Abstract
Purpose: To prospectively analyze the inter-fractional motion of the prostate in patients with prostate cancer treated with intensity-modulated radiation therapy (IMRT) using image-guided radiotherapy (IGRT) with daily cone-beam computed tomography (CBCT) as part of a rescan protocol for large offset, and to evaluate the efficacy of our protocol. Materials and Methods: Eligible patients were treated with the following protocol: 1) magnesium oxide and dimethylpolysiloxane were administered to ensure that patients had regular bowel movements; 2) the patients were instructed to have an appropriately distended bladder during the planning CT and daily irradiation; 3) the daily CBCT image was fused with the planning CT image using the prostate outline; and 4) if large offset was recognized, a rescan CBCT image was obtained after appropriate countermeasures, such as the discharge of gas and defecation, and re-registration was performed. Three shifts for the inter-fractional motion of the prostate were analyzed, in the fractions which needed the CBCT rescan; the displacement data after the final rescan were used. Results: Sixty-one patients were eligible, and a total of 2302 fractions were available for the analysis. Rescans of the CBCT for large offset were performed in 113 (5%) of the 2302 fractions. After the first rescan, the large offset was resolved in 106 (94%) of the 113 fractions. Excessive rectal gas was the reason for the large offset in 94 (83%) of the 113 fractions. The total mean and standard deviation of the inter-fractional motion of the prostate in the AP, LR, and SI directions were 1.1 ± 2.4, -0.1 ± 2.3, and 0.7 ± 3.0 mm, respectively. Conclusion: Large offset was recognized in 5% of all fractions. Daily CBCT with our rescan protocol could resolve the large offset, which was mainly caused by excessive rectal gas, and it may therefore be promising to reduce the inter-fractional motion of the prostate.
Highlights
Intensity-modulated radiotherapy (IMRT) has proven to be a useful tool because of its ability to conform high doses of radiation to the target volume
The purpose of this study was to analyze the inter-fractional motion of the prostate in the patients with prostate cancer treated with IMRT using daily cone beam computed tomography (CBCT) with a rescan protocol for large offset, and to evaluate the efficacy of our protocol
Bylund et al reported a total of 984 daily megavoltage CBCT images from 24 patients undergoing definitive IMRT for localized prostate cancer; the mean displacement of the inter-fractional motion was 4.8, 2.9, and 2.1 mm in the AP, LR, and SI directions, respectively [5]
Summary
Intensity-modulated radiotherapy (IMRT) has proven to be a useful tool because of its ability to conform high doses of radiation to the target volume. For patients with clinically localized prostate cancer, IMRT is used to escalate the dose to the prostate while minimizing the dose to the surrounding normal tissues [1]. This requires accurate daily targeting during the entire course of external beam radiotherapy to the prostate. Various targeting modalities have been used, such as two-dimensional portal imaging, and fiducial marker matching to three-dimensional (3D) volumetric computed tomography (CT) and ultrasound information [5]-[7]. These findings may offer the potential for an increased dose to be safely delivered to the target with reduced acute and late toxicity rates
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