Syed‑Neblett Template–Based GYN Interstitial Brachytherapy HR-CTV Comparison between Preplan and Treatment‑Day Plan Contouring
Abstract
Purpose
In Syed interstitial gynecologic (GYN) brachytherapy, preplanning is often used to guide applicator placement and anticipate target coverage. However, the accuracy of preplan‑based contouring relative to treatment‑day planning remains uncertain, particularly given anatomical changes after applicator insertion. This study evaluates the similarity between preplan and final plan contours for high‑risk clinical target volume (HR‑CTV) in patients undergoing Syed GYN brachytherapy.
Methods
A retrospective analysis was performed on 7 patients treated with Syed‑Neblett template interstitial Ir-192 brachytherapy with Oncentra planning system. For each fraction, HR‑CTV was contoured on the preplan CT with MRI fusion and again on the treatment‑day CT after applicator insertion with a new MRI. All sets of images were sent to MIM software for similarity study. Similarity metrics—including Dice Similarity Coefficient (DSC), Hausdorff distance (HD), and percent volume difference—were calculated to quantify agreement between preplan and plan contours.
Results
Preplan and plan contours demonstrated moderate geometric agreement. HR‑CTV showed the highest similarity, with DSC values indicating that preplan contours generally approximated final target shape but underestimated volume in cases with significant applicator‑induced deformation or tumor response with external beam treatments. The dates between preplan and treatment range from 11 to 33 days. The DSC values 0.51 ± 0.38 (mean ± standard deviation) (range of 0.18–0.78), and HD of 14.84 mm ± 8.07 (range of 4.21– 31.02). The average HR-CTV volume for preplan is 12.31+15.15 c.c., for treatment plan day is 15.72+18.60 c.c.
Conclusion
Preplanning provides a useful structural reference for Syed GYN brachytherapy, but notable differences exist between preplan and treatment‑day contours. Relying on preplan contours alone may lead to suboptimal dosimetry. These findings support the continued need for individualized, treatment‑day imaging and contouring to ensure accurate target delineation and safe OAR sparing in interstitial GYN brachytherapy.