Brachytherapy for Prostate Cancer: An Overview

LDR-BT History

In 1917, the first report of brachytherapy for PCa was published. In the 1950s, LDR-BT was performed with ¹⁹⁸ Au. By the 1970s, ¹²⁵ I seeds were used for prostate implants. However, the dose distribution of these isotopes and clinical outcomes were not ideal. The application of a transrectal ultrasound (TRUS) to guide LDR-BT was successful in Denmark and the United States in the 1980s. TRUS-guided LDR-BT became a standard technique in the United States in the 1990s because it had improved outcomes and was less invasive when compared to laparotomy-based approaches. LDR-BT has been endorsed for low-risk PCa by organizations including the National Cancer Institute (NCI), the American Cancer Society (ACS), the NCCN, and the American Urologic Association (AUA.)

The disadvantages of LDR-BT include possible seed migration (for loose seeds), high dependence on the operator skill for proper seed placement, permanent deposition of radioactive material in the body, inability to adjust seeds once they are deposited, inability to deliver high doses over a short period of time, variability of dosimetry among implants, and exposure of the staff to radiation. These disadvantages led physicians to explore other BT systems (i.e., HDR-BT) ( Table 44.3 .).

HDR-BT History

A TRUS-guided remote afterloading system (RALS) was first introduced in 1980 to deliver a high radiation dose to the prostate and helped address some limitations of LDR-BT. HDR-BT was first used as a boost with EBRT in Sweden, Germany, and the United States in the 1980s and 1990s. HDR-BT was shown to be safe and effective in Phase I/II trials. In Osaka, Japan, a systematic treatment series of HDR-BT boost was started in 1994. After a year of using HDR-BT boost, a trial of HDR-BT monotherapy was initiated. Subsequently, the Osaka group published the first report on HDR-BT monotherapy in 2000. In the United States, the group led by Alvaro Martinez at William Beaumont Hospital reported the results from a series of prospective trials of first HDR-BT boost and then HDR-BT monotherapy over a period of more than 20 years.

LDR-BT Technical Aspects

As of 2014, LDR-BT is typically performed with ¹²⁵ I or ¹⁰³ Pd; few centers use ¹³¹ Cs. The details of these radionuclides and comparison to ¹⁹² Ir (used in HDR-BT) are listed in Table 44.4 . The standard procedure for seed implantation is to use a transperineal approach under TRUS and template guidance. Patient position and the TRUS-probe angle should coincide with the preimplant planning study. A high-resolution biplanar ultrasound system (at 5–12 MHz) with dedicated prostate BT software is mandatory. Fluoroscopy is frequently used to monitor seed deposition, as a complimentary imaging modality to TRUS, and is used in some centers for intraoperative dose calculation using image fusion. However, this is not considered mandatory for successful LDR-BT.

The American Brachytherapy Society (ABS) recommends that CT-based postoperative dosimetry be performed within 60 days of the implant. A planning system generates dose volume histograms, dose volume statistics, and 2D and 3D isodose curves superimposed on CT and other images. Careful postimplant assessment provides providers with objective measures of implant quality allowing for continual technical improvement. Postimplant dosimetry is performed on the day of LDR-BT and a few weeks after the implant (once edema has resolved); satisfactory day 0 dosimetry does not obviate dosimetric calculation on a subsequent date. The time necessary to minimize edema is radionuclide-specific: 16 + 4 days for ¹⁰³ Pd and 30 + 7 days for ¹²⁵ I. Improvement of reproducibility of postimplant dosimetry, such as MR-CT image fusion, is also possible.

Regarding dosimetric analysis, the ABS recommends that the following post-BT dosimetric parameters be determined: (1) prostate – D90 (i.e., dose to 90% of the prostate volume) in Gy, V100 (volume receiving 100% of the dose) in percentage, and V150; (2) urethra – V150 in percentage, V5 in percentage, V30 in percentage; and (3) rectum – V100. For the prostate, it is recommended that the D90 is greater than 100% of the prescription dose; the V100 >90–95% of the dose; and the V150 be <60–65% of the dose. Many critical organ dose parameters have been reported. For the urethra, the V150 should be less than the prescription dose; V5 <150%; and V30 <125%. It is recognized that meeting these constraints is not always possible, especially in smaller prostates (<20 cm ³ ). For the rectum, the V100 should be <1 cm ³ on day 0 dosimetry and <1.3 cm ³ on day 30. Critical structures for postimplant erectile dysfunction have not been agreed upon, although the internal pudendal artery, penile bulb, and neurovascular bundles have been studied.