Exposing the Historical Illusion: The Minimal Clinical Significance of Lung Shunt Fraction In 90y-SIRT
Abstract
Purpose
LSF derived from MAA-based nuclear medicine imaging is a standard component of 90Y-SIRT treatment-planning. Elimination of MAA-based LSF determination has been suggested in select cases to streamline 90Y-SIRT workflows and avoid the requirement of separate planning and therapy procedures. This work proposes and evaluates a pre-treatment identification method for 90Y-SIRT cases where patient-specific LSF is clinically beneficial.
Methods
Both planar (LSFplanar) and MAA-SPECT/CT LSF (LSFSPECT) were analyzed retrospectively in glass 90Y-SIRT cases from September 2022–June 2025 at a single institution. A new metric (LSFbound) was defined as the minimum LSF value where the maximum achievable perfused volume dose (PVmax) becomes determined by a selected lung dose threshold (Lungsmax) instead of a designated whole-liver dose threshold (Livermax). LSFbound values computed using both clinical and simulated treatment-planning parameters were quantitatively evaluated relative to clinical LSF. Probability and ROC analyses were used to assess the prospective prediction capabilities of the LSFbound metric.
Results
A total of 354 cases were analyzed from 297 patients. LSFbound depends only on lung mass, Lungsmax, PV size (PVvol) and PVmax. Using observed clinical LSF distributions, the median (max) probability for LSFSPECT to exceed LSFbound was ≤1% (≤4%) for HCC ≤8 cm and non-HCC cases without macrovascular invasion (87% of all cases). ROC analysis showed that a prospective use of LSFbound could maintain perfect sensitivity and still provide specificities >70% to predict LSFSPECT > LSFbound at PVmax values up to 1000 Gy. LSFbound predictions using LSFplanar were limited by high probability thresholds (>20%) required to concurrently avoid false negatives and a high false positive rate.
Conclusion
Patient-specific, MAA-based LSF determination may be precluded in most 90Y-SIRT cases as LSF and Lungsmax play no role in limiting the achievable PV dose. Pre-treatment calculation of LSFbound provides individualized, quantitative guidance for identifying when MAA-based, patient-specific LSF assessment is warranted.