Oxford American handbook of oncology. Second Edition. Oxford University Press (2015)
Brachytherapy uses radioactive sources placed within or adjacent to a tumor. These sources typically deliver low-energy radiation with limited penetration, reducing the overall radiation dose to more distant normal structures.
Low-dose rate sources
These are used for permanent implants or emporary implants lasting 2–7 days. Sources can be placed in catheters implanted within a patient or implanted directly into tumor tissue.
Radiation staff receive exposure from preparing, placing, and removing low-dose rate (LDR) sources. To reduce this exposure, the radiation worker minimizes the time exposed, maximizes the distance from the source, and uses lead shielding. The most common sources for LDR implants are cesium-137, iridium-192 (Ir-192, for emporary implants), iodine-125, and palladium-103 (for permanent implants).
High-dose rate sources
The high-dose rate (HDR) source is guided through a catheter and delivers the prescribed radiation within minutes. HDR brachytherapy is often fractionated, giving a total of 3 –5 fractions over 1–2 weeks.
The HDR source is housed in a remotely operated machine that prevents exposure to personnel during treatment. The source is located at the tip of a wire connected to a catheter placed at the desired treatment location. The most common source for HDR implants is Ir-192.
Typical brachytherapy treatment for specific disease sites
Intracavitary sources placed in a body cavity near the target tumor
Combined with external beam radiotherapy (EBRT), a brachytherapy implant is situated with a straight tandem catheter through the cervical os and two round ovoids (thus the implant is called T&O) on either side of the cervix. This results in a pear-shaped dose distribution overlying the cervix and medial parametrial tissue. This can be done in 1–2 LDR implants or 3 –5 HDR implants.
To reduce recurrence in the vaginal cuff after hysterectomy, a cylinder is placed in the vagina, and the cuff and proximal vagina are treated with 3 –5 HDR treatments.
Bronchial recurrence is treated by threading a catheter through and past the involved bronchial site by bronchoscopy. radiation is typically given with 3 –5 HDR treatments.
Bile duct cancer
HDR radiation is used to treat biliar y stricture with the catheter placed by endoscopy into the biliar y tree.
A disc-shaped applicator containing the sources is surgically sewn onto the sclera at the site of the lesion and removed after five days of LDR radiation.
Interstitial sources placed in tissue
For early-stage prostate cancer, radioactive seeds can be permanently implanted into the prostate through the perineum using transrectal ultrasound guidance, or an HDR source is used over several fractions through catheters placed in the prostate.
An HDR emporary implant is placed, using either catheters placed through the surgical lumpectomy site or a balloon placed into the lumpectomy cavity.
Catheters are placed along the surgical bed. Either LDR or HDR sources can be used.
For early shaft disease, catheters can be placed through the tumor. Both HDR and LDR sources can be used.
Intraoperative radiotherapy is a subtype of brachytherapy in which lesions not other wise accessible are given a single treatment with either low-penetration electrons or an HDR source during surgery. It is most commonly per formed when resection is difficult and there is a high chance of microscopic or gross residual disease.
Once the tumor has been resected as much as possible, the surgical bed is prepared with lead shielding to protect nearby normal structures (i.e., bowel, kidney, nerves, blood vessels). Either a por table electron generator is placed over the target surgical bed or multiple HDR catheters embedded in a gel are placed against the surgical surface. The patient is kept under anesthesia and observed remotely while the treatment is delivered. Typically, it takes 10 – 45 minutes, depending on the size of the target field.
Because the dose is given in a single fraction, normal tissue injury is a concern, particularly for structures deep to the tumor bed that cannot be shielded. The dose given is usually between 10 and 20 Gy. The most common injury is neuropathy. Tumors treated in this fashion include abdominal or retroperitoneal tumors and recurrent rectal cancer.
Intracranial stereotactic radiosurgery
Intracranial stereotactic radiosurgery (SRS) consists of a single large-dose but tightly focused radiation treatment. For intracranial SRS a stereotactic head frame is used that facilitates exact tumor positioning. The frame creates a three-dimensional coordinate system that is linked to the planning software and treatment equipment to allow for very tight margins on the target. Both CT and Mr imaging are typically used.
SRS is offered to patients with a limited number of brain metastases and other wise well controlled disease and for benign diseases such as acoustic neuromas and arteriovenous malformations. a single fraction of 12–24 Gy is prescribed, depending on the size of the lesion and its proximity to critical normal structures such as the brainstem and optic chiasm.
Stereotactic body radiotherapy (SBRT)
In extracranial stereotactic radiotherapy, typically 3 –5 fractions are given to treat metastases or recurrent tumor. Tight margins are used to protect normal tissue when treating with high doses per fraction. Accuracy in positioning is essential. Instead of a head frame anchored to the skull, a body mold is used, often with hardware placed over the abdomen to reduce respirator y excursion.
For additional tumor targeting, use either pretreatment CT or radiographic matching to fiducial markers placed in or adjacent to the tumor.
Although SBRT is most commonly used for lung and liver metastases, it has also been used for recurrent disease previously treated by external radiation, such as spine lesions. Doses are 5 –20 Gy in 3 –5 fractions, taking into consideration size of the lesion and normal tissue tolerance. It is also becoming more common for curative treatment of early stage prostate and lung cancer.
Total body irradiation
Total body irradiation (TBI) is most typically used in conjunction with chemotherapy to prepare a patient for stem cell transplantation. The radiation serves several purposes:
- It eliminates residual disease, including sanctuary sites (CNS, testes).
- It suppresses the host immune system.
- It ablates native marrow to permit engraftment of donor marrow.
Ablative doses of radiation requiring either a full allogeneic or autologous transplant are in the range of 12–17 Gy. Low-dose TBI with a single 2 Gy fraction is used to suppress the immune system for non-myeloablative autologous transplants.
Acute side effects include nausea, mucositis, diarrhea, parotiditis, and fatigue. Possible late complications include cataracts, hypothyroidism, interstitial pneumonitis, veno-occlusive disease of the liver, sterility, and second malignancies. Children in particular are at risk for neurological deficits, growth deficits (both from impaired Gh production and epiphyseal growth plate fusion), and second malignancies.
Treatment is given with opposed fields and low energies. Patients can be positioned lying, sitting, or standing, with either lateral or anterior-posterior (aP) and posterior-anterior (Pa) beams that include the entire body.
Many dose schedules are in use, but they typically involve twice-daily treatment for 1 week. If sanctuar y sites (areas with limited chemotherapy penetration, e.g., CNS, testes) are involved, additional radiation to boost these areas is used. The dose to the lung is reduced to 10 Gy in patients with normal lung function and to 7– 8 Gy in those with reduced function. These calculations are made before treatment, and surface dosimeters are used to check the calculation on the first treatment.
Unsealed radionuclides are most often used in diagnostic nuclear medicine imaging. They can also be used therapeutically for treatment of thyroid cancer, neural crest tumors, widespread bone metastases, or intraperitoneal malignancy. Targeted radionuclide therapy with radioisotopes attached to monoclonal antibodies is used to treat lymphoma.
Dosimetry calculations for unsealed nuclides must consider both the physical half-life of the isotope and the biological half-life, or length of time before the molecule is excreted. The radionuclide is typically secreted in bodily fluids, including saliva, sweat, and urine, which can pose a radiation exposure risk to other individuals.
Thyroid cancer that absorbs iodine can be treated with radioactive iodine-131, which has a physical half-life of eight days. I-131 is selectively absorbed by differentiated thyroid cancer cells. after surgical resection of the thyroid, a low dose of I-131 is used to diagnose any residual disease. If present, a higher ablative dose can be used, with bone marrow being the limiting toxicity.
Neural crest tumors
Iodine-131 attached to metaiodobenzylguanidine (MIBG) is selectively absorbed by neuroendocrine tumors, including pheochromocytomas, neuroblastomas, and paragangliomas. This selective uptake can again be used both diagnostically and, at higher doses, therapeutically.
Bone metastases can be treated with bone-selective radionuclides that concentrate in areas of bone turnover. Radium-223 has recently been approved by the U.S. FDA for treatment of bone metastases from prostate cancer. Myelosuppression can occur up to 6 weeks after treatment, and patients should be closely observed.
Malignant ascites or metastatic peritoneal implants can be treated with an infusion of phosphoruis-32, with a physical half-life of 14 days. The radionuclide emits electron radiation with very limited penetration, so the bowel tolerance is kept while treating superficial disease.
Lymphomas expressing CD-20 can be targeted using anti-CD20 monoclonal antibodies. Two current agents, yttrium-90 conjugated to ibritumomab (zevalin) and I-131 conjugated to tositumomab (Bexxar), have been effective in treating low-grade non-hodgkin lymphoma. Bone marrow is again the dose-limiting toxicity.