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Forward Planning IMRT Targets Tumor Bed

Forward Planning IMRT Targets Tumor Bed

CHICAGO—After breast-conserving surgery, patients who receive intensity-modulated radiation therapy (IMRT) shaped to the outline of the targeted tumor bed, as determined by three-dimensional (3D) CT imaging, achieve good results with a reduced radiation dose, according to a clinical study reported at the 88th Scientific Assembly and Annual Meeting of the Radiological Society of North America (abstract 335). The technique is known as forward planning IMRT (see images).

Charles Mayo, PhD, assistant director of medical physics, University of Massachusetts Memorial Medical Center, Worcester, presented findings from a study of 120 patients treated with forward planning IMRT. The study results showed that the technique reduced the overall dose of radiation, produced excellent cosmetic results, and locally controlled the disease; there have been no recurrences of disease in more than a year of follow-up, Dr. Mayo said.

"This technique is one that can easily be applied in many clinics that already have the necessary technology, such as CT, to be more accurate in targeting the tumor bed so that we can get a higher dose to the area that really needs it," he said. The technique also opens the door to dose escalation. "We are able to cut down on the excess dose to the lung and skin, so future researchers may want to look at increasing the overall dose," Dr. Mayo said.

Conventional radiotherapy following breast conservation uses landmarks, such as the surgical scar and surgical clips, which may miss part of the tumor bed. CT imaging, however, increases the accuracy of radiotherapy targeting to the surgical excision site. "When you outline the tumor bed with CT, the breast tissue you want to treat lights up very clearly so you can be very accurate in determining where to deliver the radiation," Dr. Mayo said.

In this study, CT was performed with patients positioned using Alpha Cradle immobilization molds. The CT images were then transferred to 3D radiation treatment planning workstations, where radiologists identified the breast tissue and tumor bed (see images A and B in the figure) and developed an IMRT treatment plan.

The treatment plan was achieved by using the leaves of a multileaf collimator to block out fractions of the beam that may hit the lung or ribs and equalize the dose across the breast target, Dr. Mayo explained. Multiple iterations of progressively smaller tangential x-ray fields were then delivered to the targeted breast volumes. Once IMRT was completed, a second treatment plan was developed using the planning CT volumes to guide daily concomitant photon boosts to the tumor bed (image D in the figure).

Conventional radiotherapy (image C in the figure) typically utilizes two tangents—the medial and the lateral—which produces radiation hot spots through the lungs and at the apex of the breast. Forward planning IMRT tangents (image D) eliminate the excess doses of radiation to normal tissue, he said.

Daily Boost

All patients in the study received 4,500 cGy to the breast target. During the final iteration, patients were given a 20-cGy daily photon boost at the surgical excision site. A final 1,000 cGy boost (either electrons or photons) was then delivered, which raised the total dose to target tissue to 6,000 cGy over a period of 6 weeks.

"In the conventional approach, which uses regular tangents and electron boosts, we get a substantial dose to the lungs and hot spots laterally. With the forward planning IMRT tangents and the photon boost, we reduce the portion of the lung that gets a higher dose and eliminate hot spots near the skin," Dr. Mayo said. 

 
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