Correspondence

The application of the linear-quadratic dose-effect equation to fractionated and protracted radiotherapy

The linear-quadratic (LQ) dose-effect formalism is currently providing new perspectives into the ways in which alterations in the dose per fraction in conventional radiotherapy may be used to bring about improved results with respect to early or late normal tissue reactions. In this paper, using a model initially developed by Roesch, the LQ equations are explored further in terms of dose-rate rather than dose. By the incorporation of one other parameter, mu, which relates to the rate of repair of sub-lethal radiation damage, a more general formalism is obtained. In particular, equations are derived which can be used to examine the relative effectiveness of different treatment regimes, including those involving decaying sources. Such equations are of wider applicability than other LQ derivations which relate only to dose-response relationships. The extended equations, which are fully consistent with the existing LQ method, are also shown to lead directly to other independently established, relationships for protracted irradiation. The nature of the link between high and low dose-rate treatments is discussed, and some worked examples provide indications of how the new equations may be used to assess further the potential clinical benefits of low dose-rate treatments and permanent implants.

Treatment of experimental spinal cord compression caused by extradural neoplasms

Epidural spinal cord compression was produced in rats by injection of Walker 256 carcinoma cell suspension anterior to the T-12 or T-13 vertebral body. The tumor grows through the intervertebral foramina to compress the spinal cord and produce paraplegia in 3 to 4 weeks. The effect of several treatments upon clinical signs was assessed. Dexamethasone caused a significant but transient improvement in neurological function. Radiation therapy likewise improved neurological function, and was more effective when given by a high-dose protracted course than when given either in a single dose or a low-dose protracted course. Laminectomy was not helpful in relieving neurological symptoms. Dimethyl sulfoxide did not relieve neurological symptoms. Cyclophosphamide was most effective in relieving neurological symptoms, and most of the animals that were treated with that drug when they were severely weak but still able to move their hind limbs recovered fully. Some animals that were totally paraplegic when treatment began recovered function after radiation therapy or cyclophosphamide treatment, but recovery was better if treatment was started when animals could still move their hind limbs. This animal model appears to be a useful way of studying the treatment of human spinal cord compression produced by epidural neoplasms.

The influence of dose fractionation on acute and late reactions in patients with postoperative rasiotherapy for carcinoma of the breast

The clinical courses of 418 patients with breast carcinoma who received postradical-mastectomy radiation therapy to the internal mammary and supraclavicular are were revied in order to determine the normal tissue tolerance with various time-dose fractionation radiation regimens. Four different dose fractionation schedules wereemplyed during the period 1958-1968 at Memorial Hospital: 4900 rads/10f/32d; 4410 rads/9f/30d; 5000 rads/13f/18d; and 4600 rads/20f/28d. There was no difference in the acute reaction or local recurrence rate of the tumor in the irradiated area among the four fractionation schemes, but the late effects were quite different. Patients who received 4900 rads in 10 fractions in 32 days had a 33% incidence of subcutaneous fibrosis.A similiar dose in 13 fractions in 21/2-3 weeks resulted in about 17% subcutaneous fibrosis. No significance of these clinical findings is discussed in relation to the various formulae currently being used to express time-dose equivalence.

Comparison of skin erythemas under two different fractionation schemes

This paper reports the results of a prospective clinical trial comparing 200 rads daily five times weekly for four weeks with 400 rads twice weekly for four weeks. Total dose in the first case was 4,000 rads and in the second 3,200 rads. Thirty patients threated palliatively for bronchial carcinoma are involved in this report. Treatment was given at 220 kV and the intensity of skin erythema at the end of the treatment was the measure of comparison. Generally 200 rads daily five times weekly for four weeks produced more intense erythema than the other scheme used.