Isocenter Optimization in WBRT: Concurrent Sparing of Lens and Lacrimal Gland via Anterior Penumbra Sharpening

With improving survival for patients receiving whole-brain radiotherapy (WBRT), mitigating long-term toxicities like cataract and dry eye syndrome has become increasingly critical. The lens and lacrimal gland are highly radiosensitive and lie in close proximity to the target volume, posing a persistent challenge for achieving sharp dose gradients. While modern techniques like multileaf collimator (MLC) shaping offer some protection, the potential of classic geometric optimization principles, such as anterior beam shift, remains underexplored in contemporary treatment planning workflows.

This study aimed to systematically translate, quantitatively validate, and integrate the classic geometric principle of anterior isocenter shift into the modern treatment planning workflow for the dual protection of the lens and lacrimal gland in WBRT, utilizing 3D-conformal radiotherapy (3D-CRT) and field-in-field (FIF) techniques.

For 40 patients, conventional and isocenter-optimized plans (involving an anterior shift of the isocenter within the PTV) were generated for both 3D-CRT and FIF. We compared dosimetric parameters for the planning target volume (PTV), lenses, lacrimal glands, and other organs at risk. Plan quality, normal tissue complication probability (NTCP) for cataract and dry eye syndrome, and clinical risk stratification were evaluated RESULTS: Isocenter optimization significantly reduced the median lens Dmax by 20% and PRV_lens D0.03 cm3 by 23% (p < 0.001). Lacrimal gland Dmean, V6Gy, and V10Gy were also significantly reduced. The strategy physically improved the dose gradient, narrowing the penumbra width by 31.25% and increasing the dose fall-off rate by 15%, while reducing low-dose irradiation volumes outside the PTV. These dosimetric benefits translated into meaningful reductions in projected NTCP for both complications. The 3D-FIF plans maintained target coverage and homogeneity, mitigating the heterogeneity increase observed in 3D-CRT plans.

Anterior isocenter optimization is a practical, and hardware-free technique that seamlessly synergizes with modern MLC-based planning to provide significant, concurrent sparing of the lens and lacrimal gland in WBRT. As a readily implementable modification within existing planning systems, this strategy can be adopted immediately to enhance treatment safety without requiring additional resources.