942 POSTER Superficial dosimetry for helical tomotherapy

To determine whether perioperative high-dose-rate brachytherapy (PHDRB) implants with larger high-dose regions produce increased locoregional control.Patients (n = 166) enrolled in several PHDRB prospective studies conducted at the University of Navarre were analyzed. The PHDRB was given to total doses of 16 Gy/4 b.i.d. or 24 Gy/6 b.i.d. treatments for negative or close/positive margins along with 45 Gy/25 Rx of external beam radiation therapy. The histogram-based generalized equivalent uniform dose (EUD) formulism was used to quantify and standardize the dose–volume histogram into 2-Gy equivalents. The region of interest analyzed included: tissue volume encompassed by the prescription isodose of 4 Gy (TV100). Routine dose reporting parameters such as physical dose and single-point 2-Gy equivalent dose were used for reference.After a median followup of 7.4 years (range, 3–12+), 50 patients have failed, and 116 remain controlled at last followup. Overall, EUD was not different in the patients who failed compared with controls (89.1 Gy vs. 86.5 Gy; p = not significant). When patients were stratified by risk using the University of Navarre Predictive Model, very high-risk patients (i.e., tumors ≥3 cm resected with close <1 mm/positive margins) had an improved locoregional control with higher EUD values (p = 0.028). This effect was not observed in low-, intermediate-, and high-risk University of Navarre Predictive Model categories.In very high-risk patients, enlarged high-dose regions can produce a dose–response effect. Routine dose reporting methods such as physical dose and single-point 2-Gy equivalent dose may not show this effect, but it can be revealed by histogram-based EUD assessment.