Brachytherapy

Brachytherapy, or short-distance therapy, places a small radioactive source inside or in close proximity to the tumor. By virtue of the inverse square law, whereby dose decreases exponentially as the distance from a source increases, a steep dose gradient is achieved, resulting in a highly conformal high-dose region. As such, brachytherapy requires a high level of accuracy and precision in source placement. Permanent seed brachytherapy, which is commonly performed for localized prostate cancer, uses radioactive sources that are permanently implanted in the target volume and deliver doses through the course of their radioactive life. In temporary brachytherapy, source(s) are fed, or "afterloaded" into needles, catheters, or hollow tubes that have been placed in or near the tumor. When the desired dose has been delivered, the source(s) are retracted and the tubes removed from the patient. Brachytherapy can also be classified according to the rate with which dose is delivered, with high-dose rate (HDR) brachytherapy defined as 200 cGy/min or more.

Planning for brachytherapy is simpler than for external beam therapy. The dosimetry is dictated by the photon energies emitted from the radioactive source, and is primarily calculated with the inverse square law (39). However, as the source of radiation is physically closer to the intended target, the location of the target in relation to the source at the time of delivery is more accurate. Planning systems can now perform image-based inverse planning for brachytherapy (40-42). Only two variables (source position and source strength) need to be optimized using computer programs designed to achieve conformal dose to a planning target volume. With HDR brachytherapy, a single high-intensity 192Ir source can be placed at a variety of positions (dwell position) for a desired length of time (dwell time) within each needle, catheter, or applicator tube (Fig. 5). It is important to accurately identify the location of these tubes,

Figure 5 MRI-guided HDR brachytherapy for prostate cancer. (A) Sagittal, (B) coronal, and (C) axial images of the prostate gland after placement of 17 brachytherapy catheters under MRI guidance. Note a well-visualized urachal cyst (arrow) that was not perforated during the procedure. Isodose curves (C) displayed on axial images demonstrate that the prostate target (red) is encompassed by 100% or more of the prescribed dose. (D) 3D rendering of the spatial relationship between structures of interest and brachytherapy catheters and source positions (red: prostate target, fuchsia: urachal cyst, blue: foley catheter balloon, and pink: rectum and endorectal coil).

Figure 5 MRI-guided HDR brachytherapy for prostate cancer. (A) Sagittal, (B) coronal, and (C) axial images of the prostate gland after placement of 17 brachytherapy catheters under MRI guidance. Note a well-visualized urachal cyst (arrow) that was not perforated during the procedure. Isodose curves (C) displayed on axial images demonstrate that the prostate target (red) is encompassed by 100% or more of the prescribed dose. (D) 3D rendering of the spatial relationship between structures of interest and brachytherapy catheters and source positions (red: prostate target, fuchsia: urachal cyst, blue: foley catheter balloon, and pink: rectum and endorectal coil).

as well as the precise location of the first dwell position, as all dwell positions are determined from this reference. There are a number of X-ray and CT "markers" that can be placed inside the tubes during image acquisition, and which help identify the location of the first dwell position on the image. A small, but growing number of commercial brachytherapy devices are now MRI compatible, but no MRI "markers" have yet been developed (Fig. 5).

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