Practical needle selection for Vienna-style applicators: improving therapeutic ratio in hybrid intracavitary-interstitial brachytherapy

David A Martin; Neil K Taunk; Shibu Anamalayil; Vatsal Mangal; Jaclyn Marcel; Emily Hubley

doi:10.5114/jcb.2021.110348

Practical needle selection for Vienna-style applicators: improving therapeutic ratio in hybrid intracavitary-interstitial brachytherapy

J Contemp Brachytherapy. 2021 Oct;13(5):533-540. doi: 10.5114/jcb.2021.110348. Epub 2021 Oct 29.

Authors

David A Martin¹, Neil K Taunk¹, Shibu Anamalayil¹, Vatsal Mangal¹, Jaclyn Marcel¹, Emily Hubley¹

Affiliation

¹ Department of Radiation Oncology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania, USA.

Abstract

Purpose: Hybrid intracavitary and interstitial (IC/IS) applicators improve dose distribution compared to traditional IC applicators in cervical high-dose-rate (HDR) brachytherapy. There is a learning curve to these applicators, and initial standard needle insertion patterns have not been well-established. In this study, we quantified dosimetric benefits of IC/IS applicators, and offer practical initial interstitial needle-selection, insertion depths, and dwell position recommendations.

Material and methods: Fifteen patients previously treated with a tandem and ring IC applicator and magnetic resonance (MR)-guidance were re-planned at first fraction using a digital template of Vienna-style interstitial needles. IC/IS plans maintained identical high-risk clinical target volume (HR-CTV) D_90% while reducing dose to organs at risk (OARs). To assess the validity of planning using virtual needles, virtual needle templates were overlaid on twelve clinical IC/IS plans, and the displacements between 40 physical and virtual needles were measured at 3 cm depth.

Results: The median HR-CTV volume in the present study was 19.6 cc (range, 6.6-60.5 cc). HR-CTV D_90% was maintained in all re-plans. Median bladder D_2cc decreased from 5.4 Gy per fraction to 4.8 Gy (p = 0.003); median rectum D_2cc decreased from 2.4 Gy per fraction to 2.0 Gy (p = 0.007). We suggest that a standard loading pattern should include needles in lateral channels 4, 5, and 9, 10 inserted 3 cm deep, with dwell times < 20% of the combined tandem and ring dwells. The mean displacement between planned and physical needles was 1.8 mm. All needles but three deviated less than 3.3 mm, demonstrating the validity of re-planning with virtual needles.

Conclusions: Hybrid IC/IS applicators maintain excellent D_90% coverage while improving dose to OARs compared to IC-only applicators, even in non-bulky HR-CTVs. We offer practical recommendations for needle selection, insertion depth, and relative weighting for Vienna-style applicators in small HR-CTVs. These results support previous publications, offering practical recommendations for users of Vienna-style hybrid applicators.

Keywords: Vienna applicator; brachytherapy; cervical cancer; image-guided brachytherapy; interstitial; intracavitary.