Split-dose Y-90 microsphere therapy: Does a single TC-99m MAA mapping study suffice?
Vanderhoek M. Split-dose Y-90 microsphere therapy: Does a single TC-99m MAA mapping study suffice? Med Phys 2017; 44(6):2741.
Purpose: Treatment of hepatic malignancies with Y-90 microsphere therapy can require a “split-dose” where 2 liver sub-volumes are treated with separate vials of microspheres. Ideally, 2 separate pre-treatment Tc-99m-MAA mapping studies (1 per sub-volume) should be acquired. However, treatment planning dosimetry is typically based on a single mapping study of both subvolumes. We investigated the dosimetric uncertainties associated with a “split-dose” planned from a single Tc-99m-MAA mapping study. Methods: A standard partition model was used to plan treatment doses to liver subvolumes (VolA, VolB) and to the lungs, using a single Tc-99m-MAA mapping study. Dosimetry was determined for different mapping scenarios: equal Tc-99m-MAA activities (Scenario 1) vs. equal Tc-99m-MAA activity concentrations (MBq/mL of tissue, Scenario 2) administered to sub-volumes. Dosimetric uncertainties due to variation in lung shunting from the subvolumes were determined for cases with moderate (10%) and high (20%) lung shunting and for cases with equal (VolA = VolB) and unequal subvolumes (VolA/VolB = 2). Uncertainties were quantified via the range of possible doses to the sub-volumes (RangeA, RangeB) and to the lungs (RangeLung), expressed as a percentage of the planned dose. Results: With a single mapping study, dosimetric uncertainties stem from uncertainty on the source of lung shunting, as it may be from only one or both liver subvolumes. Cases with high lung shunting and unequal sub-volumes resulted in substantial uncertainties on sub-volume doses (Scenario 1: RangeA = 50%, RangeB = 50%; Scenario 2: RangeA = 37%, RangeB = 75%). In these cases, there was substantial uncertainty on lung dose for Scenario 1 (RangeLung = 66%) but no uncertainty for Scenario 2 (RangeLung = 0%). Overall, dosimetric uncertainties were reduced for cases with moderate lung shunting and/or equal sub-volumes. Conclusion: Substantial dosimetric uncertainties can arise when “split-dose” Y-90 therapy is planned based upon a single Tc-99m-MAA mapping study. Dual mapping studies may be warranted in cases with high lung shunting and/or unequal liver sub-volumes.