(Grant Steen, 1989; Vaupel et al., 1989). Tumour cells are able to tolerate acidic pHe by using their buffering ability, and by activating ion exchange mechanisms which are present in the cell membrane, to maintain pHi close to the physiological range. Important membrane exchangers which regulate pHi in the acidic range are the Na+/H+ antiport and

Forty-one patients with Stage III and IV squamous cell carcinomas (SCC) of the head and neck were treated preoperatively with mitomycin C and 5-fluorouracil and concomitant radiotherapy. Operation specimen were examined histologically, the percentage of vital tumor cells and devitalized tumor cells were graduated. The grade of regression was classified according to a four-stage scale. Tumor regression was elevated as good (Grades 1, 2) and bad (3, 4) response to combined preoperative therapy. After a follow-up of 18 to 30 months, 14 of 41 patients have experienced a locoregional recurrence; all these patients were bad responders (Grades 3, 4) to preoperative radiochemotherapy. There was a statistically significant correlation between tumor regression grade and probability of survival (P less than 0.001). The authors conclude that the prognosis of patients with pretreated SCC of head and neck depends on the histologic grade of tumor regression.

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