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Radiotherapy Physics - Special Procedures

Article Index

Special Procedures

Stereotactic Radiosurgery

Radiosurgery is a treatment technique to deliver sharply focused beams of radiation to intracranial targets. Multiple small beams of radiation intersect at a focal point to create a small ball or sphere of high dose radiation which then could be targeted with a high degree of precision to an intracranial lesion using a head frame unit attached to the skull. The term "stereotactic" refers to the targeting mechanisms which employ computerised tomography (CT), magnetic resonance imaging (MRI), or angiography to direct the radiosurgery beams in 3D. Stereotactic therapy is used as both a single dose radiation treatment, stereotactic radiosurgery (SRS), and multiple fraction of smaller doses called stereotactic radiotherapy, (SRT).

There are currently two types of technology for the delivery of SRS: Gamma Knife, which employs an array of over 200 concentrically positioned sources of cobalt-60 to aim at the intracranial target. Another approach is to use a linear accelerator (with small treatment cone or miniature collimators) to create a sphere of radiation by aiming an array of intersecting non-coplanar arc beams of radiation.

Malignant brain tumour, meningiomas, orbital tumours, arteriovenous malformations (AVM's), acoustic neuromas, metastatic tumours to the brain, pituitary tumours and brain stem tumours, skull base tumours, head and neck tumours including nasopharyngeal carcinoma and selected recurrent tumours near the skull base are treated with these techniques.

 

Intensity Modulated Radiation Therapy

Intensity Modulated Radiation Therapy (IMRT) is a sophisticated technology that can deliver a high dose of radiation to a tumour while sparing surrounding healthy normal tissues with good conformality.

The fundamental difference between conventional conformal radiation therapy and IMRT is beam intensity; in conventional radiation therapy, the beam intensity is uniform, but in IMRT, the beam intensity is varied across the treatment field. During IMRT treatment, when the tumour is the thickest,the beam intensity is at its maximum, and when the tumour is the thinnest, the intensity is at its minimum. This technique can also spare adjacent of overlying healthy tissues. Instead of the patient being treated with a single, large uniform beam, the patient is treated with a large number of smaller beamlets, each with different intensities or amounts of radiation.

IMRT allows the radiation oncologist to conform the radiation dose to difficult tumour volumes, yet spare eloquent areas -- for instance, treating a tumour that has wrapped itself around the spinal cord. IMRT is similar to conformal therapy, which is practiced in many radiation oncology centers, but conformal therapy is limited to a geometric shaping of the beam with no intensity modulations across the field.

When a tumour is not well separated geometrically from surrounding organs, adding modulation to the geometric shaping of the beam is more effective in conforming the dose to a tumour while minimising the dose to normal surrounding structures. This way, an optimised treatment plan can be created for a given tumour of any size, shape or anatomical location.

 

Total Body Irradiation (TBI)

Total body irradiation is often used in bone marrow transplantation. This procedure may be considered as an x-ray bath and aids in the transplant process. In this procedure the medical physicist must carefully take into account the patent’s thickness, size, shape, and treatment position. This information is used by the medical physicist to design custom shielding to protect vital organs, and to determine how long the beam should remain "on" to deliver the dose prescribed by your radiation oncologist.

 

Total Skin Irradiation (TSI)

There are few conditions that require a radiation oncologist to irradiate a patient’s entire skin surface. The most common entity is cutaneous lymphoma or mycosis fungoides. This disease entity involves malignant lymphocytes that have a propensity for lodging in the patient’s skin and causing tremendous discomfort with agonising itching and generalised skin irritation. This particular disease does not respond well to chemotherapy. In contrast, it is readily sensitive to radiation therapy.

Mycosis Fungoides presents several distinct problems that require significant attention to detail. The desire is to treat the skin surface and not damage underlying organs and tissue. For this we use a type of radiation called electrons. They have the unique property of traveling only a short distance in tissue.This creates several inherent problems. Skin or other structures shield much of one’s skin surface. For example, the region between the thighs, perianal regions, bilateral axillae and soles of the feet are normally shielded. Multiple mini radiation detection devices called TLDs are placed on the skin to allow us to accurately measure the radiation dose to multiple sites. Those sites underdosed can be given as mall boost of radiation to ensure all areas receive the prescribed dose of radiation therapy.


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