Objective To assess whether the response visible on computed tomography (CT) 21 days after the ®rst course of chemotherapy in patients with nonseminomatous germ cell tumour predicts the need for surgery and whether three-dimensional (3D) reconstruction adds to the diagnostic accuracy. Patients and methods CT scans from 52 patients treated with cisplatin-based chemotherapy were assessed for tumour shrinkage by measuring the changes of a onedimensional (1D) measurement of the maximum transverse diameter, and comparing CT scans before, 21 days after the ®rst course and at the end of chemotherapy (1D method). In a subset of patients, using a special formula, the 1D-derived 2D and 3D shrinkage (2D der and 3D der ) were compared with four other computed or calculated methods (1D, 2D, 3D calc , 3D comp ).Results At day 21, in 32 of 52 patients (62%) there was <50% tumour shrinkage using the 1D assessment; 21 of them (66%) needed surgery, compared with none of the 20 patients with >50% tumour shrinkage by day 21 (x 2 =22.83, P<0.001). The 1D method showed signi®cantly less shrinkage than all the other methods but when this was used to derive a 3D shrinkage, assuming the mass to be spherical (3D der ), it was not statistically different from that of 3D comp . Conclusions The assessment of the response from 1D CT scan estimates 21 days after initiating chemotherapy identi®es a subgroup of patients who have a high probability of needing surgery. Although expected to be more accurate, the 3D comp estimate of tumour shrinkage was no better than the 3D der estimate.

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