Familial Nasopharyngeal Carcinoma 6

198 L. Kong, J. J. Lu, and N. Leeand neck cancers, especially nasopharyngeal carcinoma.As differentiation of dose distribution towardstreatment targets and normal critical tissues/organsadjacent to the nasopharynx significantly improves,IMRT is generally accepted as a more advanced radiationtechnique for NPC. Results from retrospectiveand prospective studies have confirmed the efficacyof IMRT in disease control as well as a benefit in treatmenttoxicity profile. However, a number of issuesrelated to IMRT such as the optimal treatment targets,timing and frequency of replanning during the courseof radiation, and the significance of target volumechange during treatment remain to be addressed.The aim of this chapter is to discuss the rationaleof utilization, planning, and delivery of IMRT in thetreatment of NPC, as well as to review the clinicaloutcome reported in the literature.16.2Technical Advantages of IMRTRadiation beams delivered in conventional radiationtherapy provide universal intensity within each beam.Therefore, dose distribution generated from each radiationbeam is largely determined by the density anddepth of the tissue. In IMRT, each radiation beam issubdivided into numerous small segments of beams(pencil beams). And the intensities of the neighboringpencil beams have different intensity. Collectively, thebeams composed of segments with different intensityproduce dose distributions that conform to the requiredshape of the targets. Generally, IMRT is considered as amore advantageous radiation treatment technique as itcan deliver high-dose irradiation to defined tumor targetswhile minimizing the dose delivered to the surroundingnormal organs and tissues, thereby improvingthe therapeutic ratio of radiation therapy.The utilization of IMRT is of particular importancein nasopharyngeal carcinoma. As detailed below, anumber of critical normal structures are in close proximityto the nasopharynx. Furthermore, the propensityof disease extension to the parapharynx, base ofskull, and intracranially makes irradiation of cochlea,spinal cord, brainstem, pituitary, or optic chiasm inevident.IMRT has been shown to offer superior doseconformity to the tumor target and better sparing ofcritical organs in the treatment of NPC in several studies.And the advantage of IMRT has been demonstratedin patients with all stages (Wu et al. 2004; Hunt et al.2001; Xia et al. 2000; Kam et al. 2003). IMRT offersthe potential for improved tumor control through doseescalating to the tumor targets and high-risk subclinicaldisease regions, while spares normal structuressuch as parotid glands with sharp dose gradients.16.3Implementation of IMRT in NPC16.3.1Patient Setup and Planning CTAlthough physiologic movement is usually not a substantialissue in radiation therapy for NPC, patients’immobilization and setup accuracy are important factorsinfluencing the definition of the planning targetvolumes (PTV). The patient’s head position should beextended position, and immobilization device shouldinclude neck and shoulder (Fig. 16.1). A thermoplasticmask that covers head only may not be sufficient forneck immobilization. In a report by Gilbeau et al.(2001), the setup accuracy of three different thermoplasticmasks used for immobilization of patients withbrain or head and neck tumors was compared. Totaldisplacements were in the range of 2–5 mm (1 SD). Atthe shoulder level, setup variations are reduced whenhead and shoulder masks are used. For isocenters inthe head and in the neck, head mask is as good as headand shoulder mask, while the setup reproducibilitywas found to be significantly worse at the level of theshoulders with the head mask. Therefore, a thermoplastichead and shoulder mask is strongly recommendedfor head and neck immobilization to ensureaccurate patient daily set-up in IMRT for NPC.Treatment planning CT scan is required to definegross target volume(s) (GTV) and clinical target volumes(CTV), as well as determining the planning targetvolumes. All regions to be irradiated must beincluded in the CT scan. CT scan thickness should be0.3 cm or smaller slices through the region that containsthe primary target volumes. The regions aboveand below the target volume may be scanned withslice thickness of 0.5cm (Lee et al. 2006). As MRI issuperior to CT to demonstrate the extension of primarytumor, especially when targets extend near thebase of skull and/or intracranially, MRI fusion with atreatment planning CT is highly recommended duringtarget delineation. Emami et al. (2003) comparedCT and MRI target volumes for NPC using CT, MRI,and fused CT/MRI for defining various target volumes(GTV, CTV and PTV) for eight patients. The