Intraoperative MRI has also improved the practice of minimally invasive therapies for other organs, such as the prostate. Minimally invasive surgery typically reduces tissue damage by inserting long-necked instruments--equipped with visualization devices--through natural bodily openings (such as the nasal cavity) or small incisions close to the target area. The two big problems posed by this approach are the lack of "global" visual feedback and the lack of mechanical control when using long tools. Ultrasound is one tool that has been used to guide interventions, but the imaging is neither very clear nor very specific. And when prostate cancer is the target, precision is crucial.
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| MRI scan (left) indicates dosage distribution-- from blue (low) to red (high)--of radioactive "seeds" implanted in a cancerous prostate gland. The seeded prostate (right) contains rice-sized radioactive doses (in red). | |
"The prostate is a round gland that lives in the pelvis, close to the rectum," explains Clare Tempany, associate professor of radiology at Harvard Medical School and BWH director of clinical MRI. "The urethra goes right through the middle of the prostate, so our primary goal is to treat the cancer and not to harm other nearby structures." Men diagnosed with prostate cancer have four treatment options: radical prostatectomy, or surgical removal of the prostate; external radiation; ultrasound-guided local radiation, or brachytherapy; or, for men over 70, "watch-and-wait." Tempany, a specialist in prostate imaging, devised a localized radiation protocol with three colleagues that utilizes the intraoperative MRI system to maximize effectiveness and minimize risk. The potential side effects of prostatectomy, for example, are incontinence, impotence, and rectal bleeding. "It's very critical to treat the cancer and preserve the quality of life of the patient," Tempany emphasizes.
Brachytherapy, or seed implant therapy, the treatment protocol Tempany uses, involves inserting rice-sized radioactive "seeds" into the prostate. "Because we're putting them in with a needle, a very high dose of radiation can be put in a specific site," explains Tempany. "It's critical to get your seeds to the right place--you don't want them going into the bladder or the urethra or the rectum--so the advantages of doing this under MRI have become very apparent to us."
In Tempany's protocol, MR scans are used throughout the planning and performance of brachytherapy. Scans taken of the patient on the treatment day are used to tailor the intervention to his anatomy. The procedure takes place in the open area of the Signa SP with real-time monitoring "of each and every needle as it's going into the gland," says Tempany. A feedback loop constantly compares the original treatment plan, with its "little red dot that says the needle should be here," with the ongoing imaging during implantation. "We can see the needle in the images on the screen inside the magnet, which guides repositioning, and we keep going until the plan and the actual image match." MRI-guided brachytherapy has been performed on just over 160 men, and the preliminary results have been very successful.
