Tissue Tolerance: Central Nervous System

Abstract

IN 1958 LAMPE (13) surveyed the experience to date with radiation tolerance of the central nervous system. He concluded that, as of that time, precise radiation tolerance limits of the central nervous system were not known for clinical radio-therapeutic practice. This still holds true today. We do, however, have known ranges of tolerance which have been developed from previous experience and some knowledge of factors that may modify this tolerance in clinical situations. Radiation tolerance of the peripheral nerves, brain, and spinal cord will be discussed in this communication. Peripheral Nerves Although technically the peripheral nerves are not usually included in discussions of central nervous system tolerance, some discussion seems appropriate. Peripheral nerves are usually considered to be resistant to radiation damage at clinical levels, and concern for them rarely if ever modifies our radiotherapeutic approach. Both functional and morphological changes in peripheral nerves have been demonstrated experimentally in animals, largely with doses of radiation above the clinical level (15). There are at least two reports of damage occurring in clinical practice (16, 20). In both, megavoltage radiation was being used (4-25 MeV). Dose-time relationships included 5,100–5,700 rads in 11 or 12 increments over twenty-five to twenty-eight days (3 × weekly) and 2,000 rads in one session. Time of onset was five to thirty months, with a peak at ten to twenty-two months. Although histological sections will show demyelinization, disagreement persists (as is true also with brain and spinal cord) as to the relative role of direct damage of nerve tissue versus vascular damage. A further complicating factor here is one of mechanical pressure produced by significant local fibrosis and/ or edema, alteration of which has altered neurological findings. Brain Two basic clinical situations exist-where the disease being treated is (a) intrinsic or (b) extrinsic to the nervous system. Many examples of the latter have occurred with treatment of skin lesions with alteration of time-dose-fraction relationships (classical examples are 2,000– 6,000 R in 1 to 10 fractions over a one- to twelve-day period resulting in a well demarcated underlying area of cortical necrosis) (13,18). Where the underlying disease is of neural origin, fractionation and protraction are more likely to be conventional. Lampe's medulloblastoma cases demonstrate that a relatively low dose in a short period (2,500 R in eight days—6 fractions) is sufficient (13). Evaluation is frequently difficult because the signs and symptoms of iatrogenic damage may be the same as those of recurrent neoplasm. Many cases attributed to radiation damage may actually be recurrent neoplasm. The reverse may also be true, or a combination of both factors may be operating.