CT Simulation

Planning your Radiation Therapy

Radiation therapy must be aimed precisely at the treatment target to maximize the treatment efficacy and safety. To accomplish this, your radiation team will perform a “planning” or “mapping” session called a CT simulation. During the CT simulation, a radiation treatment planning CT scan is obtained to get an accurate mapping of your organs while you are in the same position you will be treated in each day. That map will be combined with information from your biopsy, imaging and physical exam and later used to design a radiation plan specific to your needs.

Before your CT Simulation

In preparation for your pre-treatment mapping scan for breast cancer, your radiation oncologist will need to make sure that you have healed sufficiently from your most recent surgery. Sometimes, patients experience post surgical breast swelling or issues with wound healing that need to be addressed prior to moving forward with radiation therapy. If this is the case, your radiation oncologist will often speak directly with your surgeon and share the plan with you. Once all issues have been addressed, you will be set up for your CT simulation.

When arriving for your radiation simulation, you will be asked to remove any metal that may interfere with the radiation delivery, including any necklaces. You may also be asked to refrain from putting anything on your skin in the chest area, including your armpits. There are typically no restrictions on eating or drinking prior to your simulation, but please ask your radiation oncologist for any the specific instructions they may have for you.

During your CT Simulation

Radiation treatment planning begins with a CT simulation. A CT simulation refers to the process of measuring your body to help your team direct the beams of radiation safely and accurately to their intended locations.

For external beam radiation therapy, this consists of a CT scan done in the exact same position you will receive your actual treatments.

Additionally, your radiation oncology team may use personalized support devices such as molds, casts, headrests or other devices that are customized to help you remain in the same position and stay comfortable during the entire treatment.

Once the scan is complete, the radiation therapist, under the radiation oncologist’s supervision, marks the area to be treated on the personalized support devices and/or your skin with either a bright, temporary paint or a set of small, permanent tattoos.

Please note, this CT scan is different from any other diagnostic CT scan you may have had and must be done for every patient. This scan is special because the position you are placed in is intended to be reproducible, with help of the support devices. This means, your radiation oncologist intends to put you in the same position, within about 3-5 mm in any given direction, where feasible. Additionally, the scan provides specific information about the radiation beam passes through a person’s body. This is also why a patient may need to repeat a simulation even if they already had a simulation at another facility.

If you have left-sided breast cancer or the treatment plan needs to be optimized further, breathing techniques may be used during your treatments to alter the anatomy within the chest and minimize the radiation therapy directed at the heart.

After your CT Simulation

After your CT simulation, the radiation oncologist, dosimetrist, and medical physicist will use sophisticated computer software to design your radiation treatment plan. After this, several quality assurance steps are taken to make sure that the machine that is used to administer the radiation will be able to deliver the treatment accurately. Once these steps are completed, you will be brought back to the radiation oncology department to start your treatment.

Treatment Planning and Quality Assurance

Treatment Planning

Once you have finished simulation, your radiation oncology treatment team will begin creating a specific radiation plan for your cancer. To do this, your team will review information obtained during simulation along with your previous medical tests. Next, your radiation oncologist will write a prescription that outlines exactly how much radiation should be delivered to specific parts of your body. An ideal radiation plan is one that focuses the radiation on the prostate and other targets while limiting radiation to healthy organs.

Your information is then transferred to a computer system called the treatment planning system (TPS), which is a sophisticated treatment-planning computer and associated software designed to create the best possible treatment plan. A team of experts including your radiation oncologist, medical physicist and dosimetrist will work together to develop the radiation treatment plan.

The radiation oncology team uses the images obtained during the simulation to outline both the areas we want the radiation to be directed (known as targets) and the areas we want the radiation to avoid (known as organs at risk [OAR]). For patients being treated for breast cancer, the target will depend on the type of surgery performed and the areas at risk for harboring residual cancer. This typically includes the lumpectomy bed (the area where the breast cancer used to be after it was removed during a prior partial mastectomy surgery or chest wall and mastectomy scar).

Depending on the extent of the cancer, the radiation targets can also include lymph nodes that drain the breast, including the supraclavicular lymph nodes, the axillary lymph nodes, and/or the internal mammary lymph nodes. The organs at risk that are typically accounted for include the contralateral breast, the bilateral lungs, heart, and esophagus. Ask your doctor to explain what treatment area is appropriate for you.

Sometimes, images taken at other points in your treatment journey may be better suited to determine the extent of your cancer. Specifically, some patients with breast cancer undergo diagnostic CT scans, Magnetic Resonance Imaging (MRI) scans and/or Positron Emission Tomography (PET) scans, as indicated by their treating oncologists. During a process called image fusion, your radiation oncologist can align these additional images with those taken during the simulation to get a better understanding of where the radiation should be targeted.

Radiation therapy for breast cancer is most commonly delivered with photons and occasionally via electrons via a process called External Beam Radiation Therapy. Proton therapy can also be used in selected cases. Learn more about how radiation is delivered.

Quality Assurance Before Treatment

Once your radiation oncologist approves the radiation therapy plan developed in treatment planning, members of the radiation treatment team work together to ensure that your specific treatment plan works correctly on the linear accelerator before treatment begins. Typically, this process may include an evaluation of the radiation plan where the plan is delivered to a phantom, a radiation monitor with sensors, where the actual amount of radiation from the machine is compared to what was calculated by the computer to ensure they are similar.

Your radiation therapy team may also perform a ‘dry run’. During this process, you will be asked to come in before your radiation is scheduled to start. When you come in, the treatment team will put you on the radiation table and place you in the intended treatment position. This is often needed for more complex treatments to ensure that the radiation treatment designed for you is as accurate and as safe as possible.

Radiation Delivery

During radiation treatment planning, a radiation treatment plan is developed to deliver treatment to your cancer. In most cases, the radiation comes from outside your body, and is called “External Beam Radiation Therapy”. This external radiation is most commonly delivered in the form of high-energy photons, or X-rays. In a few clinics around the country, proton beam therapy is also used to treat breast cancer. Proton therapy is a form of external beam radiation therapy that uses protons rather than photons to treat cancer cells. Protons may be considered for certain patients. The differences between protons and photons in treating breast cancer continues to be studied.

External beam radiation therapy can be delivered using a variety of techniques. A common technique used to treat prostate cancer is 3-dimension conformal radiation therapy (3D-CRT). Occasionally, intensity-modulated radiation therapy (IMRT) is also used, particularly when the doses to the OARs are lower.

With all external beam therapy, treatment is delivered in a series of daily sessions, Monday through Friday, for several weeks. Each treatment is non-invasive, painless and similar to a long X-ray. You may hear noise but will feel nothing at the time of treatment. Immediately prior to each treatment, your treatment team will ensure the areas designed to receive the radiation are indeed in the correct position. This process initially starts with making sure you are properly set up on the table. This may involve using lasers within the room and aligning them to markings put on your skin and/or the personalized support devices. Next, your radiation therapists might take a set of pictures to confirm that your internal anatomy is properly aligned. This is called Image Guided Radiation Therapy (IGRT). This may consist of a set of x-rays taken at specific angles or images taken throughout a partial or complete rotation around your body (often called a Cone Beam CT [CBCT]). These images are compared against your images taken at the time of your simulation to make sure your position is accurate.

The length of your treatment will depend on your health and the type of radiation used. In general, however, each treatment lasts approximately 15-30 minutes starting when you enter the radiation treatment area. Hypofractionated radiation is a form of external beam treatment giving slightly higher doses over one to four weeks compared to a more standard treatment time of five to seven weeks. Studies have shown similar results compared to standard treatment times.