Radiotherapy Fractionation: An Exploration
Radiotherapy, also known as radiation therapy, is a cornerstone in the treatment of cancer. One of the critical aspects of radiation therapy that ensures its safety and effectiveness is the concept of fractionation.
What is Fractionation in Radiotherapy?
Fractionation refers to the practice of dividing the total prescribed dose of radiation into smaller, equally administered doses, or fractions, over a series of treatment sessions. Rather than delivering the entire radiation dose in a single session, fractionation allows for the gradual application of radiation over several days or weeks.
Why is Fractionation Necessary?
Fractionation serves several crucial purposes in radiation therapy:
1. Minimizing Normal Tissue Damage: Radiation is not selective; it affects both cancerous and healthy tissues. Fractionation helps to spare healthy tissues by allowing time for normal cells to repair between radiation sessions. This reduces the risk of severe acute reactions and late complications.
2. Enhancing Tumor Control: Fractionation increases the likelihood of destroying cancer cells while minimizing damage to surrounding normal tissue. This is particularly beneficial when treating tumors located near critical organs or structures.
3. Optimizing Radiobiological Response: Fractionation takes advantage of the radiobiological characteristics of cancer cells. Some tumor cells are more sensitive to radiation when exposed to smaller doses over time, making fractionation an effective strategy to maximize cell kill.
Key Principles of Fractionation:
1. Dose Per Fraction: The size of each fraction varies depending on the specific tumor type, location, and clinical goals. Typically, fractions range from 1 to 2 Gray (Gy) for conventional fractionation, but higher doses per fraction may be used in specialized techniques like stereotactic body radiation therapy (SBRT).
2. Total Dose: The total radiation dose prescribed for a patient is determined based on the type of cancer, its stage, and the treatment goals. The total dose is calculated by multiplying the dose per fraction by the number of fractions.
3. Fractionation Schedule: The schedule for delivering fractions varies depending on the specific treatment plan. For conventional fractionation, treatments are usually given daily over several weeks. Hypofractionation, which delivers larger doses per fraction, may be administered over a shorter period.
The Linear Quadratic Model:
The linear quadratic model is a fundamental concept in radiation oncology. At clinically relevant doses, this model explains how cancers and early-reacting tissues respond to ionizing radiation. It highlights two critical components:
• Linear (α) Component: This component represents the linear relationship between the radiation dose and cell kill. It applies to cancers and early-reacting normal tissues.
• Quadratic (β) Component: Late-reacting tissues are significantly affected by the square of the individual dose given, known as the quadratic element.
Understanding this model is vital because it informs the choice of fractionation scheme, optimizing the balance between cancer cell destruction and minimizing damage to normal tissues.
Number of Treatments:
Traditionally, radiotherapy was administered once daily, Monday to Friday. Two main fractionation schedules have been widely employed:
Few Large Daily Fractions:
• Advantages: This approach reduces the number of patient attendances, saves resources, and leads to a fast tumor response. It also helps reduce the risk of tumor repopulation during treatment.
• Disadvantages: However, it limits the total dose that can be safely delivered, increases the risk of late normal tissue damage, and reduces the potential for reoxygenation. This may result in an inadequate total dose to eradicate all cancer cells.
Many Small Daily Fractions:
• Advantages: This strategy is associated with less severe acute reactions due to the longer treatment time. It also minimizes late normal tissue damage, maximizes the total dose that can be delivered, and fosters reoxygenation. The total dose may be sufficient to eradicate all cancer cells, and adjustments can be made if unexpected severe acute reactions occur.
• Disadvantages: However, it places more significant demands on both resources and the patient. There is also a potential for repopulation of fast-growing tumors during radiotherapy, and prolonged acute reactions may require supportive treatment.
Radiosensitivity of Tumors and Normal Tissues:
Different tumor types exhibit varying radiosensitivity. Some tumors, such as lymphoma and seminoma, can be controlled by lower doses than those required for many carcinomas. Conversely, certain tumors like gliomas and sarcomas may be resistant to even high doses.
Tolerance doses for normal tissues are crucial to prevent late damage. For example, the tolerance dose for the spinal cord is much lower than that for the lens of the eye.
A word on Inter-Fraction Interval:
The time between radiation treatments is essential for tissue repair. In once-daily fractionation, most repair processes are completed before the next treatment. To allow for maximal repair in normal tissues, an interval of at least 6 hours is recommended when more than one treatment is given in a day.