2993 - A TRAK-Normalized Reference Model for Rapid Consistency Assessment of HDR Brachytherapy Plans

Purpose/Objective(s): To develop a simple, clinical metric for rapid consistency checking of high-dose-rate (HDR) brachytherapy plans by relating Total Reference Air Kerma (TRAK, cGy·cm²) to prescription isodose volume (V_100%), enabling the use of a nomogram to assess the efficiency of dose optimization and compare with prior clinical experience to identify outlier conditions. Comparing a clinical plan against this reference curve enables a rapid consistency check of the geometry and dwell time efficiency.

Materials/Methods: Reference data points were generated using a single dwell position to establish a relationship between TRAK and a resulting prescription isodose volume, V_100%. For a given TRAK normalized to prescription dose, each isodose volume at 5Gy, 6Gy, 7Gy, and 8Gy were recorded. These data represent the minimally optimized lower boundary reference curves for isodose volume per unit delivered activity. A cohort of 27 clinical HDR intracavitary (n=9) and vaginal cylinder (n=18) plans was analyzed based on these regions. For each plan, TRAK/D_100% and the resulting V_100% was compared to the reference model.

Results: The single-dwell dataset established a bounding curve and relationship between dose-normalized-TRAK and V_100%, with the typical anisotropic dose distribution defining a clear minimum compared to patient plans. The data fit very well to a power function:

V_100% = 4.4471 * (TRAK/D_100%) ^1.5351 (R² = 0.999)

Clinical plans consistently produced higher V_100% values for a given TRAK/D_100%, reflecting increased geometric efficiency from distributed dwell positions. Plans demonstrated a mean efficiency ratio of 1.11 (range 1.03–1.28), with the majority clustering between 1.07 and 1.15, and outlier plans were readily identifiable. No plans fell below the theoretical boundary, supporting the validity of the limit.

Conclusion: The TRAK-normalized reference curve derived from a single-dwell position provides a practical lower bound for an optimized HDR brachytherapy plan. The exponent of the function is consistent with a relationship between an inverse square point source (1/r²) and the volumetric isodose (r³), yielding r^3/2 or r^1.5

This check is not intended to evaluate target coverage or organ-at-risk constraints, and an outlier is likely a sign of clinical constraints heavily influencing the overall geometry. Many plans require a careful balance of optimization to spare critical structures while adequately treating the physician-directed target. It is a rapid and lightweight quality assurance tool to identify outlier plans and assess whether dwell time is being utilized in a manner consistent with prior institutional experience, and may reduce the risk of gross planning inconsistencies prior to physician review.