MRI leaps toward primary role in radiation therapy planning

ASTRO release combines open scanner and oncology simulation

When it comes to soft-tissue visualization, MRI beats x-ray hands down. Yet most U.S. oncology departments use x-ray fluoroscopy or, increasingly, CT to gather data for radiation therapy planning (RTP).

“For a number of years, we have been able to fuse images-CT with MRI-which has allowed us to do RTP simulations based on MRI as a secondary data set,” said Terri Wimms, oncology marketing manager for Philips Medical Systems, following the acquisition of Marconi Medical Systems.

But that was not enough for Wimms and her colleagues at Philips’ newest acquisition. On Nov. 4, they unveiled technology that makes MRI a primary source of RTP data. The work-in-progress, featured at the annual meeting of the American Society for Therapeutic Radiation and Oncology (ASTRO) in San Francisco, combines flagship Marconi technology: its AcQSIM workstation, which is widely used for CT simulation in oncology, and the Proview Open MR scanner. Linking the two is software that translates MR rather than CT image findings into data that can be used to plan and simulate radiation therapy.

The MRI-based product will be featured on the exhibit floor of this month’s RSNA meeting along with the company’s mainstream wide-bore CT simulator and AcQSIM, as well as treatment-planning stations from Philips ADAC. The new product will probably sell for $850,000 to $1 million, about the same as the CT package. A final price has not been set, since the MRI simulator is pending FDA review. The ongoing corporate transition from Marconi to Philips is also raising uncertainty, although Wimms expects strong support for the product from its new owner.

“The treatment-planning products from Philips ADAC are number one in the world,” she said. “AcQSIM has the number one CT simulator software in the world. So bringing the two together is complementary.”

The expansion in capability that MRI simulation brings should give Philips a further boost. MRI better visualizes the boundaries of tumors surrounded by soft tissue, according to Rafael Vaello, global oncology sales manager for the new Philips corporate component. This promises to improve both the accuracy of radiation therapy and patient outcomes, he said. MRI will be best suited for defining the boundaries of cancers in the brain, head and neck, spine, and prostate, as well as soft-tissue sarcomas of the extremities.

“The boundaries of these tumors are difficult to discern using CT images,” Vaello said. “Three different clinicians may define the boundaries three different ways.”

The ultimate beneficiary will be the patient, because using MRI as a primary RTP data source will mean exposing less of the healthy tissue surrounding the tumor to radiation. Up to 35% of all treated cancer patients have tumors that are best visualized using MRI, Vaello said.

The software that runs Philips’ new MRI-based RTP package is an adaptation of the Marconi product that combines AcQSIM and CT. The primary difference is how the algorithms in AcQSIM analyze tissue densities, which are different in MRI compared with CT.

“The challenges were on the planning side and in making sure that we could apply the densities to the tissue that doesn’t appear well in MR, specifically bone,” Wimms said. “We also had to be certain that the dose computation software remained accurate.”

MRI is most commonly used to visualize the central nervous system. The MRI simulator will be used for brain cancers, but prostate cancer has also attracted clinicians’ attention. Oncologists at Fox Chase Cancer Center in Philadelphia have used the system to treat more than 200 patients with either of these types of cancer.

“Prostate cancer represents the number one or two cancer site being treated in oncology departments, and it presents the most difficulty when using CT and x-ray imaging,” Wimms said. “MRI brings something great to the table, because the soft tissue lights up.”

The technology to utilize this enhanced imaging capability is in hand. Intensity-modulated radiation therapy (IMRT) holds the potential for depositing radiation exactly on the cancer tumor. Clinical experience with IMRT indicates a decrease in the side effects, such as irritation of the bladder (burning upon urination) and irritation of the bowel, that often accompany injury induced by radiation of healthy tissue surrounding the prostatic tumor.

IMRT typically is based on CT reconstructions of the bladder, rectum, and prostate gland. MRI promises more exact targeting of the cancer, due to clearer delineation of the tumor boundaries. Such expectations are speculative, however, since experience with this modality in prostate imaging is limited.

Experience with MR mammography is similarly scant, but the potential for its application in oncology is enormous. Study results presented at the ASTRO meeting indicate that the availability of radiation oncology physicians and treatment services offers an attractive alternative to mastectomy. The addition of MRI and exact targeting of the tumor could make radiation therapy even more appealing.

In the ASTRO report, researchers at Duke University Medical Center compared the pathology database of a community hospital prior to and after the opening of an on-site radiation therapy consultation and treatment facility. Of the 282 patients diagnosed and treated for breast cancers during 1994 and 1995, mastectomy was performed on 201 patients (71%), while 81 patients were treated with breast-conserving therapy (29%). After an on-site radiation therapy facility became available during 1997 and 1998, the percentage of patients seeking mastectomy dropped significantly. Of 304 patients diagnosed and treated for breast cancers, 171 were treated with mastectomy (56%), while 133 received breast-conserving therapy (44%).

Oncologists at the Fox Chase Center have used MRI to guide therapy since early 2000. The Pennsylvania researchers initially began using their own MR scanner, integrating the images as a secondary data set in RTP. MR images were used as a primary data set mostly on prostate cancer patients.

The collaboration between Marconi and Fox Chase extends beyond MRI, back to a time when Marconi Medical was Picker International. Fox Chase researchers were major contributors to the development of CT simulation as a means for RTP. This project also involved an early version of AcQSIM.

While Fox Chase was a primary clinical site for moving RTP to MRI, it was not the only source of advice. Marconi recruited several clinical centers in the U.S. and Europe to explore this new idea. Before convening these advisors, the new corporate arm of Philips had proved the viability of the concept in the engineering lab.

“We wanted to test the waters with top clinicians to see if it was of interest to them,” Wimms said.

The Proview was Marconi engineers’ first choice for use with the MRI version of AcQSIM, because the openness of the scanner allows flexibility in patient positioning. The lower field product is also less costly than other scanners and easier to site. Other MR scanners will, however, be adopted for oncologic use, she said.

Now in the Philips lineup are two higher field open works-in-progress: the 0.6-tesla Infinion HFO (high field open) acquired by Philips along with Marconi and the open Panorama 1.0T, which Philips is developing in-house. More scanners may be added to the list. Prospective customers have indicated interest in conventional scanners operating at 1.5 and even 3 tesla. The MRI simulation software can be adapted to any of these, but it will probably be limited to Phillips models, according to Wimms.

“The AcQSIM software has always been tied specifically to Picker, then Marconi, and now Philips products to-frankly-sell scanners,” she said.