RADIATION MEDICINE INNOVATIONS FOR THE NEW MILLENIUM: INTENSITY-MODULATED RADIATION THERAPY

29 Jan
2010

Intensity-modulated radiation therapy (IMRT) is a revolutionary new technology in the planning and delivery of external radiation. This methodology varies the intensity of the radiation beam across the treatment field, enabling the radiation oncologist to deliver high doses of radiation to volumes that tightly conform to irregular tumor geometries. Nearby, normal structures benefit from less exposure, therefore, resulting in less toxicity.

IMRT also is ideal for treating regions of the body that have uneven surfaces (e.g. head and neck sites) by homogenizing the dose that reaches the tumor. This minimizes the “hot” and “cold” areas in the tumor volume and potentially improves local control. A common technology to modulate the radiation beam utilizes multi-leaf collimators. The multi-leaf collimator consists of small, independently moveable, high-density rods that can be programmed to sequentially block the radiation field over short time intervals. In this manner, the radiation intensity of the volume under the rod can be differentially modified or “modulated.” High-speed computers enable optimization of the rod positions with time and produce the radiation distributions prescribed by the radiation oncologist. kamagra soft tablets

IMRT has been used to treat head and neck cancer successfully for more than five years; clinical experience has been positive. The primary reason for using IMRT in these cases is to spare critical structures, such as the normal parotid gland and spinal cord. Without this technique, it is sometimes difficult to limit the dose to these structures. If the entire parotid receives greater than 30 Gy, the risk of xerostomia is dramatically increased. In these patients, dental caries, difficulty with eating, and phonation pose major challenges to the treating physician. Various drugs have been used to reduce xerostomia, with only moderate efficacy.

IMRT can produce the desired radiation coverage of the primary tumor target and nodal volume while simultaneously avoiding overexposure to the spinal cord and other adjacent normal tissues. The potential for treatment set-up errors is reduced if complex treatment field arrangements are not utilized.

For the critical structures at risk are the bladder, rectum, and small bowel. The utilization of IMRT in this disease is an attempt to decrease gastrointestinal and genitourinary morbidity. Work done by Shu et al. compared toxicity profiles in patients treated with 3-D conformal therapy vs. IMRT in patients that received >82 Gy. At Memorial-Sloan Kettering Cancer Center, Zelefsky and colleagues have been instrumental in dose escalation studies where radiation doses up to 90 Gy were delivered to the prostate. Without the use of IMRT, radiation dose to the prostate would have been significantly limited to 70-72 Gy secondary to unacceptably high bowel and bladder toxicity. Data appear to suggest an improvement in the therapeutic ratio, which allows dose escalation and a decrease in late RTOG grade 2 or higher rectal toxicity.

For pancreatic cancer, Emory University School of Medicine has led the effort in radiother-apeutic management using IMRT. IMRT allows the dose to the pancreas to be increased to 61.2 Gy, while maintaining acceptable dose and toxicity levels in the small bowel, kidneys, and spinal cord. Some reports suggest a dose-response relationship, and in a disease where outcomes can be as dismal as those for pancreatic cancer, higher doses may improve the chance of disease-free survival. This regimen, along with the concurrent administration of gemcitabine, is usually tolerated without many treatment breaks. Using conventional or 3-D conformal therapy for pancreatic cancer, patients often require treatment breaks secondary to small bowel toxicity, even at much lower doses of radiation than those used for IMRT.

Clinically, preoperative irradiation for rectal cancer is considered when tumors are bulky, and/or locally advanced, in an attempt to make them resectable. Landry and colleagues have generated protocols in which preoperative radiation doses have been increased to 60 Gy in 1.5-Gy fractions (twice daily) with 5FU-based chemotherapy. Investigations continue regarding escalation of radiation dose to 70.5 Gy, in hopes of eliminating the need for surgery. This aggressive approach still respects the 45-Gy tolerance of the small bowel, while decreasing the high dose volume. canadian antibiotics

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