Intra-operative 3D guidance and edema detection in prostate brachytherapy using a non-isocentric C-arm

Abstract

Purpose: Brachytherapy (radioactive seed insertion) has emerged as one of the most effective treatment options for patients with prostate cancer, with the added benefit of a convenient outpatient procedure. The main limitation in contemporary brachytherapy is faulty seed placement, predominantly due to the presence of intra-operative edema (tissue expansion). Though currently not available, the capability to intra-operatively monitor the seed distribution, can make a significant improvement in cancer control. We present such a system here.

Methods: Intra-operative measurement of edema in prostate brachytherapy requires localization of inserted radioactive seeds relative to the prostate. Seeds were reconstructed using a typical non-isocentric C-arm, and exported to a commercial brachytherapy treatment planning system. Technical obstacles for 3D reconstruction on a non-isocentric C-arm include pose-dependent C-arm calibration; distortion correction; pose estimation of C-arm images; seed reconstruction; and C-arm to TRUS registration.

Results: In precision-machined hard phantoms with 40-100 seeds and soft tissue phantoms with 45-87 seeds, we correctly reconstructed the seed implant shape with an average 3D precision of 0.35 mm and 0.24 mm, respectively. In a DoD Phase-1 clinical trial on six patients with 48-82 planned seeds, we achieved intra-operative monitoring of seed distribution and dosimetry, correcting for dose inhomogeneities by inserting an average of over four additional seeds in the six enrolled patients (minimum 1; maximum 9). Additionally, in each patient, the system automatically detected intra-operative seed migration induced due to edema (mean 3.84 mm, STD 2.13 mm, Max 16.19 mm).

Conclusions: The proposed system is the first of a kind that makes intra-operative detection of edema (and subsequent re-optimization) possible on any typical non-isocentric C-arm, at negligible additional cost to the existing clinical installation. It achieves a significantly more homogeneous seed distribution, and has the potential to affect a paradigm shift in clinical practice. Large scale studies and commercialization are currently underway.

Fig. 1

Overall system concept for TRUS and C-arm fusion.

Fig. 2

The main technical challenges towards intra-operative 3D seed reconstruction using C-arms.

Fig. 3

Overview of the proposed solution. The FTRAC fiducial tracks C-arms, and also registers TRUS to C-arm images, making quantitative brachytherapy possible.

Fig. 4

Images of the FTRAC fiducial (a) wire model; (b) photograph; (c) X-ray image.

Fig. 5

Mis-calibration conserves relative reconstruction between objects A and B (e.g. seeds).

Fig. 6

X-ray image before segmentation (left). There is also the FTRAC in the image. The image after segmentation (right); the blue ‘+’ symbol represent individual seeds and the blue ‘.’ indicates a seed that is a part of a multiple seed cluster.

Fig. 7

The flowchart of segmentation algorithm.

Fig. 8

The network flow formulation used to solve the hidden-seed correspondence problem.

Fig. 9

The frame transformations between the FTRAC and TRUS. The FTRAC fiducial and the needle insertion template can be pre-calibrated using a rigid mount.

Fig. 10

The FTRAC fiducial and the needle insertion template can be pre-calibrated using a rigid mount. (a) A CAD model of the FTRAC fiducial mounted on the seed-insertion needle template using a rigid connector. (b) An actual photograph of the FTRAC mounted on the template. (c) A zoomed annotated photograph of the clinical setup.

Fig. 11

GUI screen captures of the main program, offline calibration, seed and fiducial segmentation, seed matching and validation of the reconstruction by back-projection.

Fig. 12

Workflow for intra-operative dosimetry and implant optimization.

Fig. 13

(a) An image of the seed phantom attached to the FTRAC fiducial. The phantom can replicate any implant configuration, using the twelve 5 mm slabs each with over a hundred holes. (b) A typical X-ray image of the combination.

Fig. 14

An annotated image of the (a) experimental setup for the training phantom experiments; (b) overall set up of the full operating area during the Phase-I clincal trials.

Fig. 15

The system is able to detect cold spots. The seed locations (and corresponding 100%/150% isodose contours) as assumed by the planning system (top) and as computed by the proposed system (bottom), discovering two cold spots in this slice. Four seeds have drifted out of the slice, while two have migrated significantly within.

Fig. 16

The system can visualize intra-operative edema, as seen for patient 3 (mean 4.6 mm, STD 2.4 mm, max 12.3 mm). The ‘planned’ (red) vs. the ‘reconstructed’ (blue) seed positions as seen in the template view. A trend of outward dispersion from their initial locations is observed.