Design of a steerable needle for low dose rate brachytherapy of the prostate

Abstract

Prostate cancer is the most frequently diagnosed cancer type among men, and the third deathliest form of cancer. Several treatments are available to treat prostate cancer. Low Dose Rate Brachytherapy (LDR BT) is one of them, which inserts permanent radioactive seeds into the prostate using a needle. The literature review performed prior to this thesis, investigates if LDR BT is an effective method and how the quality of life is affected by the treatment compared to other treatments. Furthermore, the various anatomical variations that could intervene with the procedure are analyzed and how they are currently dealt with. It was found that during this treatment multiple complications can arise; first pubic arch interference could prevent the needle from reaching all parts of the prostate. Second, unwanted needle deflection can cause incorrect dose distribution, and third, delicate tissues like the urethra can obstruct the path of the needle. The introduction of a steerable needle in the field of brachytherapy could have the potential of treating patients with particular anatomical variations, add more maneuverability for the surgeon to steer around certain tissues and adjust unwanted needle deflections. All contributing to an accurate dose distribution and therefore an improved quality of life of the patient.

This thesis aims to design and develop a steerable needle for LDR BT of the prostate along with defining the design parameters of the steering capabilities to predict the bending of the needle. The needle should have a lumen for radioactive seeds to pass through, should use the steering mechanism of the preceding high dose rate wire needle and it should be able to steer 30mm laterally over 150mm insertion in order to reach the entire prostate.

Multiple theories are developed to predict the deflection behaviour of the needle. A theory that can predict the needle tip path, two theories that predict the distal deflection in air, and lastly a theory that predicts the deflection in tissue. One of the theories that predict the distal deflection in air is based on the proximal angle and the other one on the proximal force. To test these theories, a steerable wire needle, made of spring steel, for high dose rate brachytherapy is borrowed. From these experiments it can be concluded that the theory for the prediction of the needle tip path gives results close to the measured values and the prediction of the distal deflection based on proximal force give adequate results with an error of 1.77mm ±1.6mm. This error is in between the acceptable threshold of 2mm-5mm. The other two models presented errors of 15.1mm ±15.2mm and 14.4mm ±14.6mm, which give inaccurate predictions and are therefore not used as basis for the design of the steerable needle. An analysis is performed on the individual parameters within the two models. This provides insight into which variables need to be adjusted to design a steerable needle that complies with the requirements.

Based on the two models and the analysis of the individual parameters, a steerable needle that complies with the requirements, is designed, manufactured and tested. The manufacturing of the needle was limited by the availability of materials, causing the inner lumen to be slightly smaller than initially designed. The first tests are performed in air to compare the results with the previous experiments done with the wire spring steel needle and to verify the prediction models. The needle tip path and distal deflection based on the proximal force predictions both proof to be adequate models with the latter having an error of 2.75mm ±2.99mm. The steerable nitinol needle with a lumen, is also tested in tissue stimulant to provide a proof of principle and verify the theory of deflection in tissue. It is demonstrated that the needle can steer within the tissue stimulant, and insert an object though its lumen into the tissue. The model for the prediction of the deflection in tissue can be proven empirically. In addition, the prediction of the distal deflection based on the proximal force can also be applied for bending in tissue, if a constant is added to account for the bevel tip of the needle.

To provide an additional substantiation for the prediction model, a third needle is tested in air. This needle is borrowed and is intended to be used in high dose rate brachytherapy. This needle, made of tungsten, also proofs that the model can make adequate predictions with an error of 1.76mm ±1.4mm.

One of the requirements of the needle is that the inner lumen must accommodate the passing of a radioactive seed. Due to the unavailability of certain materials this was not accomplished. Therefore, a theoretical design is made with a larger inner lumen. For further research this design can be manufactured and tested.

The steerable needle presented in this thesis provides a proof of principle for a steerable needle with a lumen. It could have the potential of overcoming the anatomical obstacles presented in the literature review and therefore aiding in an accurate dose distribution. Furthermore, it can also provide the surgeon with the capability of adjusting unwanted deflection created by the needle. In addition, by analyzing the variables that influence the steering, a model is made that can predict the deflection based on the proximal force applied.