Transfer rate index as a new parameter of [ 201 Tl]-SPECT in brain tumors

Introduction

201Tl-SPECT has been useful for imaging of brain tumors, especially for malignant gliomas and metastatic brain tumors. It is known that 201Tl is taken up by tumor cells through Na-K ATPase and Na-K-2Cl-cotransport with the biologic behavior of 201Tl as a potassium analog. A new parameter of 201Tl-SPECT, transfer rate index, reflecting the unidirectional transfer into normal brain or tumor tissue is presented to evaluate intra tissue uptake of 201Tl.

Patients and methods

Twenty-one patients with intracranial lesions (10 malignant gliomas, 6 patients with 8 metastatic brain tumors, 2 meningiomas, 2 brain abscesses and 1 malignant lymphoma) received a dose of 111MBq of 201TlCl i. v. as a bolus. SPECT was initiated 10 min (early) and 3 h (delayed) after the injection using a three-head gamma camera (GCA-9300A/DI, TOSHIBA, Japan). The mean counts in ROIs placed over lesions (Ct), contralateral normal brain (Cn) and sagittal sinus (Ca) were measured from early and delayed SPECT images respectively. The Ct/Ca or Cn/Ca was assumed as the volume of distribution (DV) in lesions or normal brain. A time (è) reflecting exposure time was calculated from Ca and median scanning time (t1; early, t2; delayed) of each SPECT. The slope of relationship between DV and è measured from the two points of early and delayed SPECT was defined as the transfer rate index in lesions (Kt) and normal brain (Kn). The transfer rate index ratio was applied as Kt/Kn. Retention index was also calculated as [Ct/Cn of early SPECT]/ [Ct/Cn of delayed SPECT] ratio.

Results

Transfer rate index in normal brain (Kn) was 0. 14±0.02 (n=21) and tended to decrease with age. Transfer rate index in the lesions (Kt), transfer rate index ratio (Kt/Kn) and retention index in various lesions are shown in Table 1. Kt/Kn in benign lesion (meningioma and brain abscess) was under 1.0. Kt, Kt/Kn and retention index in malignant lesions (malignant gliomas, metastatic brain tumors and malignant lymphoma) were higher than those in benign lesion. Kt and Kt/Kn were highly sensitive especially in malignant gliomas.

Table 1

Transfer rate index in the lesions (Kt), Transfer rate index ratio (Kt/Kn) and Retention index in various lesions

	Kt	Kt/Kn	Retention index
Malignant glioma (n=10)	0.39±0.12	2.90±1.00	0.81±0.15
Metastasis (n=8)	0.17±0.08	1.09±0.49	0.60±0.11
Meningioma (n=2)	0.13	0.95	0.48
	−0.67	−6.51	0.39
Asscess (n=2)	0.10	0.76	0.65
	0.02	0.18	0.36
Malignant lymphoma (n=1)	0.17	1.91	0.80

Mean ± S.D.

Comments

The present study has provided a new parameter in 201Tl SPECT, transfer rate index, for evaluation of intra tissue uptake of 201Tl. The transfer rate index could be a good indicator for differentiating brain tumor and malignancy grading of the tumor.