CHICAGOAfter 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
"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
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 tangentsthe medial and the lateralwhich 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.
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.