Chorioretinal temperature monitoring during transpupillary thermotherapy for choroidal neovascularisation

Abstract

Aims: To investigate the difference in temperature rise between normal choroid and choroidal revascularisation (CNV) during transpupillary thermotherapy (TTT) and the relation between laser spot size and power in the rat fundus.

Methods: A modified slit lamp, which was installed with two laser wavelengths (490 nm for illumination and fluorescein excitation and 810 nm for hyperthermia), was developed for TTT and temperature monitoring. Temperature rise during TTT was monitored by observing fluorescence released from thermosensitive liposomes encapsulating carboxyfluorescein. Two types of liposomes were prepared; their phase transition temperatures were 40 degrees C and 46 degrees C, respectively. Laser power settings required to observe fluorescence released from 46 degrees C liposome in normal choroid or CNV were compared. Next, the power settings with 0.5 mm and 0.25 mm spot sizes were compared following administration of 40 degrees C liposome or 46 degrees C liposome.

Results: The minimum power values when release from 46 degrees C liposome was observed showed a significant difference in distribution of power values between normal choroid and CNV. CNV required significantly higher power than normal choroid. With 40 degrees C liposome, the power was 9.7 (1.9) mW (mean (SD)) at a spot size of 0.25 mm, and 12.1 (1.6) mW at 0.5 mm, respectively. When using 46 degrees C liposome, the power setting was 10.2 (1.2) mW at a spot size of 0.25 mm, and 14.6 (2.2) mW at 0.5 mm, respectively.

Conclusions: CNV demonstrated varying heat conduction, compared with normal choroid. Laser power required to raise the temperature should not necessarily be doubled, even when the spot size is doubled. Close attention should be given to the selection of power settings when performing TTT for CNV.

Figure 1

Principle of liposomal temperature monitoring (LTM). (A) Immediately after injection of CF liposome, an illuminating argon laser was delivered to the rat fundus. (B) TTT started. No release of fluorescent bolus was observed, when the temperature increase was below Tc in the heated lesion. (C) The TTT power setting was raised to an adequate value. When the temperature in the lesion was above the Tc of the injected liposome, the emitted fluorescent bolus was observed in and around the heated lesion.

Figure 2

Temperature profiles of CF liposome. Note that CF release rate significantly increased at Tc of each liposome. Squares; 40°C liposome, diamonds, 46°C liposome.

Figure 3

LTM applied to the normal choroid. (A) Background fluorescence. Immediately after injection of CF liposome, an illuminating laser slightly delineated major retinal vessels. However, no fluorescence leaked in the retinal vasculature. Arrow: a weak aiming laser at 810 nm. (B) TTT started after injection of CF liposome. No release of fluorescent bolus was observed, because the temperature rise was inadequate in the heated lesion. (C) When the temperature ruse was above the Tc of the injected liposome, patchy fluorescence around heated lesion was observed.

Figure 4

The minimum power values when release of CF from 46°C liposome was observed. Note the significant difference in distribution of power values between normal choroid and CNV (#: p<0.00001). CNV required significantly higher power than normal choroid (*p<0.05).

Figure 5

The power values at release of CF from 40°C liposome (triangles) and 46°C liposome (squares) was observed (*p<0.05).