GSOR09 Presentation Time: 12:10 PM: Big Gains for Small Volumes: Dosimetric Benefits of Adding Supplemental Interstitial Needles for Small Volume Cervical Tumors

• Published in: Brachytherapy Vol. 23; no. 6; p. S77
• Main Authors: Niedermayr, Thomas, Ngo, Nicholas, Kidd, Elizabeth
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.11.2024
• ISSN: 1538-4721
• DOI: 10.1016/j.brachy.2024.08.107

Previous data has shown the benefit of supplementary interstitial needles for large volume tumors (>30cc) for increasing D90 to the high-risk clinical target volume (HR CTV) and improving local control without increasing dose or toxicity of adjacent normal tissues. At our institution, we have consistently been adding supplemental interstitial needles for all cervical brachytherapy cases since the end of 2019 to improve dose shaping, so we aimed to evaluate the dosimetric benefit of the interstitial needles for small volume cervix tumors. We identified 57 hybrid interstitial cases treated from November 2019 through November 2023 whose HR CTV was less than 30 cc at the time of first brachytherapy. For all cases the second fraction high dose rate (HDR) brachytherapy plans were then re-planned to use the intracavitary applicator only (tandem and ovoid/ring/cylinder) with the goal of optimizing coverage while maintaining similar minimum doses to the most exposed 2cc of bladder, rectum and bowel. Specifically, improvements in tumor coverage would be finalized when any of the OARs of the no-needle plan would reach the corresponding metric of the plan with needles. Tumor extent metrics at point A were recorded (lengths in the anterior, posterior, left and right directions) as well as the percentage of needle dwell times relative to the plan total for the with needle plans. The patient group included the following 2018 FIGO stage distribution: 7% stage I, 23% stage II, 56% stage III, and 14% stage IV with 75% squamous cell carcinoma, 14% adenocarcinoma, 2% adenosquamous, and 9% other unfavorable. The average tumor volume was 19.0 cc (range 4.3-29.8 cc). The HR-CTV V100 and D90 were 92.9%, vs. 80.1%, 7.32 Gy vs. 6.15 Gy for the clinical vs. no-needle plans, with a t-test p-value <10-9. Normal tissue metrics showed no statistically significant differences, with average rectum, bladder and bowel D2cc of 3.23 Gy, 4.95 Gy and 3.68 Gy respectively for the clinical plans. For the with needle plans, an average of 2.8 needles were used with an average 12.0% of needle contribution to the total dose. Combining the external beam and brachytherapy doses, the clinical plans achieved an average EQD2 dose > 90.1 Gy with needles vs. 80.9 Gy for the no-needle plans, to compare to the EMBRACE 2 recommended threshold of 90 Gy and 85 Gy limit. Even for volumes <20 cc (n=31) and <15 cc (n=15), the differences in HR-CTV V100 and D90 were statistically significantly improved with added needles, with p<10-6 and p<10-5. The D90 improvements in the clinical plans relative to the no-needle plans were most correlated to a higher needle contribution as well as to a larger anterior extent of the tumor at point A, but not strongly correlated with volume of HR CTV. These results suggest a dosimetric benefit of adding supplementary interstitial needles to intracavitary implants for small volume cervical tumors. Adding interstitial needles allows for enhanced dose shaping to improve HR CTV coverage (Figure) while maintaining normal tissue sparing.