CHICAGOIntensity modulated radiation therapy (IMRT) offers
dramatic advantages for selected pediatric patients, such as those with
sarcomas of the chest, abdomen, or pelvis, as well as children with tumors of
the head and neck or brain, said Suzanne Wolden, MD, a radiation oncologist at
Memorial Sloan-Kettering Cancer Center.
Preliminary findings in 43 children show that IMRT effectively
reduced the dose of radiation to normal tissue and improved tumor target
coverage, Dr. Wolden said at the 86th Annual Meeting of the Radiological
Society of North America (RSNA).
IMRT is a computer-optimized technique for distributing
radiotherapy intensity by cross-firing a patient with many small beams of
radiation, each with a varying intensity. As a result, IMRT can be designed to
deliver a uniform dose of radiation to a tumor while limiting the amount of
radiation delivered to normal, surrounding tissue.
"In many cases, this represents a dramatic improvement
over more traditional, 3D conformal radiotherapy by delivering high degrees of
conformality to the target, greater dose homogeneity within the target, and
sharper dose falloff at the boundaries of the target," Dr. Wolden said.
The goal of IMRT in pediatric patients is somewhat different
than it is for adults, she added. IMRT in adults seeks to escalate the dose of
radiation and reduce acute side effects.
"In pediatrics, we’re not as interested in escalating
dose, because we already have good tumor control for most of the common tumors.
But we are interested in decreasing some of the potentially devastating late
effects these children can have because of irradiation of normal tissue,"
Dr. Wolden said. "So our primary goal is to decrease late complications,
and our secondary goal is to improve target coverage and potentially increase
the cure rate."
43 Diverse Patients
Radiation oncologists at Memorial Sloan-Kettering Cancer Center
used IMRT to treat 43 pediatric patients with a median age of 10 years (range,
1 to 26) between 1998 and 2000. Most of the patients had rhabdomyosarcoma or
medulloblastoma. However, a number of patients had Ewing’s sarcoma,
osteosarcoma, lymphoepithelioma, acinic cell carcinoma, and anaplastic
The most common treatment location was in parameningeal sites
or at the base of the skull (25 patients). Six patients were treated for tumors
in the pelvis, five for malignancies in head and neck sites other than
parameningeal, four for tumor sites in the thorax, and three for tumors in
paraspinal locations. Patients were selected for IMRT if they required
relatively high radiation doses, ranging from 3,000 to 7,000 cGy.
Treatment planning was conducted using spiral CT with and
without contrast. Whenever possible, initial magnetic resonance imaging (MRI)
or positron emission tomography (PET) scans were fused with CT data to further
characterize anatomy. The gross tumor, or the target for the planning target
volume (PTV), was defined as the preoperative or pre-chemotherapy tumor volume
plus an additional margin of tissue depending on the clinical situation.
A computerized plan optimization using an inverse planning
algorithm specified the dose for all target tumors as well as dose volume
constraints for normal tissue within the region of interest, and radiotherapy
was administered using dynamic multileaf collimation.
Because the patient population treated with IMRT has been so
diverse, Dr. Wolden did not summarize data accumulated thus far. Rather, she
reported on the results of this form of radiotherapy in several patient
One of these patients is a 17 year old with osteosarcoma of the
second vertebral body who achieved local control of the disease a year and a
half after treatment with IMRT.
This patient had gross residual tumor in the bone and soft
tissue after resection. The PTV developed for this patient was horseshoe-shaped
and folded around the spinal cord. The dose was calculated to deliver at least
60 Gy radiation to the tumor but to restrict exposure of the spinal cord to 40
Dr. Wolden pointed out that years of follow-up and additional
experience will be needed to discover the potential long-term clinical
consequences of IMRT on side effects in pediatric cancer patients. "But we
are very hopeful that IMRT and other conformal therapy will improve the health
and quality of life for survivors of childhood cancer," she