Optimization in brachytherapy

Frequently asked questions by the health professionals

» What advantages are there to using remote afterloading devices, compared with manual brachytherapy procedures? 
» Iridium-192 has proven to be an often-used source for high dose rate applications. Why is this, and why is it not used for all brachytherapy? 
» Why are brachytherapy sources encapsulated? 
» Is HDR brachytherapy better and safer than LDR brachytherapy in all aspects? 

» What advantages are there to using remote afterloading devices, compared with manual brachytherapy procedures?

Foremost, staff will be less exposed to radiation.

When circumstances allow remote afterloading to be used, there are some advantages from a radiation protection perspective and also from other practical points of view. There is reduced radiation exposure to staff, since radioactive sources do not have to be handled manually but are driven by the remotely controlled afterloading device. Furthermore, treatment can potentially be better optimized through enhanced reproducibility.

» Iridium-192 has proven to be an often-used source for high dose rate applications. Why is this, and why is it not used for all brachytherapy?

While iridium 192 (¹⁹²Ir) has many good qualities, different radionuclides are good for different applications.

¹⁹²Ir is a radionuclide with a high specific activity, or activity per unit mass, which means that a very small source can provide a very high dose rate (HDR) – essential for HDR-applications. The picture shows some typical dimensions for a HDR source of ¹⁹²Ir. The effective photon energy of around 350 keV ensures a sufficient absorbed dose at a sufficient distance from the source to treat the target homogeneously.

A drawback, however, is the short half-life of 74 days. This means that sources typically need to be replaced every three to four months in order to maintain an acceptable treatment time. In typical low dose sate (LDR) -applications, size is of somewhat less importance. An often-used radionuclide in gynaecological applications is caesium 137 (¹³⁷Cs), which has a much longer half-life (30.2 years) than ¹⁹²Ir and, thus, only needs to be replaced every 10-15 years, while the specific activity is only a hundredth of that of ¹⁹²Ir. For permanent implants, an often-used source is iodine 125 (¹²⁵I) due to its low photon energy making radiation absorbed within the patient.

There is no such thing as a perfect radionuclide for all brachytherapy applications. One has to look at the application in question and consider issues such as specific activity, half-life, type and energy of emission, and shielding requirements.

» Why are brachytherapy sources encapsulated?

To ensure containment of the radioactive material and sometimes to act as a filter of unwanted radiation.

Brachytherapy sources are usually sealed so that the radioactive material is contained fully encapsulated within a protective capsule. This capsule is designed to prevent leakage or escape of the radioactive source and it makes the source rigid. Furthermore, for photon emitting sources, the capsule can serve the purpose of absorbing alpha and beta rays produced through the source decay. A tiny brachytherapy seed (with a size roughly equivalent to a grain of rice) such as radioactive iodine 125 (¹²⁵I) in such a seed is encapsulated in titanium.

» Is HDR brachytherapy better and safer than LDR brachytherapy in all aspects?

No.

While HDR brachytherapy may offer advantages such as more practical procedures with outpatient treatment, increased opportunities to optimize the absorbed dose and enhanced radiation protection of staff under normal conditions, there are still other factors where LDR brachytherapy has an advantage. LDR can be said to be less technically complex than HDR brachytherapy, where a large absorbed dose is given to the patient in a short time span. HDR treatment thus requires more training and advanced knowledge in terms of operating the equipment and optimizing protection in the treatment, including keeping the irradiating organs at risk to the minimum necessary to achieve the objective. While the potential for error might not be greater for HDR brachytherapy, the consequences of HDR error might be exacerbated due to the high activity of HDR brachytherapy sources.