[Mesoblastic nephroma--a malignant tumor with low grade of malignancy].

The mesodermally differentiated and only in newborns and young infants occurring mesoblastic nephroma is regarded as benign. Recidivations with malignant course, inclination to infiltration and richness in mitosis make arise doubts about the benignity despite good prognosis. The results of a proliferation-kinetic examination received by an autoradiographic in vitro method of a mesoblastic nephroma which was observed in a 20-day-old infant are characteristic for a malignant tumour of low degree of malignity: 3H-thymidine marking index 5.8%, DNA synthesis time 18.2 hours, mean generation time 141.1 hours (= 5.9 days). Growth fraction 0.28 and potential doubling time of the tumour 16.4 days. These data decisively deviate from those of the Wilms-tumours, which are cell-kinetically characterized as extraordinarily quickly growing, highly malignant tumours. The position of the mesoblastic nephroma in the system of the dysontogenetic renal tumours is briefly discussed. The ureteronephrectomy performed in time is the only necessary therapeutic measure, since also the mesoblastic nephroma follows the general rule of dysontogenetic tumours that the prognosis is the better the younger the infant is.