Prostate IGRT: Fiducials, Soft Tissue, and Imaging Dose
A 70-year-old with favorable intermediate-risk prostate cancer is treated with 78 Gy in 39 fractions using VMAT. Three gold fiducials were implanted 2 weeks before planning. Daily CBCT is used with a 5 mm PTV margin. At fraction 12, CBCT shows the prostate displaced 4 mm posteriorly and 3 mm inferiorly relative to the fiducials; there is marked rectal gas. The fiducials appear unchanged. The CBCT dose is approximately 3 mGy per scan to the prostate. The planning CT and T2 MRI were fused using the prostate capsule; the fusion was within 2 mm. The patient has bilateral hip replacements that cause streak artifact on CT. The physician asks whether to use fiducials or soft-tissue matching and whether the imaging dose is a concern.
1. How should you align the prostate at fraction 12, and what would make you re-plan or re-implant?
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At fraction 12 I would use soft-tissue CBCT matching to the prostate capsule rather than fiducials alone, because fiducials can migrate independently of the gland and rectal gas can distort the prostate position. The 4 mm posterior and 3 mm inferior displacement is within the 5 mm margin but close to its edge, so I would repeat the CBCT after bowel preparation or ask the patient to empty the rectum, then re-match. If the displacement is systematic over several fractions, if fiducials show migration, or if soft-tissue matching cannot be achieved because of artifact, I would re-plan with a new CT/MRI and consider re-implantation. A single day of displacement is usually managed by re-positioning, not re-planning.
2. Calculate the total CBCT dose to the prostate and compare it with the prescribed dose. Does ALARA require a change?
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Total CBCT dose = 3 mGy x 39 = 117 mGy = 0.117 Gy. Compared with 78 Gy, this is about 0.15% of the prescribed dose. That is small relative to the treatment benefit and is not a reason to omit daily imaging. However, ALARA still applies: I would optimize the CBCT scan length and exposure, use kV planar imaging when soft-tissue information is not needed, and avoid repeat scans when a single well-positioned scan is adequate. The risk is low, but the principle is to use the minimum imaging necessary for accurate delivery.
3. How do hip prostheses affect CT-based dose calculation, and how would you mitigate?
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Bilateral hip prostheses cause streak artifacts and incorrect CT numbers, which can affect heterogeneity correction and dose calculation, especially for beams passing through the prostheses. I would use metal artifact reduction, contour the prostheses and assign appropriate densities, and compare the calculation with an independent algorithm or measurement. MRI is useful for prostate and OAR contouring, but CT density is still needed for dose calculation unless an MRI-only workflow is validated. I would also verify daily setup with CBCT and review the dose distribution for artifact-related hot or cold spots. The effect is usually modest for prostate VMAT, but it should not be ignored.