GSOR4 Presentation Time: 9:15 AM: Predicting the Benefit of Hybrid Intracavitary + Interstitial Needle Applicators in Patients with Locally Advanced Cervical Cancer

Abstract

<h3>Purpose</h3> The purpose of this study is to determine whether pre-treatment and brachytherapy (BRT) characteristics are associated with benefit from hybrid intracavitary + interstitial needle (IC/IS) applicator, as opposed to IC applicator, use in patients with locally advanced cervical cancer (LACC). <h3>Methods</h3> 53 LACC patients treated with IC/IS BRT from August 2014 to March 2020 using HDR were included. Patient variables included pre-treatment characteristics (age, histology, grade, T Stage, N Stage, max lateral, A-P, S-I, and overall tumor dimensions, tumor symmetry, max distance from endometrial canal, tumor volume, tumor max SUV on PET) and BRT characteristics (max lateral, A-P, S-I, and overall tumor dimensions, tumor symmetry, max distance from tandem, GTV and high-risk CTV (CTV HR) volumes). For each patient, an IC plan prescribed to match the IC/IS plan (CTV HR) coverage and optimized based on volumes was created. In addition, a standard loading IC plan prescribed to point A was also created. These plans were compared to the IC/IS plan. Benefit for IC/IS was defined by organ at risk (OAR) D2cc's transitioning from unacceptable to acceptable based on American Brachytherapy Society (ABS) guidelines. Patients were then placed into 3 categories: those having 0, 1, or 2 or more organs that benefited from IC/IS. Wilcoxon rank sum and Fisher's exact tests, proportional odds models, and linear regression models were used to determine which characteristics were associated with benefit from IC/IS. <h3>Results</h3> Analysis showed that 33 (62%) patients showed benefit from IC/IS compared to optimized IC plans. IC/IS resulted in an average decrease in EQD2 to normal structures of 11.4 Gy (bladder), 6.2 Gy (rectum), 5.7 Gy (sigmoid), and 4.4 Gy (small bowel). No pre-treatment characteristics were associated with an increase in the number of organs benefiting from IC/IS. Increasing BRT max S-I and overall dimension as well as GTV and CTV HR volumes were associated with increased organ sparing with IC/IS. The bladder had the greatest reduction in dose from IC/IS with greater than 10 Gy benefit with S-I extension or max tumor dimension greater than 5 cm, GTV greater than 27 cc, and CTV HR greater than 35 cc. Standard IC plans prescribed to point A had inferior coverage of the CTV HR compared to IC/IS plans above 2.9 cm max distance from tandem, 5.3 cm max lateral dimension, 4.1 cm max A-P dimension, 5.4 cm max S-I dimension, and 5.6 cm overall dimension. Standard IC plans prescribed to point A also began to treat the CTV HR inferiorly to IC/IS plans above a GTV and CTV HR of 37.4 and 43.1 cc, respectively. <h3>Conclusions</h3> Various BRT characteristics are significantly associated with benefit from IC/IS, and our data suggests patients having tumors with max overall dimensions > 5 cm, CTV HR > 35 cc and GTVs > 27 cc at the time of BRT would benefit from IC/IS. However, further analysis using independent data is needed to confirm these thresholds.