The high quality of anatomic images achieved with MRI technology (7
) provides a strong rationale for the integration of MRI data with prostate brachytherapy, a technique that demands a high level of accuracy in target delineation. To this end, two general approaches are actively under investigation. The first involves performing brachytherapy procedures directly under the “real-time” guidance of MRI, which to date has focused on using MRI scanners with an “open” architecture. These systems have lent themselves well to interventional procedures because they afford a greater spatial freedom than conventional scanners. A technique for permanent seed implants of the prostate using an “open” MRI scanner has been designed, implemented, and reported (15
). The results confirmed that transrectal ultrasound-guided implants may be improved on using MRI. However, because “open” MRI scanners have a significantly lower field strength and less gradient performance than standard cylindrical magnets, less signal is available to generate the MRI data. This results in anatomic images of lesser quality than those acquired in standard MRI scanners and severe limitations to the integration of emerging techniques in biologic imaging (e.g., dynamic contrast-enhanced MRI, MR spectroscopy), all of which demand high signal levels.
A second approach is to “fuse” the diagnostic MRI data to the interventional images that guide needle placement, most commonly ultrasonography (22
). This approach may be more broadly applicable, because ultrasound guidance for brachytherapy procedures demands less technical support and is readily available in most centers. However, it introduces registration errors and mandates two separate imaging sessions, one to acquire the diagnostic data and one for brachytherapy.
We chose to address the challenges involved with performing HDR prostate brachytherapy in a standard 1.5T MRI scanner for two main reasons. The first was to circumvent the errors introduced with deformable image registration of MRI data sets in an effort to improve the accuracy of anatomic target delineation, and in turn, the quality of the treatment. More importantly, however, we sought to establish a procedural platform in which new techniques for imaging tumor biology could be investigated simultaneously, compared with needle core tissue samples obtained under the same image coordinate reference, and directly applied to cancer therapy. Reducing the interval between the acquisition of the investigational imaging data, biopsy, and therapy, therefore, became important.
We have shown that HDR prostate brachytherapy in a 1.5T MRI scanner is feasible and can achieve favorable dosimetry in a reasonable period with exceptional image guidance. This procedure may offer a therapeutic advantage for those patients with extracapsular disease extension visualized on MRI, because extracapsular disease may be included in the radiation target volume. Our dosimetric results compared favorably with those published for a permanent seed implant technique performed under “open” MRI guidance, for which the median V100
was 96% (range, 89–99%) (21
), and for HDR implants performed under ultrasound guidance with ISPA (mean V100
). However, this standard dosimetric measure of implant quality does not take into account the accuracy of the target delineation, which we surmise to be superior with the present technique. Also, the urethra and distal urethral muscles are more visible on MRI than on ultrasonography (images not shown); therefore, the urethral contours may be bigger on MRI. The same dosimetric index values (V125
) may result in less dose to the urethra using our technique.
Both the dosimetric parameters and the overall procedure time improved with the introduction of IPSA for treatment planning. As we continue to gain experience with this new technique, we anticipate additional improvements in the overall time required for this procedure. As we achieve consistency in the overall procedure time, alternative anesthetic approaches such as spinal blocks and conscious sedation may be considered, because they may be better suited for some patients.
One of the challenges with this technique has been safe positioning and immobilization in the left lateral decubitus position. We chose this approach because it permitted maximal perineal exposure within the scanner bore size constraints. Although the scanner bore diameter is 60 cm, the space available in the AP dimension is limited to 45 cm because of the patient table. By placing patients in the lateral decubitus position, we took advantage of the much-needed additional 15 cm to better expose the perineum. For anesthetic considerations, this position was also favored over a prone position.
To our knowledge, no prior published experience is available to draw from regarding transperineal brachytherapy with patients in the left lateral decubitus position. A major concern with this position for a prolonged period is the stability of the position and the potential for brachial plexopathy and injuries to cutaneous pressure points. This risk, caused by traction on the brachial plexus from weight bearing on the dependent shoulder, can be reduced by placing an axillary roll that redistributes the weight to the chest wall (26
). This standard technique is frequently applied during other surgical procedures performed in the lateral decubitus position. We have found that when sufficient time and care is taken during patient positioning and immobilization, pressure injuries and problems with patient shifts during the procedure can be largely circumvented.
Another concern was the position and stability of the prostate gland relative to the bony pelvis with patients in the left lateral position. To date, we have not experienced difficulties with pubic arch interference and/or displacement of the prostate gland to the left side of the pelvis. Although limitations will surely exist in the eligible gland size and body habitus with this technique, we have not yet encountered them and they may not differ significantly from those applicable to a dorsal lithotomy approach. We suspect that the patient's shoulder width may become the most important limiting factor for this technique because of collision with the anterior edge of the scanner bore. Finally, our feasibility results to date must be confirmed with a larger cohort of patients.
As in all HDR brachytherapy procedures, preventing organ and/or catheter motion between the acquisition of the treatment planning images and RT delivery is of critical importance. Relatively small shifts in the catheter position relative to the target can potentially lead to important errors in delivery (27
). In our case, the catheters were locked to the template, which was locked to the table overlay. The patient was kept under general anesthesia and in the same position until the treatment was delivered without releasing any of the immobilization devices. Patient transfer for RT delivery was achieved by simply sliding the table overlay from the MRI table to the stretcher. This table transfer, organ swelling, and a small amount of bladder filling in the 45–60-min interval between image acquisition and RT delivery could conceivably result in catheter shifts. For that reason, portable X-rays were obtained immediately before RT delivery, documenting the location of the catheter tips relative to the Foley catheter balloon. No subjective systematic shifts in catheter tip location relative to the Foley balloon between the MRI and X-ray images were appreciated. In the future, we hope to quantitate this observation by obtaining CT images in our department immediately before RT delivery.
Although this procedure is technically complex and requires more equipment and personnel resources than ultrasound-guided HDR prostate brachytherapy, it is ideally suited to the study of emerging MR-based biologic imaging techniques. Examples of novel imaging techniques currently under investigation include MR spectroscopic imaging (28
), dynamic contrast-enhanced MRI (30
), diffusion-weighted imaging (34
), and bold oxygen level-dependent imaging (36
), all of which require additional histopathologic and molecular validation before they are applied to the design of radiation targets. Once the diagnostic accuracy of these techniques is better determined, specific areas with a high burden of disease or with biologic features indicative of radioresistance within the prostate gland may be specifically targeted for higher radiation dose delivery. In essence, the basic knowledge gained from the clinical imaging research that can be conducted during this procedure may be translated directly back to patient care. This avenue of research is now being integrated with the HDR brachytherapy procedures and will be the subject of future work.