KEY POINTS
- This technical study evaluated five commercial lung-equivalent inserts manufactured in 2010, 2015, and 2024 using mass measurements, computed tomography numbers, stoichiometric analysis, and recalculation of three centrally located lung stereotactic body radiotherapy plans.
- The two 2010 inserts had approximately 17.1% lower mass than the 2024 reference insert despite dimensional differences of no more than 0.2 mm. Their theoretical densities were 0.141-0.150 g/cm³, versus nominal values near 0.2 g/cm³.
- Plans were created using a nonconforming Hounsfield unit look-up table and recalculated with a reference table while preserving monitor units and beam parameters. Treatment used volumetric modulated arc therapy with 60 Gy in 8 fractions and Acuros XB.
- In the worst-case inspiration breath-hold plan, recalculation reduced gross tumor volume mean dose by 2.1% and planning target volume D95% by 3.0%, from 60.0 Gy to 58.2 Gy. Planning target volume V100% decreased from 95.0% to 89.3%.
- Global gamma passing rates remained high despite the target underdose: 99.3% at 2%/2 mm and 96.9% at 2%/1 mm in the worst-case plan, showing that conventional gamma analysis may mask localized calibration errors.
CLINICAL TAKEAWAY
Commercial tissue-equivalent inserts should not be accepted solely on nominal manufacturer specifications, because hidden density variation can propagate through the Hounsfield unit look-up table and exceed dose-calculation tolerances. Initial mass screening, independent density verification, longitudinal computed tomography monitoring, and external calibration audits are warranted, although the study examined only five inserts and three clinical cases.