What is the alpha/beta ratio of normal tissues?

From the Guidelines

The alpha/beta ratio of normal tissues is typically low, ranging from 1-4 Gy, similar to that of prostate cancer, as reported in a study published in the Journal of the National Comprehensive Cancer Network in 2014 1. This ratio is crucial in understanding the response of normal tissues to radiation therapy, particularly in the context of hypofractionation.

• Normal tissues with low alpha/beta ratios, such as the rectal mucosa, are more sensitive to larger radiation doses per fraction and demonstrate greater sparing with smaller fraction sizes.
• The low alpha/beta ratio of normal tissues suggests that they have a greater capacity for DNA repair between fractions, making them more sensitive to the quadratic component of cell killing that dominates at higher doses per fraction.
• Understanding these ratios is essential for radiation treatment planning, as it helps balance tumor control against normal tissue toxicity, and clinicians must consider these tissue-specific sensitivities to minimize long-term complications while maximizing therapeutic effect, as seen in the use of stereotactic body radiotherapy (SBRT) in prostate cancer treatment 1.
• The use of SBRT, which delivers highly conformal, high-dose radiation in 5 or fewer treatment fractions, has shown excellent biochemical progression-free survival and similar early toxicity compared to standard radiation techniques, highlighting the importance of considering the alpha/beta ratio of normal tissues in radiation treatment planning 1.

From the Research

Alpha Beta of Normal Tissues

• The alpha/beta ratio is a measure of the curviness of dose-effect curves, with low values indicating a greater sparing of radiation damage with smaller dose fractions 2.
• Normal tissues have been found to have low alpha/beta values, ranging from 1.5 to 5 Gy, indicating that they are more sensitive to changes in dose fractionation 2.
• In contrast, tumors have been found to have higher alpha/beta values, often greater than 8 Gy, suggesting that they are less sensitive to changes in dose fractionation 2, 3.
• The alpha/beta ratio can be used to compare the effectiveness of different radiation therapy regimens, with higher ratios indicating a greater therapeutic gain for tumors relative to normal tissues 4, 3.

Comparison of Radiation Therapy Regimens

• Hyperfractionation, which involves delivering smaller doses of radiation more frequently, has been shown to confer a therapeutic gain relative to late responding normal tissues 2, 3.
• The use of proton therapy has also been shown to reduce the dose to critical normal tissues, potentially improving quality of life for patients with head and neck malignancies 5.
• Intensity-modulated radiation therapy (IMRT) has been compared to proton therapy, with results showing that proton therapy can significantly reduce the integral dose to head and neck critical structures 5.

Implications for Clinical Practice

• The alpha/beta ratio can be used to inform treatment planning decisions, with the goal of minimizing damage to normal tissues while maximizing the therapeutic effect on tumors 4, 3.
• The use of hyperfractionation and proton therapy may offer therapeutic advantages for certain types of tumors, particularly those with high alpha/beta values 2, 3, 5.
• Further research is needed to fully understand the implications of the alpha/beta ratio for clinical practice and to develop more effective radiation therapy regimens 4, 2, 3, 5.