Radiation therapy (RT / RTx / XRT) utilizes high-energy radiation to control or kill malignant cells
- can be part of a curative treatment in several types of cancer
- may be part of a palliative treatment to control symptoms such as pain or progression progression of disease
- Radiation is used to kill cancer cells by damaging their DNA, but normal cells can also be damaged and therefore, treatment must be carefully planned to minimize radiation exposure to normal organs
palliative treatment = specialized care focused on providing relief from symptoms, regardless of diagnosis or stage
The radiation used for cancer treatment may come from a machine:
- external beam radiation therapy = external to the body
- Brachytherapy = radioactive material placed in the body near tumor cells
- Systematic radiation therapy / Nuclear medicine = radiopharmaceutical injected/ingested
A patient may receive RT before, during, or after surgery, depending on the type of cancer being treated. Some patients receive radiation therapy alone, and some receive RT in combination of chemotherapy and/or surgery
Successful RT requires a uniform dose distribution within the target (tumor). The criteria of a uniform dose distribution within the target:
- Recommendations regarding dose uniformity, prescribing, recording, and reporting of photon beam therapy are set forth by the International Commission on Radiation Units and Measurements (ICRU).
- The ICRU report 50 and 62 recommends a target dose uniformity within and relative to the dose delivered to a well defined prescription point within the target.
- Modern RT has required some variations from this for special treatment techniques.
Radiotherapy techniques
RT is carried out with a variety of:
- beam energies
- beam types
- field sizes
- delivery mode (static, dynamic, or rotational)
Uses one of two set-up conventions:
- a constant source to surface distance (SSD) for all beams
- an isocentric set-up with a constant source to axis distance (SAD)
isocenter = fixed, 3D point in space where the central axes of the medical imaging / RT beams intersect. It acts as the reference point for centering the patient’s target volume to ensure accurate treatment delivery and high-quality, undistorted imaging
SSD technique = distance from the source to the surface of the patient is kept constant for all beams. Each beam requires repositioning of the patient
SAD technique = center of the target volume is placed at the machine isocenter i.e. the distance from source to the target point is kept constant
- the SAD technique requires no adjustment of the patient setup when turning the gantry from one field to the next field