Intensity-modulated radiotherapy of head-and-neck cancer: Encouraging early results

Intensity-modulated radiotherapy (IMRT) involves the delivery of optimized, nonuniform irradiation beam intensities. It achieves a higher target dose conformity compared with previous technologies. This characteristic provides the potential for improved tumor irradiation and sparing of the organs in the vicinity of the targets to an extent that was not possible before. On the other hand, the technology is complex, work-intensive, and prone to potential errors and difficulties in quality assurance and radiation safety. Highmonitor-unit exposure and interleaf leakage result in a higher total-body low-dose radiation exposure to the patient, the consequences of which are not yet known. Multiple reports dealing with the planning, delivery, and quality assurance of IMRT have been published. These topics have recently been summarized in a report by the IMRT Collaborative Working Group (1). However, clinical data about the outcome of treatment that would verify the potential gains and drawbacks of this technology are sparse. The report by Lee et al. (2) in this issue of the Journal provides important clinical data about a large number of patients treated with IMRT for nasopharyngeal cancer between 1995 and 2000. Three consecutive intensity-modulation methods were used during this short time span, attesting to the rapid evolution of IMRT technology. Such an evolution is likely to continue in the near future, as more efficient and reliable technologies are introduced, delivering treatment that is easier to assess and monitor. Head-and-neck cancer represents an attractive site for IMRT. Organ motion is practically absent, and only the setup uncertainties need to be addressed. Many critical and radiation-sensitive organs are in close proximity to the targets. Tight dose gradients around the targets, limiting the doses to the noninvolved tissue, offer the potential for a therapeutic gain. Noninvolved tissue, the partial sparing of which may achieve tangible gains, includes the major salivary glands, minor salivary glands dispersed within the oral cavity, the mandible, the pharyngeal musculature, and, in cases of nasopharyngeal and paranasal sinus cancer, the inner and middle ears, temporomandibular joints, temporal brain lobes, and optic pathways. IMRT offers the potential for improved tumor control by reducing the constraints on the tumor dose imposed by limiting critical organ maximal doses. In addition, IMRT eliminates the need for posterior neck electron fields, commonly used in conventional irradiation, and their associated dose deficiencies. The locoregional recurrence-free survival rate of the 67 patients in the series reported by Lee et al. (2) was 97% at a median follow-up of 31 months. It represents a benchmark in the therapy of nasopharyngeal cancer and in the clinical practice of head-and-neck cancer IMRT. It has been achieved without an apparent increase in acute toxicity compared with the toxicity expected after conventional RT and chemotherapy. The authors stated that no increase in late tissue toxicity was observed. However, the follow-up period was not yet sufficient for an accurate late toxicity assessment. Several important factors are relevant to these successful results. The first is an accurate delineation of the gross tumor volume (GTV) and the volumes containing potential subclinical disease (clinical tumor volume [CTV]). In most cases, the simulation contrast-enhanced CT is the only imaging modality required for the delineation of head-andneck cancer GTVs. However, MRI is a necessary adjunct to CT for tumors that are close to the base of the skull (i.e., nasopharyngeal and paranasal sinus cancer), where it provides clearer details of the tumor extension and of the parapharyngeal and retropharyngeal spaces, compared with CT (3). Lee et al. (2) have rightly emphasized their use of MRI in all patients and their efforts to overlay the MRI and planning CT images for accurate delineation of the GTVs. A major potential pitfall of IMRT is the failure to select and delineate the CTVs accurately. Adequate outlining of the primary tumor CTV requires knowledge of the anatomy of the head and neck and the clinical pattern of local tumor extension. Adequate selection and delineation of lymph nodes at risk of metastatic disease (nodal CTVs) requires knowledge of the pattern of tumor metastases to the various