By Yolanda Smith, BPharm
When radiation therapy is used to treat cancerous cells in the body, it is important to measure the dose correctly to avoid unnecessary damage to normal cells in the body. Radiation is not selective to tumor cells and targets any cells that are in the process of replication when the therapy is applied, which accounts for the importance of the correct dose to ensure optimal efficiency with minimal side effects.
Gray (Gy) is the unit used to measure the total about of radiation the patient is exposed to. This can also be recorded as centigray (cGy), which is 0.01 of a single gray unit.
Adjuvant therapy doses typically range from 45 to 60 Gy for cancer of the breast, head and neck, which is divided into multiple smaller doses given over a period of one to two months. The specific dose for each patient depends on the location and severity of the tumor and is at the discretion of the oncologist responsible for therapeutic decisions.
The total radiation dose is usually divided into several fractions. For most patients that require radiation therapy, the total dose is broken up into daily doses five times a week for a total period of five to eight weeks. Some cancers, however, require treatment more often than once per day.
Each fraction contains a small amount of radiation that gradually accumulates to form the total dose. This technique allows the cancerous cells to be treated effectively, whilst leading to less damage that affects normal tissues.
Hyperfractionated radiation divides the daily dose into two treatments each day, which means that the patient is subjected to smaller but more frequent doses of radiation over the same period of time.
Conversely, hypofractionated radiation breaks the total dose into larger doses, often giving a dose less that once each day.
Standard treatment with radiation therapy lasts for five to eight weeks, depending on the specific type of cancer being treated and is at the discretion of the oncologist supervising the therapy.
Accelerated radiation refers to when the total dose is administered over a shorter period of time than usual. This involves more frequent doses, usually more often than once daily, to administer the equivalent total dose over a shorter period of time. This can be useful in some types of cancer, when a more aggressive treatment regimen is required.
Changes in the dose frequency and treatment length do not alter the total exposure to radiation and, as a result, the long-term effects remain similar. However, different treatment fractioning and accelerated treatments are often associated with a faster onset of effects, both on normal and cancerous cells.
Patient Positioning During Doses
The exact position of the patient whilst the radiation is of utmost importance to ensure that the correct dose of radiation is emitted to the intended area of the body.
It is common practice for skin to be marked to indicate where the treatment should be focused and the patient should be instructed to remain in the same position without moving for the duration of each treatment fraction.
A mold or cast can be helpful to assist patients in maintaining the correct position and stay still while radiation therapy is in progress. Additionally, areas of the body that do not contain tumor cells should be subjected to as little radiation as possible, often necessitating blocks or shields to protect other parts of the body.
Last Updated: May 4, 2015