Detection of 90Y extravasation by bremsstrahlung imaging for patients undergoing 90Y-ibritumomab tiuxetan therapy

Extravasation of therapeutic (90)Y-ibritumomab tiuxetan can cause significant injury. Detection of extravasated (90)Y using a γ-camera for patients undergoing (90)Y-ibritumomab tiuxetan therapy is a challenge because of the inherently low efficiency of bremsstrahlung imaging and the interference of prompt and scattered photons from (111)In that are still present in the body at the time of (90)Y injection. We have configured a γ-camera to image bremsstrahlung radiation from superficial (90)Y in the presence of (111)In and evaluated the effectiveness using phantoms.

Phantoms were constructed to contain (90)Y and (111)In with activity levels and with a geometry approximating conditions in a patient being scanned for evaluation of possible extravasation in the antecubital fossa. Imaging was performed using a camera equipped with medium-energy general-purpose (MEGP) and high-energy general-purpose (HEGP) collimators.

The contrast that developed between the patch representing extravasated solution and the background was comparable for MEGP and HEGP collimators. With MEGP collimators and 5-min acquisitions, a patch containing 8.3 MBq (220 μCi) distributed over an elliptic area of 7 × 11 cm was clearly discernable.

With our experimental arrangement, the lower limit of detection is approximately 8 MBq. We calculate that an extravasation of this much (90)Y would result in an absorbed dose to the skin and subcutaneous tissue of 2.5 Gy, which is close to the threshold for skin damage. This technique is therefore sensitive enough to be of use in the clinic when extravasation of (90)Y is suspected.