What are the characteristics of a brachytherapy source?

Characteristics of a Brachytherapy Source

Brachytherapy sources are radioactive materials placed directly into or adjacent to tumor tissue, characterized by their radionuclide type, energy emission, half-life, dose rate, and tissue penetration range—with the fundamental property being short-range radiation that delivers high doses to localized targets while sparing surrounding normal tissues. 1

Physical Characteristics

Radiation Type and Energy

• Brachytherapy sources emit either gamma or beta radiation, with gamma sources (like Iridium-192, Cesium-137) providing better tissue penetration but requiring more shielding, while beta sources have more limited penetration 2
• Low-energy photon sources emit radiation with a short range, allowing adequate dose delivery to the cancer within the prostate while avoiding excessive irradiation of the bladder and rectum 1
• The absorbed dose falls off very rapidly with increasing distance from the sources, enabling high doses to be safely delivered to a localized target region over a short time 1

Common Radionuclides and Their Properties

• Conventional sources in the United States include Cesium-137, Iridium-192, Iodine-125, and Gold-198 2
• Newer sources include Americium-241, Palladium-103, Samarium-145, and Ytterbium-169 2
• For permanent prostate implants, the recommended prescribed doses are 145 Gy for Iodine-125 and 125 Gy for Palladium-103 1
• High-dose-rate (HDR) brachytherapy uses Iridium-192 at dose rates of 20 cGy per minute (12 Gy per hour) or more 3

Half-Life Considerations

• Longer-lived radioactive sources can be used multiple times in different patients, reducing their effective cost compared with shorter-lived radioisotopes 2
• For permanent implants, shorter-lived radionuclides provide higher initial dose rates, which may have biological advantages 2
• Sources are either permanently implanted and gradually lose their radioactivity, or temporarily placed and then removed 1

Dose Rate Classification

Low Dose-Rate (LDR) Brachytherapy

• LDR brachytherapy consists of permanent seed implants that emit low-energy radiation over an extended period 1
• The short range of radiation from these low-energy sources allows delivery of adequate dose levels to the cancer while avoiding excessive irradiation of adjacent organs 1
• Very high doses are not possible with brachytherapy because radiation is delivered at a much slower dose rate than with external-beam RT, which reduces biologic effectiveness 1
• A typical LDR brachytherapy schedule consists of 55–60 Gy given in 4–6 days 1

High Dose-Rate (HDR) Brachytherapy

• HDR brachytherapy involves temporary insertion of a high-intensity radiation source (typically Iridium-192) that delivers radiation at rates ≥20 cGy per minute 3
• Common HDR boost doses include 9.5 to 11.5 Gy times 2 fractions, 5.5 to 7.5 Gy times 3 fractions, or 4.0 to 6.0 Gy times 4 fractions 1
• A commonly used regimen for HDR treatment alone includes 13.5 Gy times 2 fractions 1

Delivery Methods

Placement Techniques

• Brachytherapy involves placing radioactive sources directly into the tissue (interstitial) or in body cavities in close proximity to the tumor (intracavitary) 2, 4
• Intracavitary radiation is always temporary and usually takes about 1 to 4 days, while interstitial brachytherapy can be temporary or permanent 2
• The treatment can be delivered via surface, intracavitary, intraluminal, or interstitial application 3

Dose Distribution

• Current brachytherapy techniques attempt to improve radioactive seed placement and radiation dose distribution 1
• The dose distribution is highly conformal to the tumor volume but more inhomogeneous within this volume compared to external beam radiation 1
• Postimplant dosimetry should be performed to document the quality of the implant 1

Radiation Protection Characteristics

• Sources emitting higher energy photons have better tissue penetration but are more difficult to shield for radiation protection purposes than sources emitting lower energy photons 2
• Gamma radiation sources require long treatment times and high radiation exposure, necessitating special shielding and removal of staff from the treatment room during dwell times 1
• Beta radiation has more limited tissue penetration, reducing radiation protection concerns 1

Clinical Implications

• The fundamental advantage of brachytherapy results from superior localization of dose to the tumor volume 2, 4
• This technique delivers very high doses to the tumor focally while minimizing the probability of normal tissue complications 4
• As radiation is continuously delivered over time, repair of sublethal and potentially lethal damage, proliferation, and other cell kinetic effects modify the response of tumor and normal tissues, resulting in complex dose rate effects 2