Stem-cell depletion and grid therapy
The stem-cell depletion hypothesis postulates that there exists for each organ a limiting critical volume, which can be repopulated by a single surviving stem cell and for which damage can be repaired by repopulation (Yaes & Kalend, 1988; Yaes et al, 1988). When the critical volume is totally depleted of stem cells, irreversible damage occurs. Certain organs such as the spinal cord yield very interesting experimental results, where the frequency of transverse myelitis has varied with the minimal critical length of cord irradiated (Hopewell et al, 1987; van der Kogel, 1987; Hopewell & van der Kogel, 1988). The critical length of spinal cord has been determined to be approximately 2–5 mm in WAG/Rij rats and <4mm in Sprague-Dawley rats. Both sets of data indicated a critical length of spinal cord that must be irradiated to cause myelitis and that lies between 2 and 5 mm for the rat. Similar predictions have been made using a model based on functional subunits (Withers & Taylor, 1988; Withers et al, 1988).