Radiation Therapy: Two Common Photon Treatments

There are several common types of radiation therapy that are used in treating cancer today.  Radiation therapy is classified according to the type of radiation particles or waves that are used in treatment which include: photons, electrons, or protons.  The most commonly available treatments using photons and electrons.  In this installment we will look further into treatments using linear accelerators, LINAC, and CT Scanners, in the treatment of cancerous tumors.

3D Conformal Radiation Therapy

3D conformal radiation therapy is a method of treating cancer that uses CT imaging, Cat scan-based, in the treatment of cancerous tumors.  In 3D conformal radiation therapy, the tumors and organs are well-defined 3D images as opposed to flat images that are gotten from x-rays.  Tumors are outlined in three dimensions on a CT scan. Healthy organs are defined as well when scans are performed so that technicians can see areas that need to be avoided. 

Radiation beams are then organized in the best possible way to avoid healthy organs while delivering the highest dose of radiation possible to the cancerous tumor.  Patients are placed in the same position as they were when their CT scan was performed.  This allows for accurate placement of beams according to the scans that were obtained. 

Special software is used that calculates the total amount of radiation that will be delivered to the tumor and normal tissues to assure that the tumor is sufficiently covered in radiation while healthy tissues and organs receive as little radiation as possible.   The radiation beams are adjusted even further based on the software’s calculations to give healthy doses of radiation while being manipulated into varying positions and shapes as defined by the tumor.  Radiation beams are directed using one of two types of machines: Cerrobend blocks or multi-leaf collimators. 

Cerrobend blocks are individually shaped to form specific, custom-made shapes that create the correct beam for treatment.

Multi-leaf collimators are skinny, metal blocks that move independent of one another in a quick, swift manner to form complex patterns to shape radiation treatment beams.  They are commonly used in IMRT.

Intensity Modulated Radiation Therapy (IMRT)

IMRT is another form of delivering photons in the treatment of cancerous tumors with the possibility of lowering the dose of radiation delivered to non-cancerous tissues.  Planning for IMRT begins in a similar fashion as 3D conformal radiation therapy in that it starts out with simulation.  The tumor and organs are all outlined and shown as three-dimensional objects. Several beams of radiation are situated at varying points around the person in order to deliver the optimal amount of radiation. In IMRT, the beams are divided into a grid pattern.  The large radiation beam is split into numerous smaller beams known as beamlets. 

Software is used to establish the appropriate pattern to break the beamlets down into from the larger beam to prevent radiation targeting healthy tissues while delivering the optimum radiation to the cancer.  The multi-leaf collimators often form more than fifty different shapes during radiation treatment.  The main advantage to this type of radiation therapy is that the patterns the beamlets form are precise and the radiation controlled.  IMRT is often utilized when cancerous tumors are in positions that are difficult to treat. 

Often tumors that are directly adjacent to or wrapped around normal healthy organs, IMRT is used.  Intensity modulated radiation therapy reshapes the radiation in the best possible way to avoid normal organs while delivering large doses of radiation to the tumor.  IMRT is used commonly in the treatment of head and neck tumors where there are many other significant structures and organs that are near the tumor such as the spinal cord.

The one downside to IMRT is that it can take longer to plan and deliver treatment than other 3D conformal therapy.  Radiation can also be a bit more uneven as well because of the small beamlets that are being used.  Although normal organs are out of danger of high doses of radiation larger number of normal organs receive low doses of radiation which can be a disadvantage.  As of today, low doses of exposure to radiation are unknown.  Tumors must continually be monitored for movement and shrinkage to ensure that radiation is being delivered to the cancer. 

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