Optimal contouring of seminal vesicle for definitive radiotherapy of localized prostate cancer: comparison between EORTC prostate cancer radiotherapy guideline, RTOG0815 protocol and actual anatomy

Abstract

Background: Intermediate- to-high-risk prostate cancer can locally invade seminal vesicle (SV). It is recommended that anatomic proximal 1-cm to 2-cm SV be included in the clinical target volume (CTV) for definitive radiotherapy based on pathology studies. However, it remains unclear whether the pathology indicated SV extent is included into the CTV defined by current guidelines. The purpose of this study is to compare the volume of proximal SV included in CTV defined by EORTC prostate cancer radiotherapy guideline and RTOG0815 protocol with the actual anatomic volume.

Methods: Radiotherapy planning CT images from 114 patients with intermediate- (36.8%) or high-risk (63.2%) prostate cancer were reconstructed with 1-mm-thick sections. The starting and ending points of SV and the cross sections of SV at 1-cm and 2-cm from the starting point were determined using 3D-view. Maximum (D1H, D2H) and minimum (D1L, D2L) vertical distance from these cross sections to the starting point were measured. Then, CTV of proximal SV defined by actual anatomy, EORTC guideline and RTOG0815 protocol were contoured and compared (paired t test).

Results: Median length of D1H, D1L, D2H and D2L was 10.8 mm, 2.1 mm, 17.6 mm and 8.8 mm (95th percentile: 13.5mm, 5.0mm, 21.5mm and 13.5mm, respectively). For intermediate-risk patients, the proximal 1-cm SV CTV defined by EORTC guideline and RTOG0815 protocol inadequately included the anatomic proximal 1-cm SV in 62.3% (71/114) and 71.0% (81/114) cases, respectively. While for high-risk patients, the proximal 2-cm SV CTV defined by EORTC guideline inadequately included the anatomic proximal 2-cm SV in 17.5% (20/114) cases.

Conclusions: SV involvement indicated by pathology studies was not completely included in the CTV defined by current guidelines. Delineation of proximal 1.4 cm and 2.2 cm SV in axial plane may be adequate to include the anatomic proximal 1-cm and 2-cm SV. However, part of SV may be over-contoured.

Figures

Figure 1

Drawing the central line of SV. A. P0 indicates the starting point of the SV, located in the first axial slice where both prostate and SV are visible. It is determined by referencing the coronal and the sagittal views to ensure that it is centered in the SV contour in both planes. B. Pend indicates the ending point of the SV in the last slice. The exact location needs to be determined by referencing the morphology in both the coronal and the sagittal planes. C. After locating the starting and ending points, an oblique coronal plane containing both endings is obtained by rotating the cut lines in the sagittal window. The central line is determined by drawing a curve connecting P0 and Pend along the middle line of the SV.

Figure 2

Delineation of anatomic proximal 2-cm SV. A. Locating a point 2-cm from the starting point of the SV on its central line (P2), cut lines in the oblique coronal plane are centered at P2 and adjusted until the sagittal cut plane becoming tangent to the central line. B. Oblique sagittal plane including the SV. C. Cut planes orthogonal to the central line. H2 and L2 indicate the points with maximum and minimum vertical distance to the starting plane of the SV. “*” indicates the vas deferens. D. Maximum diameter (R2) of the 2-cm cross section (R2).

Figure 3

Different volumes of proximal SV CTV. Yellow line indicates VANAT-1, red line indicates VRTOG, dark green line indicates VEORTC-1, orange line indicates VANAT-2, blue line indicates VEORTC-2, and light green line indicates the prostate.

Figure 4

Relationship between CTV extent and anatomic SV included. Schematic diagram indicates the cross section of the SV. P0Pend indicates the central line. HH’ and LL’ indicate the maximum and minimum vertical distance to the starting plane of the SV. Dotted area indicates the part of SV that might be over-contoured for irradiation.