient specific.

FIG. 1.

CT image slice in transverse plane for rectum (solid-line contour) and corresponding (D_r)_i,k = (F_SD)_r,k ⋅ (F_CT)_r,i with |(F_SD)_r,k| = 1 for display purposes.

FIG. 2.

(a) Percentage degraded dose (%ΔD_98,95) and (b) DVH variability (%ΔDV H_98,5–95) at prescribed dose of CTV_prostate (black bins) and CTV_SV (upward diagonal bins) for 19 zero-PTV-margin plans with delineation uncertainties considered using the ASSD model.

FIG. 3.

Axial, sagittal, and coronal dose distributions of CP_COP versus CP_OM versus FM plans on one slice for patient 8 when delineation uncertainties are considered. The thick isodose surfaces are the 78 Gy (seashell) TV for the CTV_prostate (red colorwash) and the 66 Gy (aquamarine) TV for CTV_SV (green colorwash). The yellow and magenta colorwashs are the bladder and rectum. The thin isodose surfaces are 75 Gy (maroon), 69 Gy (slateblue), 57 Gy (lightblue), and 40 Gy (green). Compared to CP_OM and FM plans, the CP_COP dosimetric margin (the distance between the CTV_prostate and the 78 Gy TV) on the sides of bladder and rectum is, in general, smaller due to CP_COP’s inclusion of OAR uncertainties in the objective function. This explains why a CP_COP plan is more likely to have better normal tissue sparing and sometimes lower target coverage.