Answer to Question #443 Submitted to "Ask the Experts"

Mature muscle is one of the most resistant tissues in the mammalian organism to deterministic effects of radiation. Because of this, there have been few studies reported that deal with radiation effects in muscle. Most information comes from studies of patients who have been treated for malignant disease with therapeutic doses of radiation, generally about 60 Gy fractionated over a period of about six weeks. Clinically significant problems from radiation injury to muscle have been rare. Muscle fatigue has been a frequently reported early effect of patients from therapeutic radiation. The mechanism is not well understood but is thought to relate to creatine metabolism. The effect is transient. The effects of alpha, beta, and gamma radiation are qualitatively similar in muscle as well as most other tissues. However, alpha particles from external sources will not penetrate more than a few micrometers in depth, so their effects are limited to superficial layers of skin. Only internal emitters of alpha radiation will deliver any significant dose to muscle. These are heavy atoms and are not likely to be distributed to muscle tissue. Effects of beta and gamma radiation are indistinguishible. Acute effects of radiation on muscle tissue are not generally observable directly unless the dose is very large. Acute necrosis requires doses in excess of 500 Gy. Most of our information on human effects has come from study of patients undergoing radiation therapy for treatment of cancer. Early effects are creatinuria, or excretion of creatine in the urine, which indicates disturbed muscle metabolism, and fatigue or muscular weakness. Late effects, occurring months to years after exposure, may include atrophy of muscle. This is more prominent in children than adults. Impaired blood supply may result in fibrosis, or scarring. The magnitude of these effects is dependent on dose and on volume of tissue exposed. Exposure of an entire small muscle may be more damaging than that of a small part of a long muscle.  Significant muscle atrophy also requires massive doses--up to 500 Gy in animal experiments.

An old but still useful reference is Chapter 19 in Volume 2 of Clinical Radiation Pathology by Rubin and Casarett (Saunders 1968). S. Julian Gibbs, DDS, Ph.D.