November-December 2000 > Features

Image-Guided Brachytherapy

 

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.



       

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.