IMRT Reduces Radiation to Normal Tissue in Pediatric Patients

CHICAGO—Intensity 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 86^th 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.

IMRT Reduces Risk of Hearing Loss in Medulloblastoma

At her RSNA presentation, Dr. Suzanne Wolden described the evolution of treatment for medulloblastoma using IMRT.

The standard method of treating the posterior fossa boost in patients with medulloblastoma is to use opposed lateral fields of radiation, she said. Even a highly conservative estimate of the target area for the posterior fossa boost will almost always include the cochlea or the inner ear in the area of high dose volume.

Because children with medulloblastoma also receive cisplatin (Platinol) therapy, severe hearing loss is a major potential long-term side effect of treatment.

Dr. Wolden and her colleagues at Memorial Sloan-Kettering Cancer Center have developed an IMRT technique using five posterior oblique fields to deliver 100% of the radiation therapy dose to the target area.

"If you look at the cochlea when lateral fields are used, there is a very high dose of radiation going to the inner ear. If you use a traditional, noncoplanar 3D plan, you can lower the dose significantly. The dose to the cochlea with IMRT is even lower," she 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 astrocytoma.

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 examples.

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 Gy.

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 concluded.