An evaluation of near infrared spectroscopy and cryospectrophotometry estimates of haemoglobin oxygen saturation in a rodent mammary tumour model

Haemoglobin oxygen saturation in subcutaneous rat mammary tumours was measu red using near infrared spectroscopy (NIRS) in vivo and in rapidly frozen s ections from the same tumours using cryospectrophotometry, which reports ox ygen saturation in individual blood vessels to depths of 4 mm from the tiss ue surface. Measurements were performed on tumours while animals breathed e ither room air or carbogen. In five of nine tumours, the average saturation calculated From cryospectrophotometric measurements agreed with that deter mined from NIRS to within 13%, and in Four of these five tumours agreement was 5% or better. In the remaining four of nine tumours, where agreement wa s poor, the volume-averaged saturations estimated from NIPS were consistent ly higher than those calculated From cryospectrophotometry. Monte Carlo sim ulations demonstrated that the depth of tissue probed by NIRS was significa ntly greater than that sampled by cryospectrophotometry. Analysis of the fr equency of severely hypoxic vessels showed that when NIRS reported a satura tion of approximately 70% or higher, the fraction of tumour vessels with sa turations less than 10% was limited to 0.06 or less. Sensitivity and specif icity analysis suggests that NIRS and NIRS imaging may identify clinically relevant hypoxia, even when its spatial extent is below the resolution limi t of the NIRS technique.