Impact of radiotherapy on normal brain tissue: Semi-automated quantification of decrease in perfusion

Purpose: We attempted to ascertain the impact of Co-60 conventional externa l radiotherapy (cRT) on the perfusion of normal brain tissue in relation to the radiation doses delivered to the tumors in patients with primary brain tumors. Materials and Methods: After surgery 18 patients (pts) were due to undergo cRT with a total dose of 5400 - 6400 cGy. All the patients had a Tc-99m-HMP AO SPECT study prior to cRT (basal), 15th and 30th days of cRT as well as 1 (in 6 pts), 3 (in 9 pts), and 6 (in 3 pts) months after cRT. For quantitat ive evaluation, the entire set of transverse slices were divided into 4 reg ions as frontal, parietal, occipital and temporal regions by means of a com puter software program. Semi-automated quantification was performed on a to tal of 1392 regions in 87 studies to determine left to right ratios. An int erregional difference of at least 10% was considered abnormal. Results: After elimination of tumor sites, 80 normal brain regions showed d ecreased perfusion after cRT. The percent decrease in perfusion was (mean 2 2.5 +/- 9.9) significantly higher in areas irradiated with doses > 3000 cGy (p < 0.05). Conclusion: cRT has adverse effects on the perfusion of normal brain tissue for doses > 500 cGy. Our findings justify treating patients with small and limited lesions with stereotactic radiotherapy in order to minimize the ad verse effects of cRT on normal tissues.