CT-Guided Pelvic Lymph Nodal Brachytherapy

Abstract

Purpose: This is a report of our initial experience using computed tomography (CT)-guided interstitial high dose rate (HDR) brachytherapy to treat bulky pelvic nodal metastases as a part of definitive radiotherapy.

Material and methods: Between February 2015 and April 2019, 14 cervical/endometrial cancer patients presenting with bulky pelvic node(s) underwent nodal interstitial brachytherapy boost in our institution. In total, 17 nodes were treated. The median maximum diameters of the positive nodes at the time of diagnosis and at the first nodal implant were 25 mm (range: 10-65 mm) and 16 mm (range: 9-51 mm), respectively. Dosimetry data of the lymph nodal target volume and small bowel were collected and compared using the paired-sample t-test. Treatment-related toxicities were classified using the Common Terminology Criteria for Adverse Events version 4.0.

Results: The median follow-up time for all patients was 26 months. Local recurrence in pelvic nodes occurred in one patient (7%) after 16 months. One patient experienced grade 3 bladder bleeding, and one patient experienced grade 2 pubic bone fracture. No patient had grade 2 or greater gastrointestinal toxicity. In the dosimetric analysis, the mean nodal brachytherapy D_90% in terms of the total equivalent dose of 2 Gy (EQD2) was 65.6 Gyαβ10. The mean small bowel dose (SBD)_0.1cc and SBD_1cc in terms of the total EQD2 were 60.4 and 56.5 Gyαβ3, respectively. Nodal D_90% was significantly higher in terms of the total EQD2 than the SBD_0.1cc (p = 0.003) and SBD_1cc (p < 0.001). The Kaplan-Meier 2-year pelvic control estimate was 90%.

Conclusions: CT-guided interstitial HDR pelvic nodal brachytherapy appears to be well tolerated with excellent local control in cervical or endometrial cancer patients with bulky pelvic nodes. This approach may offer a useful therapeutic option for unresected bulky pelvic nodes.

Keywords: CT guidance; gynecological malignancies; interstitial brachytherapy; nodal brachytherapy; pelvic lymph node.

Figure 1

(A) Multi-slice axial CT images of the right obturator node (cyan line), uterine target (pink line), bowel (green line), external/internal iliac vessel (yellow line), rectum (brown line), and bladder (purple line) receiving 50% (blue lines), 100% (red lines), and 150% (white lines) of the prescription dose (6 Gy) in a representative patient with a right obturator node metastasis. One interstitial flexible catheter (white arrows) was inserted in the lithotomy position passing from the inside of the pubic bone through the outside of the bladder up to the front of the right sacroiliac joint using TRUS and CT guidance in the HDR room with dedicated CT, while avoiding bladder and external/internal iliac vessels. TRUS is commonly used for implantation from the perineum, and CT is used to adjust catheter positions at a deeper level. An iterative approach of catheter adjustment and CT image acquisition allows precise placement of the catheter in the target volume. (B) A coronal CT image of the uterine target (pink dashed line), the right obturator node (cyan dashed line), bowel (green dashed line), external/internal iliac vessel (yellow dashed line), rectum (brown line), and bladder (purple dashed line) receiving 50% (blue lines), 100% (red lines), and 150% (white lines) of the prescription dose (6 Gy) from interstitial and intracavitary applicators in a representative patient with a right obturator node metastasis. It is apparent that one interstitial flexible catheter (light green) is implanted, passing from the inside of the pubic bone through the outside of the bladder, so that the catheter can be inserted to the central part of the node.

Figure 2

A coronal CT image of the bulkiest obturator node (blue dashed line), uterus (pink dashed line), bowel (green dashed line), external iliac vessel (yellow dashed line), bladder (purple dashed line) receiving 50% (blue lines), 100% (red lines), and 150% (white lines) of the prescription dose (6 Gy) from two implanted needles.

Figure 3

Multi-slice axial CT images (A) of implanted needles and a sagittal CT image (B) of the right internal iliac node (cyan dashed line), bowel (green dashed line), common/internal iliac vessels (yellow dashed line), and bladder (purple dashed line) receiving 50% (blue lines), 100% (red lines), and 150% (white lines) of the prescription dose (6 Gy) in a representative post-operative patient. Pre-treatment CT/¹⁸F-FDG-PET/CT images (C) and post-treatment CT/¹⁸F-FDG-PET/CT images (D). Both insertion points and directions were decided using pre-treatment CT. Two flexible interstitial needles (white arrows) were obliquely implanted in the prone position, passing from under the right sacroiliac joint through the front of the piriformis muscle up to the front of the second sacral vertebrae under CT guidance (no contrast medium) in the HDR room with dedicated CT. Implantation was performed under local anesthesia alone, with care taken not to damage the bowel or common/internal iliac vessels. The catheter was inserted into the central part of the node using an iterative approach of catheter implantation and adjustment with serial CT image acquisition. MPR CT was used to reconstruct the axial images into oblique anatomical planes. The patient was irradiated in the prone position in the HDR room with dedicated CT.