Early detection of tumour response to radiotherapy using MRI

The response of tumours to radiotherapy is normally determined by measurement of volume or rate of growth. We have investigated the possible use of magnetic resonance imaging (MRI) to detect changes in the apparent diffusion coefficient (ADC) of tumour water following exposure to x-rays. Two experimental tumour models, a T50/80 mouse mammary tumour and a Dunning rat AT6 prostate tumour, both implanted subcutaneously in nude immune-suppressed mice. were chosen because they have markedly different ADC values prior to treatment. Statistically significant increases in ADC were observed, for both tumour types, within 2-3 days of a single, curative dose of x-rays. Using a more clinically relevant model of fractionated irradiation, both tumour types again showed significant ina eases in ADC prior to any reduction in tumour volume. Magnetization transfer contrast (MTC) measurements of the tumours showed a marked difference in the proportion of mobile, non-exchangeable water in the two tumour types, but this proportion was unchanged by irradiation. This suggests that the increase in ADC following irradiation is not due to an increase in extracellular water, but reflects a decrease in the barriers to diffusion, in particular, increased cell membrane permeability. Measurement of ADC by MRI appears to offer a valuable, non-invasive method for the early detection of the response of tumours to radiotherapy.