Familial Nasopharyngeal Carcinoma 6

224 J. J. Lu, V. Grégoire, and S. LinFig. 17.9. Illustration of thegross tumor volume (GTV),clinical target volume(CTV), planning targetvolume (PTV), and internaltarget volume (ITV)defined by ICRU Report 50and ICRU Report 62.Irradiated VolumeTreated VolumePTVCTVIrradiated VolumeTreated VolumePTVITVCTVGTVGTVICRU 50 ICRU 62and CT scan can be used when MRI is not readilyavailable. In addition, MRI images should be used(through fusion with planning CT) for better definingand delineation of the primary disease. In a studyaimed to compare CT and MRI target volumes forNPC and evaluate the role of IMRT in treating compositeCT + MRI targets, Emami et al. (2003) foundthat that MRI-based targets were 74% larger, moreirregularly shaped, and did not always include the CTtargets. When CT-based plans were compared withthose based on CT + MRI targets, 14% underdosingwas found, and doses to the OARs were significantlysuboptimal. Approximately 20% of dose reductioncould be achieved using targets delineated based onCT and MRI fusion.Functional imaging, especially FDG-PET/CT, hasbeen studied in the diagnosis and staging ofnasopharyngeal carcinoma. However, whether FDG-PET/CT is more superior to MRI or enhancing CTin detecting and staging of NPC is pending for furtherinvestigation. In a study of 52 patients withstage III and IV NPC, King et al. (2008) found thatMRI demonstrated more extensive disease innasopharynx, skull base, brain, and/or orbit whencompared with FDG-PET/CT. The addition of FDG-PET/CT to MRI did not change the overall stage ormanagement strategy in any patient. Similar findingswere reported in a series of 111 patients withhistologically confirmed NPC. PET/CT showed adiscrepancy with head-and-neck MRI in nearly onethirdof the patients, and MRI appeared to be superiorto PET/CT for the assessment of locoregionalinvasion and retropharyngeal nodal metastasis (Nget al. 2009). Whether PET/CT in addition to MRIcould facilitate the definition and delineation of theGTV in NPC treatment planning is largely unknown.Further investigation is needed to study the optimalutilization of PET/CT in the delineation of tumorvolumes and radiation planning in NPC treatment.One intriguing issue in defining and delineationof the primary disease in IMRT is whether the entiremucosa of nasopharynx should be included as theGTV for high-dose irradiation. Because of the invasivenature of the disease and its high probabilityof submucosal extension, the entire nasopharynxwith a safety margin is usually encompassed in thedefinitive dose region in radiotherapy. However,whether to include the mucosa as part of the GTV,or defining the disease area visualized on enhancedCT, MRI, or FDG-PET/CT as GTV but include the“normal” nasopharyngeal mucosa as a high-riskregion to be irradiated in high-dose coverage inIMRT is debatable, and probably possesses no clinicalsignificance. Either method of GTV definitionand delineation has been reported in studies onIMRT for NPC. It seems that substantial differencesin treatment outcome is unlikely after high-doseradiation therapy using IMRT (Lee et al. 2002; Kamet al. 2004; Lin et al. 2009; Wolden et al. 2006;Tham et al. 2009a).