Types of Radiation Treatments

There are a variety of different treatment techniques in modern radiation therapy, ranging from simple to highly complex in terms of procedures and delivery. The complexity and technique of treatments depending on the disease site, complexity, cancer progression, and patient health.

But first, let’s define radiotherapy simply as treatment of a disease using radiation. If this is what we think of as radiotherapy then we can separate radiotherapy into three broad categories of radiotherapy/radiation treatments. These three categories are External Beam Therapy, Brachytherapy, and Radiopharmaceutical treatment.

External Beam therapy is radiation therapy delivered from a radiation beam which is external to the patient. The external radiation beam can be in several different flavors of radiation; protons, electrons, heavy ions, and x-rays. X-rays are by far the most common, followed by electrons, and protons and heavy ion particle beams are normally referred to as exotic beams, that’s how rare they are. External Beam Radiation Therapy (EBRT) is by far the most common catergory of radiotherapy

Brachytherapy is where treatments are performed by position a radioactive source inside or near the body, close to the tumor. The radiation used here is in the form of gamma radiation, since a radioactive source is used. Just to recape that there is not a big difference between gamma radiation and x-ray radiation, they are both photons and can have similar energies. The difference is in where they originate from. Gamma rays come from radioactive material, and x-rays are created by deaccelerating electrons inside a target material.

Radiopharmaceutical treatments are rare. Often they are not performed in radiotherapy departments, rather they are held in nuclear medicine departments. Nuclear medicine is a specialist department in the hospital, where they used radioactive material in medicine, mainly for diagnostic purposes (think PET imaging), but occasionally for therapy. The therapy in question is using radioactive iodine to treat thyroid cancer or hyperthyroidism.

The three categories or radiotherapy are vastly different in terms of deliver and technique. But this post is not going to deal with brachytherapy and iodine therapy, this is just a roundup of techniques used in external beam radiotherapy using x-rays and electrons. Heavy Ion Therapy and Proton Therapy are also excluded, and along with Brachytherapy and Radiopharmaceutical Therapy these topics deserved to be discussed in more detail by themselves. So let’s start with techniques in external beam radiation therapy.

Superficial Radiation Treatments

Superficial radiation treatment uses lower energy radiation beams to target diseases that are superficial- close to the surface of the body. Superficial treatments are useful for skin cancers and melanomas- if they have been caught early. Low energy radiation is used as the radiation does not need to go too deep to cover the entire disease site. This helps by not delivering too much radiation to the healthy tissue deeper than the disease.

To further protect the surrounding healthy tissue the radiation beams are shaped using special cones that fit onto the machine. These cones will define radiation fields of different sizes, such as 5 cm circle, or 3 cm square. A department will have a range of these standard sizes to chooses from which will best cover the treatment site. Otherwise, if no cone will correctly cover the site, a lead shield can be created that matches the site perfectly while shielding all the healthy tissue. Patients may have several treatment sites at a time, but normally only one beam is used per treatment site.

An image of a superfical treatment machine used to treat cancers which are close to the surface such as skin cancer

Electron Beam Therapy (EBT)

Electron beams are similar to superficial x-ray treatments, as they only penetrate to a certain depth depending on the electron energy selected. They have advantages over superficial treatments as the dose delivered beyond the treatment site drops off much faster sparing more healthy tissue.

Secondly, they can be delivered using a Linear Accelerator, a machine that every radiotherapy department will have, therefore there is no need to by a specialist machine to treat sites close to the surface.

Just like superficial treatments, electron beams are defined by different applicator- one of the most common size is the 10 cm square applicator. And again, personalized shaped lead alloy shapes are cast to shape the electron beam to the treatment site to prevent healthy tissue from receiving any radiation. Electron beam therapy often uses one beam per treatment site like superficial treatments.

There are downsides to electron beam therapy. One of the main downside is that the radiation dosimetry is more challenging than superficial x-rays. This stems from physics, mainly the fact that x-rays scatter less than electrons- as electrons have an electric charge they interact more. This means that in certain cases calculating the dose delivered from electrons is much harder than superficial x-rays- but not entirely impossible.

Three Dimensional Conformal radiotherapy

An aperture to shape the radiation beam to the outline of the tumor use in 3D conformal radiation therapy

Three-dimensional (3D) conformal radiation therapy is technique where several beams of radiation are used to treatment a tumor. Using a CT scan of the patient, the radiation beams are optimally directed to the tumor from different directions to best avoid any healthy tissue and critical organs, secondly the beams are shaped to match the tumor outline.

It is important to sculpt the radiation dose to cover the tumor as this avoids delivering radiation dose to the healthy tissue surrounding the tumor. 3D conformal radiation therapy is ideal for tumors that have irregular shapes or that lay close to healthy tissues and organs.

Before the invention of CT and advances in Linac technology, radiation treatments only matched the height and width of the tumor, delivering radiation beams in squares and rectangles, resulting in healthy tissue receiving radiation dose. This limited the dose that could be delivered, as delivering more would increase the chances of radiation side effects. Using 3D conformal techniques, higher doses can be delivered as the healthy tissue is better avoided, which improves the ability to kill the tumor cells.

You will see that 3D conformal therapy is, in many ways, very similar to IMRT (below). Both are used to target cancer while sparing the surrounding healthy tissue. The Radiation Oncologist will determine which is best for each patient, depending on their tumor’s shape and location.

Intensity Modulated Radiation Therapy (IMRT)

As mentioned above Intensity Modulated Radiation Therapy is very similar to Three-dimensional (3D) conformal radiation therapy. IMRT is again technique where several beams of radiation are used to treatment a tumor. The beams are directed at the tumor from different directions. The beam is shaped using several different fields at each direction- unlike 3D conformal, which generally uses one, or sometime two, field shapes from the different direction.

A set of segments used to defined the radiation beam during IMRT deliver.

A single IMRT beam can have anywhere from 10 to 30 separate fields within it- generally these individual fields are called segments. The dose delivered by each of these segments is varied in terms of intensity. All these segments are added up to deliver the final dose. Using several segments allows the dose to be modulated over the field- 3D conformal generally uses uniform fields. This modulation give more ability to sculpt the dose to the tumor and avoids healthy tissue.

Because the dose is modulated across the radiation field, correct positioning is critical. Precise CT information is also required to be able to plan an IMRT treatment. IMRT treatments are too complex for a dosimetrist or radiation therapist to plan manually. They are planned using computer algorithms which select the best sequence of segments to deliver the highest amount of dose to the tumor while delivering the lowest amount of dose to the healthy tissue.

A 5 beam IMRT treatment, showing the different directions of the beams and the resulting dose distribution

Because most of the treatment planning for IMRT is done using a computer, multiple check are performed along the process to ensure that the data has been transferred correctly and that the dose shown on the planning computers screen is actually the dose being delivered by the Linear Accelerator. This task is usually performed by the medical physicist.

Volumetric modulated arc therapy (VMAT)

Volumetric modulated arc therapy, or VMAT, is a subset of IMRT. The radiation is modulated just like in IMRT. The difference is that the segments are continuously changing as the machine rotates in an arc around the patient, rather than at static points as in IMRT. A similar number of different field segments are used in both VMAT and IMRT deliver.

The main difference is that VMAT has more ways in which it can adjust where the dose is delivered by constantly varying the rotation speed of the machine’s gantry, the shape of the treatment aperture using the movement of multileaf collimator leaves, and the delivery dose rate.

An illustration of the VMAT delivery technique of radiation therapy, where the gantry of the linac rotates continuously around the patient.

Stereotactic Radiation therapy (SRT)

Stereotactic radiation therapy is best suited for small tumors, normally inside the brain, with very high doses in a shortened treatment regime. Stereotactic radiation therapy uses small field and tight margins to precisely target the tumor, because of this the patient needs to be exactly is the same position as when they were imaged for treatment planning. This is done using patient immobilization devices such as head mask moulded around the patient’s head. This insures the dose is correctly delivered.

The advantageous of stereotactic radiation therapy is to deliver a high amount of dose in a short amount of time precisely to the treatment target minimizing the dose to nearby organs. The disadvantage is that the technics is only suited for small round targets which do not have much movement. This makes brain tumors very suitable for this technique of radiation therapy. As well as this a specialist team is required to deliver stereotactic treatments.

Stereotactic Radiation therapy (SBRT)

Stereotactic Body Radiation Therapy (SBRT) is a treatment procedure similar stereotactic radiation therapy, except that it deals with tumors outside of the brain. Stereotatic radiation therapy to ta target tumor outside the brain is difficult. Targets in certain organs, such as lung and liver, tend to move more than ones in the brain. This requires more precision and the ability to determain if the target is in the correct position will the radiation is being delivered.

Stereotactic radiation therapy and stereotactic body radiation therapy can be delivered on linear accelerators, but there are also specialized machines dedicated to this type of treatment. These include the gamma knife, which uses hundreds of small radiation beams from radioactive cobalt 60 to precisely target the tumor. There is the gamma knife, which is uses a robotic arm to target the tumor from any direction using circular beams.

A CyberKnife robotic stereotatic radiation therapy machine used for SRT and SBRT treatment techniques.

These devices are specialist machines with large price tags which can only perform these types of treatments. Therefore they are normally only located in larger departments, training hospitals, or centers which participate in research.

I hope that give you some more insight into the different types of radiotherapy treatment techniques available in radiation therapy. As always if you have any comments or questions please leave them below.

And Check out my pinterest page if you would like to see some pictures of these machines.

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Originally published at www.radicalradiationremedy.com on January 16, 2017.