Daily bone alignment with limited repeat CT correction rivals daily ultrasound alignment for prostate radiotherapy

Purpose: To compare the effectiveness of daily ultrasound (US)- and computed tomography (CT)-guided alignments with an off-line correction protocol using daily bone alignment plus a correction factor for systematic internal prostate displacement (CFID). Methods and Materials: Ten prostate cancer patients underwent CT scans three times weekly using an integrated CT-linear accelerator system, followed by alignment using US for daily radiotherapy. Intensity-modulated radiotherapy plans were designed with our current clinical margins. The treatment plan was copied onto the repeat CT images and aligned using several methods: (1) bone alignment plus CFID after three off-line CT scans (bone+3CT), (2) bone alignment plus CFID after six off-line CT scans (bone+6CT), (3) US alignment, and (4) CT alignment. The accuracy of the repeated US and CT measurements to determine the CFID was compared. The target dosimetric effect was quantified. Results: The CFID for internal systematic prostate displacements was more accurately measured with limited repeat CT scans than with US (residual error, 0.0 +/- 0.7 mm vs. 2.0 +/- 3.2 mm). Bone+3CT, bone+6CT, and US provided equivalent prostate and seminal vesicle dose coverage, but bone+3CT and bone+6CT produced more precise daily alignments. Daily CT alignment provided the greatest target dose coverage. Conclusion: Daily bone alignment Plus CFID for internal systematic prostate displacement provided better daily alignment precision and equivalent dose coverage compared with daily US alignment. The CFID should be based on at least three repeat CT scans, which could be collected before the start of treatment or during the first 3 treatment days. Daily bone alignment plus CFID provides another option for accurate prostate cancer patient positioning. (c) 2008 Elsevier Inc.