As mentioned in the introduction, a number of novel robotic brachytherapy devices will be leaving the lab and heading toward the clinic. Each system has its strengths and weaknesses. Of particular interest for all these systems is the technology to (1
) introduce needles into the body causing minimal distortion of the organs and their relative position and orientation and/or (2
) compensate for these changes. For example, Moerland et al. have discussed a robot (23
) that minimizes distortions by utilizing a tapping method. The robot discussed here uses a very fast insertion stroke to take advantage of the rest inertia of the organs to minimize distortions on insert. Item 1 touches on an area of research that hopefully will produce results beneficial to the field as a whole – namely modeling of the needle-tissue interaction – and there have been some promising results including the results of (34
) and that for curved needles (35
). Of interest in this paper however is addressing item 2 and compensating for these inevitable changes via an adaptive planning workflow.
To make this transition successful and to fully tap the potential these robotic devices have to offer, a clinical workflow must be developed that can incorporate seamlessly these devices. This work here accomplished precisely this. The workflow demonstrated here incorporates the robotic hardware, imaging devices, and planning systems into a procedure that takes full advantage of the software planning abilities of a modern brachytherapy clinic. Designing and building a robotic brachytherapy device is a significant accomplishment. But in order to be used efficiently and effectively in the clinic, the robotic device must be able to communicate with other hardware, imaging and planning software, and the human operators of these devices. This workflow demonstrates a method for achieving this synergy: A synergy that will allow for further improvement in brachytherapy care.
Reducing uncertainties is critical to further advancement of brachytherapy. In addition, the range of therapeutic outcomes reported in the literature is wide – ranging from the upper ninetieth percentile for recurrence free survival to as high as 30% biochemical failure. This dichotomy may be a result of a correlation between physician skill and therapeutic outcome for prostate permanent-seed implants. Placing seeds accurately and precisely is a skill which takes years of practice to hone, but is also the key factor in mitigating the radiation-induced side effects that are common after brachytherapy treatment.
The most common prostate brachytherapy practice incorporates the use of a regularly-spaced template for the positioning of needles and seeds. One of the benefits of incorporating a robotic device is the ability to remove this template from the clinical workflow. Without the restriction of a fixed template more degrees of freedom are available for the insertion of the needles or catheters (36
). It has been shown (40
) that this may aid in reducing the deleterious side effects that can be caused by trauma to the critical structures near the penile bulb like the cavernous arteries along the side of the penile bulb (41
), by increasing the ability to avoid these structures.
A stereotactic robotic device will also mitigate this needle puncture trauma. This robot and the workflow presented here will have the largest impact in those clinics where the physician is new to the field or lacks the decades of experience necessary to master the art of brachytherapy seed placement. During a PPI procedure, it is not uncommon for the physician to have to insert each needle multiple times in an effort to ensure it is in precisely the correct position for the deposition of the seeds. Clearly in the hands of well established physicians, the average number of corrections per needle will be close to zero. However, observation of medical residents clearly indicates that one to three redirections per needle are common for those with little hands-on experience. For large prostates an implant can contain up to 30 needles and a total number of punctures (initial insertion + redirections) of two to four per needle leads to 60–120 full or partial needle paths. Because of its explicit knowledge of the relevant coordinate systems, a stereotactic robot like the one used for these experiments can execute an implant using only one needle insertion per needle. This would reduce the volume of tissue cut by the needles and therefore has the potential to significantly reduce trauma and puncture-induced edema.
In addition to trauma another factor of standard brachytherapy that can benefit from improved delivery techniques is seed placement uncertainty. This uncertainty can lead to inadequate dose coverage of the target organ or overdosage of the healthy structures. One method to account for uncertainty that is already starting to be implemented in the clinic (7
), but would benefit from a stereotactic robotic device is adaptive planning,
in which the plan is modified as more information is gathered during the course of the implant procedure. Foster et al. demonstrated a PPI procedure which allows for updating the dose planning software with the actual position of the placed needle tip immediately after its insertion and prior to dropping the seeds. It is done in this way since the needle is clearly visible on the ultrasound image. The repositioning of the needle tip slightly effects the coordinates of the seeds to be deposited in that pre-loaded needle. This information is automatically registered by the TPS. Westendorp et al. showed that intraoperative adaptive brachytherapy procedures can have an impact on the dosimetry of the final implant. Both of these procedures allow for more accurate post-implant dosimetry and can allow a physician to determine whether there are parts of the prostate which do not receive a sufficient dose. If so, additional seeds can be placed to fill in the gaps in the coverage. These works suggest a clear impact on toxicity. It has recently been shown that a robotic device can be used in an MR environment to implant individual radioactive seeds in the prostate of live canines (43
). In that study, images were acquired and inert seeds locations were determined one-by-one by eye. This study, in contrast, demonstrates that the robot can access the entirety of the generally larger human prostates and used computer-based planning to generate seed positions.
IGRIP brachytherapy requires the integration of a robotic device into the clinical workflow and it is necessary to be able to seamlessly transfer coordinate system information between all the elements in the process. These experiments addressed each element of the workflow depicted in :
- Validated the communication of reference systems between imaging, planning, and robotic delivery;
- demonstrated the ability to operate the robot in a clinical CBCT & MR environments;
- demonstrated the ability to use MR Spectroscopy to guide the robot to a target location;
- demonstrated the ability to use the MR-guided robot to deliver a seed to a desired location in non-homogeneous soft tissue (bovine muscle);
- demonstrated the ability to use quasi-real-time image guidance to validate the location of the brachytherapy needle at the delivery site prior to depositing the seed;
- implemented a full brachytherapy treatment procedure in a prostate phantom, including imaging, robot registration to the image coordinate system, generating a dose plan using IPSA, transferring the planned seed coordinates to the robot, delivering the seeds with the robot, and imaging to verify seed placement locations.
Incorporating robots, image guidance, and inverse planning can mitigate the disparity between more-practiced and less-experienced clinicians. Some of the most promising advancements that can immediately be addressed in the current brachytherapy paradigm are reducing needle-puncture-induced edema and improving the conformality of the delivered dose to the diseased tissues through improved seed placement accuracy. However, stereotactic robotics devices will also be able to push brachytherapy into a new paradigm by enabling access to body sites previously unavailable for brachytherapy treatment. The current permanent seed brachytherapy paradigm relies on real-time image guidance and often a pre-fabricated regular-grid template of possible needle positions for placement of the radioactive seeds. Because of this, brachytherapy is mostly limited to areas of the body that have predictable disease topology (prostate cancer), and/or easily accommodate naked-eye or ultrasound imaging: Prostate and gynecological cancers (transrectal ultrasound), eye and skin lesions (naked-eye). However, a stereotactic device – because of its explicit link to the reference frame of the patient – can both do away with the template guide to allow for vastly more degrees of freedom in needle positioning and not require real-time imaging for implantation. This could allow access to a broad range of anatomical sites like the liver, pelvic side wall, or the densely packed structures of the head and neck.
Through the course of the work with this particular robotic device, elements of the robotic design that can still be optimized for use in the clinic were identified. To obtain the best possible access to the prostate target while avoiding sensitive structures, the conical needle pattern was employed, which, along with other robot-deliverable non-standard catheter patterns, has been shown to provide treatment benefits (40
). The range of needle angles achievable was limited slightly by the range of motion of the robot in the anteroposterior direction (translation). In addition the range of angles in the anteroposterior direction is adequate for an average human prostate (as exemplified by the CIRS phantom – 30 cc), provided a precise mounting of the robot on the treatment couch is employed. A larger range of motion would allow for less stringent setup positioning. This can be remedied by extending the range of the actuator motors and/or moving the motors closer to each other and will be implemented in the next design of MRBot.
The seed magazine is located on the control device, which can lead to difficulties associated with sterilization and radiation protection. Although the robot is designed such that the same needle is used for the insertion of all seeds and therefore the needle needs only be exchanged at the end of the implant procedure, it may be needed on occasion to exchange the needle. This process requires approximately 15 minutes with the current design: A costly period during an operating room procedure. Finally, the needle insertion mechanism operates on the principle that a fast insertion utilizes the rest inertia of the organs to minimize tissue movement and deformation. However in some circumstances, it would be clinically beneficial to be able to have control of the needle insertion speed. The next version of the robot will be designed to address these issues.