Brachytherapy

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Brachytherapy is a form of radiotherapy where a sealed radiation source is placed inside or next to the tumor.

Brachytherapy is classified according to:

Location of implants
Type of loading
Dose rate
Duration of treatment
Type of emission.

Types of brachytherapy according to location of implants

Three common techniques of radioactive implants applied in brachytherapy:

Molds/ plaques: Molds or plaques are used for superficial lesions whereby the radioactive source is placed on the skin 1-2 cm from the lesion. It is mostly used in treating tumors of the eye.

Interstitial: Interstitial implant involves temporary placement of a radioactive source into the tumor volume example in a breast tissue.

Interstitial Breast Implant

Intracavitary: In intracavitary implant, sealed radioactive source is placed in the body cavity. Cancer of the cervix, uterus and vagina are often treated by this procedure. A cervical applicator consists of central tube (tandem) and lateral capsules (ovoids).

A Cervical Applicator

A Vaginal Applicator

Intracavitary Procedure on Cancer of the Cervix

Types of brachytherapy loading

Hot loading: In hot loading, an applicator is preloaded and contains radioactive sources at the time of placement.
Afterloading: In afterloading an applicator if placed first into the patient and the radioactive sources are loaded later either by hand (manual afterloading) or by machine (automatic remote afterloading).

1. Manual afterloading: The radiation sources are manually afterloaded into aplicators or catheters that have been placed within the target volume. At the end of the treatment the sources are removed gain. The manual loading and offloading of the radiation source result into high radiation exposure to the medical staff.

2. Remote afterloading: In remote afterloading, machines or systems automatically administer a radiation source directly to the tumor minimizing radiation exposure to the medical staff and to the surrounding normal tissue.

Remote afterloading technique

Types of brachytherapy emission

Gamma emitters:Gamma emitters emit gamma rays through gamma decay and possibly characteristic rays through electron capture and internal conversion. T here are two types of gamma emitters; High energy and Low energy radioisotopes. High energy radioisotopes have deep penetration power examples; Radium, Cobalt, Gold, Cesium and Iridium. While low energy radioisotopes have less penetration power, example, Iodine-125, and Lead-103.

Beta emitters: Beta emitters are unsealed radioactive sources absorbed within few millimeters in tissue. They emit electrons following beta source decay. Examples of beta emitters are: Lead-32, Rhodium-106, Strontium-89 and Yttrium-90.

Neutron emitters: Neutron emitters emit neutrons following spontaneous nuclear fission. They are more effective against hypoxic tumors. Example of neutron emitters is Californium-252.

Brachytherapy classification with respect to Dose Rate

There are two main types of dose rates:

Low Dose Rate (LDR) (0.4-2.0 Gy/h): Low Dose Rate is mostly used where radioactive seeds are permanently implanted into the tumor.

High Dose Rate (HDR) (>12 Gy/h): High Dose Rate is mostly used where radioactive seeds are temporarily implanted into the tumor or placed close to the tumor

Duration of Treatment

Permanent implant: In a permanent implant, radioactive source remains in the tissue indefinitely. Short half-life isotopes are used, example; I-125, Pd-103, Au-198.

Temporary implant: In a temporary implant, radioactive source is removed from patient within a short period of time once desired dose has been delivered.

Principles of Brachytherapy:

Brachytherapy has higher Tumor Control Probability due to higher tumor dose.
There is lower dose to surrounding normal tissue.
Morbidity and complications increase when treating large volumes, thus the treatment volume is minimized.
Total treatment duration is kept minimal because shorter time intervals between treatments reduce repopulation of tumor cells.

Advantages of Brachytherapy

Sharp dose fall-out outside the target volume because of attenuation by inverse square law hence minimizing dose to the surrounding tissue.
Short duration of treatment (20-70 Gy in 2-7 days) unlike EBRT which takes 4-7 weeks.
Improved localized dose delivery to the target.
Continuous radiation by LDR allows redistribution of cells in cell cycle.
Better conformal therapy since dose distribution can be manipulated to make irregular shaped tumors.

Disadvantages of Brachytherapy

It is only suitable for well localized tumors.
There is no therapeutic dose due to positive surrounding lymph nodes.
Sharp dose fall-off might not adequately cover tumor volume.
Great risk of late complications using HDR given with inadequate fractionation.
Requires special skills and training.

The most commonly used systems in Brachytherapy:

Quimby (Memorial) System: In this system, the radioactive sources are distributed uniformly over a plane or a volume of tissue. The dose near the center of the implant is much greater than at the edge of the implant. Thus in Quimby system, a uniform distribution of radioactive sources is used to give a non-uniform dose distribution.

Manchester (Patterson-Parker) System: In Manchester system, the radioactive sources are distributed non-uniformly over the area or the volume of the implant to give a uniform dose distribution. The aim of the system is to deliver a uniform dose of ± 10 % in the implanted area or volume.

Paris System: Paris system was developed for linear sources of iridium wire. It is used for single or double plane implants. In this system, sources are distributed uniformly for a planar implant but follow a particular pattern for volume implants.

Internal radiation therapy is a form of treatment where a source of radiation is put inside your body. One form of internal radiation therapy is called brachytherapy. In brachytherapy, the radiation source is a solid in the form of seeds, ribbons, or capsules, which are placed in your body in or near the cancer cells. This allows treatment with a high dose of radiation to a smaller part of your body. Internal radiation can also be in a liquid form. You receive liquid radiation by drinking it, by swallowing a pill, or through an IV. Liquid radiation travels throughout your body, seeking out and killing cancer cells.

Radioactive isotopes used in brachytherapy:

The radioactive isotopes in brachytherapy,

Name Half-life

1. Radium 226 1600 years

2. Iridium 192 73.8 days

3. Cobalt 60 5.26 years

4. Cesium 137 30 years

5. Gold 198 2.7 days

6. Iodine 125 59.4 days

7. Radon 222 3.83 days

Various techniques used in brachytherapy:

Brachytherapy sources are applied in three ways: external applicators or molds, interstitial implantation, and intracavitary therapy (fig a). A choice of one technique or the other is dictated primarily by the size and location of the tumor. For example, surface molds are used to treat small superficial areas, such as the ear or the lip; interstitial therapy is indicated when the tumor is well localized and can be implanted directly according to accepted rules of distribution; intracavitary therapy is used when applicators containing radioactive sources can be introduced into body cavities. In all these cases, owing to the short treatment distance, the geometry of source distribution is critical.

Figure a shows the images various techniques used in brachytherapy

Brachytherapy may be used with people who have cancers of the head, neck, breast, uterus, cervix, prostate, gall bladder, esophagus, eye, and lung. Liquid forms of internal radiation are most often used with people who have thyroid cancer or non-Hodgkin's lymphoma. You may also get internal radiation along with other types of treatment, including external beam radiation, chemotherapy, or surgery.

Questions:

1. Brachytherapy will used for the cancer of

a) Cervix

b) Breast

c) Tongue

d) Oesophagus

e) All

Answers:

1. e) All

References:

1. http://www.thegreenjournal.com

2. www.cancer.org