Advances in radiotherapy have been tied to advances in technology. In the early days, low-energy orthovoltage devices and radium 226 sources were used to treat HNSCC. Then came megavoltage beams from linear accelerators and cobalt 60 units and brachytherapy isotopes such as cesium 137, iridium 192 and iodine 125 produced in nuclear reactors. Modern radiotherapy centers use sophisticated linear accelerators producing photon beams of different energies and megavoltage electron beams that can easily treat posterior neck lymph nodes without risk of spinal cord damage. Computer-controlled multileaf collimators facilitate custom blocking techniques to spare uninvolved normal tissues and sequential changes in field geometry as a patient progresses through treatment. CT and MRI are used to locate tumors, with many radiation oncology centers having dedicated scanners used exclusively for simulation and treatment planning. CT, MRI, and PET scans are fused to give the clinician a broader perspective in locating regions at risk for tumor. Noncoplanar field configurations, often using vertex presentations, are standard techniques.
Intensity-modulated radiotherapy (IMRT) is the next step in the saga to improve dose localization. In this approach, many different treatment fields are used, with each field being divided into multiple segments and each segment delivering a prescribed amount of radiation. Sophisticated computer programs are used to calculate the doses given by each of these field segments,and the clinician is able to specify normal tissue dose constraints to be used in obtaining the “solution” for a particular patient.
Head and neck cancer represents an ideal situation in which to apply this technique because with proper immobilization techniques, organ motion is not a major problem. There often are critical normal structures in close proximity to the tumor, and achieving a sharp dose gradient around the target while limiting the normal tissue dose offers the potential for significant therapeutic gain. Lee and colleagues have reported on the UCSF experience using this approach for nasopharyngeal cancer, which yielded a 97% locoregional recurrence-free survival rate in 67 patients, with a median follow-up time of 31 months. There was less toxicity than would have been expected using concomitant chemotherapy and conventional radiotherapy. The ability to reduce the dose to the parotid glands and to reduce the subsequent xerostomia experienced by the patient is an important advantage of this technique. A comparison between the isodose distributions produced by optimized, three-dimensional conformal radiotherapy and by IMRT for a particularly challenging clinical situation is shown in Figures 90-7 and
90-8. The patient shown has a nasopharyngeal primary with clinically positive cervical and retropharyngeal nodes. Note the ability to spare the parotid glands without sacrificing tumor dose, using the IMRT approach.
Why are these advances important clinically? The advantages of improved tumor imaging is obvious as there should be fewer “marginal misses” than in the past. Another advantage relates to being able to give higher radiation doses to the tumor in a safe manner. Other things being equal, higher doses of radiation lead to a higher tumor control probability. In the case of HNSCC, dose-response curves generally exhibit a steep region wherein modest increases in radiation dose will give rise to significant improvement in outcome.
Revision date: July 4, 2011
Last revised: by Tatiana Kuznetsova, D.M.D.