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Prostate Seed Implant
22.1
Introduction
Prostate brachytherapy is one of several treatment techniques available for patients with localized disease. Prostate brachytherapy is currently practiced using two different techniques: (1) prostate seed implant (PSI) using low dose rate (LDR) sources and (2) high dose rate (HDR) brachytherapy. Because prostate HDR is one variation of interstitial implants, it is grouped with other interstitial implants in Chapter 20 .
Like other technologies in Radiation Oncology, PSI has undergone major changes both in implementation of technology and in clinical recommendations based on knowledge from clinical trials. Initially, 103 Pd loose seeds were the only available radiation source for implant. In 1999, the National Institute of Standards and Technology (NIST) changed its calibration standard, leading to a 9% adjustment in dose calibration. 125 I, 131 Cs, and 198 Au expanded the practitioner's choice of sources. Better ultrasound technology and computing technology allowed the move to 3D-based preplanning or live planning in the operating room (OR). Vendors developed preloaded needles, stranded seed configurations, and stranding technology that could be used to assemble strands in the OR itself. Third-party vendors are now offering independent assays of seeds, which led AAPM to change the recommendation for clinical seed assays.
Patient selection criteria and treatment regimens for PSI are constantly evolving as more clinical data become available. The “classic” PSI monotherapy patient has low grade, early stage disease. PSI is also a choice for intermediate risk patients, either as monotherapy or in combination with other therapies. The American College of Radiology (ACR)-American Society for Radiation Oncology (ASTRO) practice guideline for the transperineal permanent brachytherapy of prostate cancer suggests “that each facility establish and follow its own practice guidelines. Ongoing clinical trials will help to better define indications.” The American Brachytherapy Society's (ABS) recommendations for transperineal permanent brachytherapy of prostate cancer published in 1999 discusses in more detail patient selection criteria based on clinical rationales drawn from literature review and the authors' extensive clinical experience. This guideline was updated in 2012. In general, patients with lower risk disease, good life expectancy, acceptable urinary function, and favorable anatomy are good candidates for prostate brachytherapy as monotherapy. Select patients with intermediate or high risk disease might also be candidates for brachytherapy, especially as boost therapy in conjunction with external beam radiotherapy.
Medical physicists must abide by the legal requirements of using radioactive by-product material for medical purposes in their region of practice. In the United States, agreement states adopt the Nuclear Regulatory Commission (NRC) Regulations Title 10 Part 35 Use of Byproduct Material within the Code of Federal Regulations (10 CFR 35). Non-agreement states ( http://nrc-stp.ornl.gov/rulemaking.html ) adopt their own regulations, which are usually based on the NRC regulations.
Several societies have developed recommendations on many aspects of PSI. The ACR and ASTRO have collaborated to publish practice guidelines aimed at the whole treatment team and high level technical guidelines for physicists. The ABS has a series of recommendations addressed to both physicians and physicists. American Association of Physicists in Medicine (AAPM) Task Group (TG)-64 on PSI brachytherapy and AAPM TG-128, Quality Assurance Tests for Prostate Brachytherapy Ultrasound Systems and white papers offer recommendations on dosimetry and quality assurance (QA) for the physicist.
22.2
Sources
LDR Sources, Characteristics, Advantages, and Disadvantages
Cesium is the most recent isotope developed for PSI (see Table 22.1 ). Because of its very short half-life, there may be a biological advantage in faster dose delivery with possibly lower long-term complication rates. This rationale is similar to arguments driving clinical trials for prostate stereotactic body radiation therapy (SBRT) and HDR.
TABLE 22.1
List of Sources and Source Characteristics
| Source | 103 Pd | 125 I | 131 Cs | 198 Au |
|---|---|---|---|---|
| Half-life (days) | 17 | 59.4 | 9.7 | 2.7 |
| Energy (keV) | 21 | 28 | 30 | 412 |
| Initial dose rate (monotherapy) (cGy/h) | 18-20 | 7 | 1.9 | N/A |
| Relative biological effectiveness (RBE) | 1.9 | 1.4 | Not known | Not known |
While some studies have shown an advantage of one isotope over another in very select clinical presentations, other studies have contradicted these findings. Clinical studies have not shown any statistically significant difference between the isotopes based on outcome or toxicity. The time to urethral and rectal toxicity development is shorter for shorter half-life isotopes. Lower energy isotopes cause the delivered dose to be more sensitive to the seed spacing.
AAPM TG-43 on brachytherapy, Dosimetry of Interstitial Brachytherapy Sources, specifies the calculation of dose to tissue for the various source isotopes and source models (see Chapter 8 ). In 1999, NIST discovered a significant error of about 9% in determining the air kerma strength for 103 Pd. The new 1999 NIST air kerma strength standard, S K,N99 , was subsequently implemented and 103 Pd dose calculations adjusted accordingly. The reader should be aware of this when referencing publications on the dosimetry and clinical use of 103 Pd published before the corrected standard.
Loose and Stranded Seeds
In the early years of PSI, sources were only available as loose seeds, either unsterilized or sterilized. Seed designs vary between vendors, which causes differences in dose distribution that are large enough that they need to be accurately modeled in treatment planning, but not so large that they would impact the decision about which seed to buy. AAPM TG-43-U1, A Revised AAPM Protocol for Brachytherapy Dose Calculations, contains detailed drawings for several seed models. The general seed schema is shown in Figure 22.1 . The radioactive material is packaged inside a titanium capsule with welded end caps; the cap shape and outside surface texture may vary. The isotope is mounted on a carrier substrate and packaged into pellets, rods, or cylinders. Most capsules also contain a high electron density material such as lead, silver, gold, or tungsten as markers to improve seed visibility on x-ray images.
Loose seeds provide ultimate flexibility in designing an implant. Specialized devices (e.g., the Mick applicator, Figure 22.2B ) are available to automate the process of loading loose seeds into the applicator needle and to reduce staff exposure. However, loose seeds do migrate more easily along the needle track and potentially could be lost into the bladder or, more rarely, embolize into the lung or even the brain. Different surface textures and shapes were developed to prevent seed migration. Over time, stranded seeds became available in different configurations: strands/braids of 10, chains of up to 70 seeds, preloaded needles, and devices to custom-strand seeds in the OR. The material used for strands or braids is bioabsorbable suture material. More recently, echogenic strands have been developed to increase the visibility of seed strand placement during implant ( Figure 22.3 ). These strands consist of braided strings of bioabsorbable material, thereby increasing the complexity of the surface, which increases the visibility on ultrasound (US) (U.S. Patent US7874976 B1).
Figure 22.2 shows loose seeds (A), loose seeds in Mick cartridges (B), and stranded seeds (C). Each system has advantages and disadvantages depending on the technology and implant technique used. For example, preloaded needles require a preplan; this results in source order efficiency because the exact numbers of sources with a small additional margin of seeds needed for a patient are ordered versus creation of a source estimate using a nomogram. It should be noted that these nomograms are seed model and potentially institution specific. On the other hand, with preordered strands there is inflexibility in adjusting the plan while in the OR. Such adjustments might be necessary in the following situations:
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Previously undetected pubic arch interference with the placement of anterior lateral needles
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Change in gland size between initial planning study and actual implant date
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Positioning change (e.g., between magnetic resonance imaging (MRI) preplan and transrectal ultrasound (TRUS) imaging during implant) causes the urethra and rectal avoidance geometry to change
Table 22.2 summarizes the uses, advantages, and disadvantages of currently available seed systems.
TABLE 22.2
Uses, Advantages, and Disadvantages of Different Seed Systems
| Seed Type | Loose | Stranded | ||||
|---|---|---|---|---|---|---|
| Unsterilized | Sterilized | Stranded in OR | Strands of 10 | Chain of up to 70 Seeds | Preloaded Needles | |
| Use |
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|
|
# of needles, seeds/needle, and spacing determined by preplan |
| Advantage |
|
|
|
|
|
|
| Disadvantage |
|
|
|
|
|
No flexibility to adapt plan in OR |
Seed Assay
AAPM TG-40, AAPM TG-56, Code of Practice for Brachytherapy Physics, and AAPM TG-64 all contain recommendations on prostate seed source calibrations and physicist responsibilities. These task groups were published before third-party independent source assays became available. At the time, preassembled sterile sources were not in use; third-party independent assay services were not available yet. To update and clarify recommendations, the AAPM Low Energy Brachytherapy Source Calibration Working Group (LEBSC WG) was formed. The white paper on third-party brachytherapy source calibrations and physicist responsibilities published by this working group contains updated recommendations for currently available source configurations and supersedes recommendations made by earlier task groups (see Table 22.3 ).
TABLE 22.3
Overview of Assay Recommendations from AAPM TG-40, AAPM TG-56, and AAPM TG-64, with LEBSC WG Superseding Earlier Recommendations
| AAPM TG-40 (1994)/AAPM TG-56 (1997) | AAPM TG-64 (1999) | LEBSC WG (2008) | |
|---|---|---|---|
| Loose, unsterilized | 10% | Random sample of at least 10% of seeds | ≥10% of total or 10 seeds, whichever is larger |
| Cartridge, unsterilized | N/A | N/A | ≥10% of total, either whole cartridge assay or individual sources |
| Stranded/cartridge, unsterilized | 10% or 2 strands, whichever is larger | N/A | 10% or 2 strands, whichever is larger; or 5% or 5 loose seeds from same batch, whichever is larger |
| Stranded, sterilized | 1 single source from each strength batch | N/A | 10% of total, whole cartridge assay in sterile environment; or 5% or 5 loose seeds from same batch, whichever is larger |
N/A denotes no specific recommendations given.
After a vendor manufactures the seeds, third-party assay services have the technical capabilities to assay all seeds within an order and then assemble them into the customer order either as preloaded needles or strands. The user is provided with an assay report, which provides valuable information on the distribution of seed strength within the order. The International Standards Organization/International Electrotechnical Commission (ISO/IEC) standard 170325 covers requirements for testing and calibration laboratories; accreditation under this standard can serve as an indicator for the quality of the third-party assay vendor. As the authors of AAPM LEBSC WG points out, there is still a residual risk of errors made by the third-party assay vendor. An example is event No. 54 in the IAEA report on accidental exposures in radiotherapy, where a mismatch in units specified in the order versus vendor delivery reached the patient because the vendor assay was not double-checked. Therefore, it is the responsibility of the clinical medical physicist to verify the assay. Seed assays are performed at least a day before the implant procedure to give enough time to respond to survey results that may be outside the recommended tolerance. Table 22.4 summarizes the tolerance levels given by the AAPM LEBSC WG.
TABLE 22.4
Recommended Actions Based on the Sample Size Assayed and the Relative Difference, Δ S K , Found Between the Manufacturer's Source Strength Certificate and the Physicist's Source Assay a
Adapted from AAPM LEBSC WG.
| Sample Size for Assay of Sources by End-User Medical Physicist | Δ S K | Action by End-User Medical Physicist |
| Individual source as part of an order of ≥10 sources b | Δ S K ≤ 6% | Nothing further. |
| Δ S K > 6% | Consult with the radiation oncologist regarding use of the outlier source: Dependent on the radionuclide, intended target, source packaging, and availability of extra sources. | |
| ≥10% but <100% of order, or batch measurements of individual sterile strands, cartridges, or preloaded needles | Δ S K ≤ 3% | Nothing further. |
| 5% ≥ Δ S K > 3% | Investigate source of discrepancy or increase the sample size. | |
| Δ S K > 5% | Consult with manufacturer to resolve differences or increase the sample size. For assays performed in the operating room, consult with the radiation oncologist regarding whether to use the measured source strength or to average with the manufacturer's value. | |
| 100% of order, or batch measurements of each and every individual sterile strand, cartridge, or preloaded needle | Δ S K ≤ 3% | Nothing further. |
| 5% ≥ Δ S K > 3% | Investigate source of discrepancy. | |
| Δ S K > 5% | Consult with manufacturer to resolve differences. For assays performed in the operating room, consult with the radiation oncologist regarding the consequences of proceeding with the implant using the measured source strength. |
a Assay results obtained at sites other than the end-user institution should not replace the source strength value on the manufacturer's certificate. The source strength value to be used in planning may be either that stated on the manufacturer's certificate or the value determined by institutional medical physicist when the difference is ≥5%.
b For orders consisting of <10 sources, the action threshold is Δ S K > 5% for individual sources.
22.3
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