Integrated optimization of needle paths and dwell time for individualized template-guided interstitial brachytherapy

3D-printed individualized template (3D-PIT) guided interstitial brachytherapy (ISBT) is an effective treatment modality for cervical cancer. However, the current practice of pre-operative planning requires manual needle paths design, which is highly dependent on operator experience and may lead to unnecessary insertion trauma.

To address this issue, this study introduces and validates an automated integrated optimization method for needle paths and dwell times for an institutional protocol of 3D-PIT based ISBT for locally recurrent cervical cancer.

To automatically generate a treatment plan, a candidate needle path set was first generated based on patient's anatomy and the transvaginal template. Then, a two-layer optimization strategy was introduced for automated integrated optimization of the needle paths and dwell times, with the outer layer employing simulated annealing to optimize the needle paths, and the inner layer to optimize the dwell times. A total of 40 patient cases (with the prescribed dose to HR-CTV D_90% being 6 Gy (physical dose) per fraction, and 2-8 fractions per case, totaling 219 independent fraction plans) with locally recurrent cervical cancer previously treated with 3D-PIT-assisted ISBT were enrolled in this study. Automatic plans for all fractions were generated and evaluated using the same number of needles as in the clinical plans, with the dosimetric results of the clinical plans used as constraints. In addition, the quality of automatic plans using fewer needles was also investigated.

The method generated clinically acceptable plans in 1.5 ± 1.2 min, requiring 145 ± 74 iterations, according to institutional protocols based on the EMBRACE II. In terms of dosimetric quality, the automatic plans met the constraints and generally outperformed the clinical plans in protecting organs-at-risk (OARs). When normalizing both plans to the same HR-CTV D_90% and comparing the dosimetric outcomes of OARs, the D_2cc of rectum, bladder, colon, and small intestine were reduced from 363 to 350 cGy, 397 to 376 cGy, 226 to 204 cGy, and 140 to 123 cGy, respectively (all p < 0.001). The D_0.1cc of urethra was reduced from 406 to 351 cGy (p < 0.001). V_100% of HR-CTV remained the same at 78% (p = 0.28), and V_150% of HR-CTV was increased from 42% to 44% (p < 0.001). Total dwell time decreased from 278 to 271 seconds (p < 0.001). In addition, plans with fewer needles generally had higher doses to OARs.

The automated integrated optimization method provides a fast, standardized pre-planning tool for 3D-PIT-assisted ISBT. By reducing operator dependence, it serves as a valuable clinical baseline to help determine optimal needle configurations and maintain consistent dosimetric quality.