A critical evaluation of the planning target volume for 3-D conformal radiotherapy of prostate cancer

Purpose: To determine an adequate planning target volume (PTV) margin for three-dimensional conformal radiotherapy (3D CRT) of prostate cancer, the uncertainties in the internal positions of the prostate and seminal vesicles (SV) and in the treatment setups were measured. Methods and Materials: Weekly computed tomography (CT) scans of the pelvis (n = 51) and daily electronic portal images (n = 1630) were reviewed for eight patients who received seven- field 3D CRT for prostate cancer. The CT scans were registered in three dimensions to the original planning CT scan using commercially available software to measure the center-of volume (COV) motion of the prostate and SV. The daily portal images were registered to the corresponding simulation films to measure the setup displacements. The standard deviation (SD) of the internal organ motions was added to the SD of the setups in quadrature to determine the total uncertainty. Positive directions were left, anterior, and superior. Rotations necessary to register the CT scans and portal images were minimal and not further analyzed. Results: The mean motion for the COV of the prostate ± the SD was ± 0.9 mm in the left-right (LR), 0.5 ± 2.6 in the anterior-posterior (AP), and 1.5 ± 3,9 mm in the superior-inferior (SI) directions. The mean motion for the COV of the SV ± the SD was 0.3 ± 1.7 mm in the LR, 0.7 ± 3.8 mm in the AP, and 0.9 ± 3.5 mm in the SI directions. For all patients the mean isocenter displacement ± the SD was 0 ± 3.1 mm in the LR, 1.4 ± 3.0 mm in the AP, and -0.4 ± 2.1 mm in the SI directions. The total uncertainty for the prostate was 3.2 mm, 4.0 mm, and 4.4 mm in the LR, AP, and SI directions, respectively. For the SV, the total uncertainty was 3.5, 4.8, and 4.1 mm in the LR, AP, and SI directions, respectively, Conclusions: PTV margins of 10 to 16 mm are required to encompass all (99 %) possible positions of the prostate or SV during 3D CRT. PTV margins of 7 to 11 mm will encompass the measured uncertainties with a 95% probability. PTV margins of 5 mm may not adequately cover the intended volume.