Familial Nasopharyngeal Carcinoma 6

Advances in the Technology of Radiation Therapy for Nasopharyngeal Carcinoma 199Fig. 16.1. Thermoplastic mask system extending from vertexof scalp to shoulder for immobilization of the patient in thetreatment of nasopharyngeal carcinomaresults revealed that MRI-based targets were 74%larger, more irregularly shaped, and did not alwaysinclude the CT targets when compared with CT-basedcontouring. CT/MRI fusion improved the determinationof target volumes in NPC. If possible, the patientimmobilization device should also be used for theMRI scan.16.3.2Definition and Dose Specificationsof Target VolumesIMRT offers superior dose conformity to the tumortargets with relative sparing of critical organs and tissuesin the treatment of all stage NPC (Wu et al. 2004;Hunt et al. 2001; Xia et al. 2000; Kam et al. 2003).However, as IMRT provides sharp dose fall-off gradientbetween the tumor targets and surroundingnormal tissues/organs, adequate and accurate targetvolume delineation is absolutely essential. Incorrector insufficient delineation can result in tumorrecurrence.A detailed description of delineation of GTV, theselection and delineation of CTV, and the definitionof PTV is beyond the scope of this chapter, and isdetailed in chapter 17.Briefly, GTV is defined as all detectable tumor tissueobserved on imaging studies and physical examination.CTV is defined as a tissue volume that contains ademonstrable GTV and/or subclinical malignant diseasethat must be eliminated. The CTVs of NPC include thesubclinical disease surrounding the GTV and regionallymphatics. At present, CTV is very much based on ourclinical knowledge for potential spread of NPC and patternof failure after treatment; however, no universalagreed guideline is available for clinical use. The RTOGhas set guidelines to use in multiinstitutional protocols.GTV-P (for primary tumor) and GTV-N (for nodal disease)with a margin of >=5mm are called CTV 70-Pand CTV 70-N, respectively. This margin can be reducedto as low as 1 mm for tumors in close proximity to criticalstructures, e.g., tumors abutting the brainstem.Certain regions are at high risk for microscopicdisease. These regions include the areas in closeproximity to the GTV(s) and the more direct draininglymph nodal regions from nasopharynx: Theentire nasopharynx, anterior 1/2 to 2/3 of the clivus(entire clivus, if involved), skull base (including bilateralforamen ovale and rotundum in all cases), pterygoidfossae, parapharyngeal space, inferior sphenoidsinus (in T3–T4 disease, the entire sphenoid sinus),and posterior fourth to third of the nasal cavity andmaxillary sinuses (to ensure pterygopalatine fossaecoverage). The cavernous sinus should be included inhigh-risk patients (T3, T4, bulky disease involvingthe roof of the nasopharynx). Lymph nodal drainagein NPC usually follows an orderly pattern. Therefore,the high-risk lymph nodal regions (CTV 59.4-N)include level II, III, V, and retropharyngeal nodes.Level IB nodes are at higher risk if ipsilateral level IInodes are clinically involved. Once level III nodes areinvolved, the low jugular (level IV) and supraclavicularlymph nodes should be considered at high risk.As higher radiation dose in the range around 59.4 Gyis recommended to treat these subclinical regions, it isknown as CTV for high-risk subclinical disease or CTV59.4. The outermost boundary of CTV 59.4 of the primarytumor should be at least 10 mm from the GTV.The low-risk CTV (CTV 54) includes bilateral uninvolvedlower neck nodal regions for patients with N0disease or with Level II node adenopathy only.The PTV should provide a margin around the CTVsto compensate for the variabilities of treatment set upand internal organ motion. Studies should be implementedby each institution to define the appropriatemagnitude of the uncertain components of the PTV. Aminimum of 3–5 mm around the CTVs is usuallyrequired in all directions to define each respective PTV.Although GTVs and CTVs form the most clinicallyrelevant target volumes, radiation doses are prescribedto PTVs. Various dosing regimens have beenused and reported. Although outcomes reported inliteratures cannot be directly compared, nosubstantial differences regarding tumor control and