Imaging Application in Radiation Therapy

Introduction to Radiation Therapy

X-ray was first discovered by Wilhelm Roentgen on November 8, 1895. About two and half months later, on January 29, 1896, an X-ray was used to treat breast cancer patients. Fast forward over 100 years, clinicians are using various types of radiation (e.g., x-ray, electrons, protons, carbons, and others) to treat not only oncological diseases but also non-oncological diseases like heterotopic ossification, arterial venous malformation and trigeminal neuralgia. Radiation therapy uses highly energetic radiation (ionizing radiation) to induce DNA damage to the cancer cells while minimizing radiation damage to the normal tissue. Radiation can be used for curative intent and palliative purposes depending on the stage of the disease and location. There has been a long history of evidence based clinical trials and clinical experiences to show that radiation is highly effective at treating cancer with a highly favorable survival rate. Recently, there has been tremendous increase use of imaging modalities to better target cancers before, during and after treatment for the purpose of radiation treatment planning, target localization, and treatment outcome evaluation.

Computed Tomography (CT)

Computed tomography is an imaging system that utilizes ionizing radiation to generate volumetric images that are anatomically accurate. In radiation oncology, CT has become a standard imaging modality to capture three-dimensional images of the patient’s anatomy to identify the location of the tumor and surrounding critical structures. From it, physicians can digitally contour the tumor(s) and critical structures. Using this information, clinical staff can create a radiation treatment plan that is customized to the patient’s anatomy and tumor geometry and location. In addition to providing the patient’s anatomy, CT images are also used to simulate radiation treatment and calculate radiation dose distribution. In addition to stand alone CT system, a radiation therapist can use a cone beam CT (CBCT) that is integrated into a treatment machine to view the internal anatomy of the patient and help the therapist to set up the patient for the radiation treatment. The advantage of CBCT is the ability to visualize the tumor inside the patient at the time of the treatment and adjust the patient’s position to maximize the accuracy of the treatment. Lastly, recent advances in dual energy CT (DECT) system has provided the ability to enhance the soft tissue contrast with or without the need for an intravenous contrast agent.

  Magnetic Resonance Imaging (MRI)

Similar to CT, a magnetic resonance imaging system is an imaging system that can provide a volumetric image of a patient. However, the main difference between MRI and CT is that the former uses non-ionizing radiation (magnetic field) to generate three dimensional images of internal anatomy. The main advantage of MRI is that it has superior soft tissue contrast images that no CT can provide. The soft tissue contrast images of the MRI are so exquisite that it is a standard imaging modality for any brain and spine cancers. 

Typically, in radiation oncology, MRI is used to supplement the CT when identifying the tumor location and other neighboring critical structures at the time of treatment planning. This is especially true for any brain and spinal cancers.

Positron Emission Tomography With CT (PET/CT)

Positron emission tomography generates images of the metabolic activities of cancer cells in the patient. This is done by intravenous injection of sugar that is bounded to a radioactive tracer (18-fluorodeoxyglucose) to the patient. Once the patient is positioned inside the tomography unit, the sugar is metabolically absorbed by the cancer cells, and the radioactive tracers (annihilation radiation) are detected by the ring of detectors located inside the tomography unit. The end result is the volumetric images of metabolically active cancer cells fused with CT images of the patient, hence called PET/CT. The reason why image fusion of CT is necessary is that PET itself does not provide adequate information about the physical anatomy of the patient to properly identify and locate the metabolically active cancer cells within the patient. By fusing the CT images with the PET images, the clinician can clearly identify and orient where the metabolically active cancer cells are located with respect to the patient’s anatomy. The advantage of PET/CT is the ability to see the metabolically active cancer cells. Images like CT and MRI may show the solitary mass, but the mass may not necessarily be an active cancer (for example, it can be fibrotic tissue that looks similar to the solid tumor in CT images). PET/CT can provide supplemental information about the metabolic activity of the cancer cells. Typically, PET/CT is used in lung cancer and metastatic cancers to supplement the CT images in radiation therapy.

Imaging Modality Integrated Within Linear Accelerator

Because the localization of cancers at the time of treatment is critical to the treatment outcome, multiple imaging modalities have been integrated with a medical linear accelerator. A medical linear accelerator is a machine that generates high energy X-rays that can deliver radiation with pinpoint accuracy to the target, provided that the target inside the patient can be localized. For this reason, there has been a significant push for the integration of various volumetric imaging modalities with linear accelerators. The most common integration is with cone beam CT. MRI has also been integrated into the linear accelerator in order to exploit the benefits of soft tissue contrast. Most recently, the integration of PET has been introduced to the market. By integrating various imaging modalities with the treatment machines, clinicians are able to better localize the tumor at the time of the treatment. The advantage of the integration is the ability to increase the radiation dose to the cancer while minimizing the dose to critical structures.