Experimental evaluation of the usefulness of 201Tl-chloride scintigraphy for monitoring radiotherapeutic effects.

The level of Na(+)-K+ adenosine triphosphatase (ATPase) has been demonstrated to be correlated with tumour growth potential, and accelerated active transport of K+ carried out by Na(+)-K+ ATPase is said to be a trigger of 201Tl affinity for malignant tumour cells. Therefore, 201Tl scintigraphy appears to be a good indicator for evaluation of changes of tumour proliferative potential after treatment. In the present study, the usefulness of 201Tl scintigraphy for monitoring radiotherapeutic effects was evaluated in VX-2 tumours irradiated with variable doses (10-40 Gy of radiation). Changes in 201Tl uptake were compared with tumour growth, and 201Tl uptake on day 7 after irradiation was compared with tumour bromodeoxyuridine (BrdU) uptake, which reflects DNA synthesis. All the treated tumours showed dose-dependent diminished 201Tl uptake, accompanied by either dose-dependent tumour growth delay or tumour regression/resolution. The diminished 201Tl uptake had already appeared on day 7 after irradiation, accompanied by diminished BrdU uptake, although, at this time, the tumour volumes were increased, or were not significantly decreased compared to pre-irradiation. Moreover, the 20 Gy tumours demonstrated two different tumour growth patterns, each accompanied by a different degree of reduction of 201Tl uptake. These findings suggest that the degree of reduction of tumour 201Tl uptake following irradiation can reflect the degree of suppressed proliferative activity in the treated tumours, and that assessment of altered 201Tl uptake at a relatively early time following irradiation allows prediction of subsequent tumour growth, regardless of the tumour volume.