ABSTRACT
Low dose-rate permanent implant brachytherapy is widely used in the management of patients with early stage prostate cancer. An assessment of the implant quality is usually carried out 30 days after the implant is delivered, using computed tomography (CT) to identify the prostate and seeds. This is difficult due to poor contrast of the prostate and the superposition of seeds in the CT images. Magnetic resonance (MR) imaging offers superior contrast but inferior visualization of seeds. At our centre, patients are imaged using both CT and T2 weighted MR 30 days after an implant, and the image sets are fused using a commercial software package. The seeds are identified on CT and the prostate volumes are contoured on MR, with fusion performed by matching seeds on CT with seed signal voids on MR. The purpose of this study was to compare standard prostate post-implant dosimetric parameters (D90, V100, etc.) for prostates contoured on CT alone (MR blinded) versus MR/CT fusion. 25 patients were evaluated with all contouring performed by the same physician. We found that the prostate volume was overestimated using CT alone as compared to MR/CT fusion (mean: 37.2cc vs. 35.0cc respectively, p = 0.033). We also found that dosimetric parameters were underestimated for CT alone compared to MR/CT fusion, including D90 (mean: 144.3Gy vs. 150.8Gy respectively, p = 0.005) and V100 (mean: 89.2% vs. 91.0% respectively, p = 0.01). Centres using CT alone for post-implant dosimetry may therefore be underestimating their implant quality.
ABSTRACT
We have investigated the influence of the linear accelerator (LINAC) monitor chamber calibration on the dosimetry of Siemens Virtual Wedge (VW.) The doses delivered in the three phases of wedge delivery (initial gap, sweep portion, and open field) utilize the ionization current generated in two dose monitoring ion chambers (MONITOR 1 and MONITOR 2) in the LINAC to control the wedge delivery. We intentionally offset the calibration of each of these chambers by +/- 3% and observed up to a 13% change in the dose along the wedge profile for a 6 MV beam at a field size of 20 x 20 cm2. If the calibration of one of the two dose monitoring chambers changed independently then the relative dose at points along the wedge profile were affected. Furthermore, the percentage change in dose varied across the wedge profile thereby affecting the wedge angle as well as the central axis wedge factor. We also present equations for calculating the change in dose at a position along the wedge profile as a function of monitor chamber calibration. A comparison with measurement showed that our theoretical predictions were accurate to within +/- 1.7%. The equations have proven useful tools in evaluating periodic drifts in VW dosimetry.
