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Interfractional Needle Displacement During HDR Brachytherapy for Patients with Gynecological Cancer Using Interstitial Needle Template

S Benhabib*, R Kim, J Duan, L Burnett, R Popple, X Wu, R Cardan, I Brezovich, S Shen, Univ Alabama Birmingham, Birmingham, AL

SU-E-T-344 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
HDR Brachytherapy using interstitial needle templates for gynecological cancer is commonly delivered in 4-5 fractions. Potential movement of afterloading needles between fractions requires verification before each treatment. The current study analyzed the interfractional displacement of the implant needles relative to implanted fiducial markers (for tumor border) positions.

Methods:
15 patients with interstitial needle implants were reviewed. Treatment plans were based on CT images and the superior and inferior borders of the target volumes were defined by fiducial markers. The implant position was verified with kV orthogonal images before each treatment. The 3D positions of the needle tip and fiducial markers were determined from the orthogonal images. Needle displacement was defined by the change of the 3D distance between needle tip and 2 fiducial markers between planning and before the last fraction. The relative 3D distances between 2 fiducial markers were also measured to assess possible markers migration.

Results:
The median needle was 1.1 mm and the displacement ranged from 0.2-7.8 mm for the needle that near superior fiducial marker, and was typically in the inferior direction. One patient had significant needle displacement (7.8 mm) and positions were corrected before treatment. The needle displacements in the remaining patients were less than 4 mm and were not corrected because of adequate treatment margins. The displacement between 2 fiducial markers changed from 0.2-4.9 mm with a median change of 1 mm.

Conclusion:
There was detectable displacement of implant needles between HDR fractions, typically in the inferior direction. Pretreatment verification using orthogonal imaging is required and used to correct needle position based on implanted fiducial markers and needle tip positions.

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