Daily Bone Alignment With Limited Repeat CT Correction Rivals Daily Ultrasound Alignment for Prostate Radiotherapy

Abstract

Daily ultrasound (US) alignment is commonly used to correct for internal prostate displacements. However, the accuracy of US is user-dependent. In a previous study, we found that daily CT-guided alignment, via on-board kV imaging or in-room CT, is a more accurate option but could be time-consuming. In this study, we investigated a third alignment solution: daily bone alignment plus a correction factor for the systematic internal prostate displacement (CFID) as described by de Boer et al. [de Boer 2005]. Ten prostate patients were CT-scanned 3 times per week prior to radiotherapy using an integrated CT-LINAC system. They were aligned using US for their daily treatments. An IMRT plan was designed on the planning CT image with current clinical margins (8 mm anterior and lateral, 7 mm superior/inferior, 5 mm posterior). The treatment plan was copied onto the repeat CT images and aligned using several daily alignment schemes: (1) bone alignment plus CFID after 3 offline CT scans (bone+3CT), (2) bone alignment plus CFID after 6 offline CT scans (bone+6CT), (3) bone alignment, (4) US alignment, and (5) CT alignment. The accuracy of repeat US and CT measurements to determine the CFID was compared. The dosimetric impact on target coverage for each scenario was reported. Statistical significance was calculated with the Wilcoxon Matched Pairs Signed Ranks test. The CFID for internal systematic prostate displacements was more accurately measured with repeat CT than with repeat US (residual error: 0.0 ± 0.7 mm and 2.0 ± 3.2 mm respectively); therefore the CFID from repeat CT scans were used. Average prostate minimum doses (0.1 cm3) from the bone+3CT and bone+6CT alignments were equivalent to each other and to the prostate minimum doses from daily US alignment (Table). The alignment precision (1StDev in daily prostate minimum dose) was better with bone+3CT and bone+6CT than with daily US alignment (Table). Daily bone alignment plus CFID for internal systematic prostate displacements provided better daily dose precision and equivalent prostate dose coverage when compared to daily US alignment. The CFID should be based on at least 3 repeat CT scans, which could be collected before the start of treatment or during the first three treatment days. Daily bone alignment plus CFID provides another option for accurate prostate patient positioning.