Reirradiation tolerance of the human brain

To give an overview of current available clinical data on reirradiation of glioma with respect to the tolerance dose of normal brain tissue.

Clinical brain reirradiation studies from January 1996 to December 2006 were considered on radiation-induced late adverse effects-i.e., brain tissue necrosis. The studies were analyzed by using the linear quadratic model to derive information on the cumulative biologic effective tolerance dose (BED(cumulative)) and equivalent doses in 2-Gy fractions (normalized total doses, NTD(cumulative)) for the healthy human brain.

The NTD(cumulative) in conventional reirradiation series (NTD(cumulative) of 81.6-101.9 Gy) were generally lower than in fractionated stereotactic radiotherapy (FSRT) (NTD(cumulative) of 90-133.9 Gy.) or LINAC-based stereotactic radiosurgery series (NTD(cumulative) of 111.6-137.2 Gy). No correlation between the time interval between the initial and reirradiation course and the incidence of radionecrosis was noted. The analysis showed the prescribed NTD(cumulative) to increase with decreasing treatment volume, which is allowed by modern conformal radiation techniques.

Radiation-induced normal brain tissue necrosis is found to occur at NTD(cumulative) >100 Gy. The applied reirradiation dose and NTD(cumulative) increases with a change in irradiation technique from conventional to radiosurgery re-treatment, without increasing the probability of normal brain necrosis. Taken together, modern conformal treatment options, because of their limited volume of normal brain tissue exposure, allow brain reirradiation for palliative treatment of recurrent high grade glioma with an acceptable probability of radionecrosis.