Familial Nasopharyngeal Carcinoma 6

Advances in the Technology of Radiation Therapy for Nasopharyngeal Carcinoma 201conventional radiation for NPC. As IMRT has the abilityto minimize the dose received by the normal surroundingtissues and organs, parotid gland sparing isof particular importance in the treatment of NPC withIMRT. Approximately 70% of the saliva are producedby the parotid glands in a healthy individual.Xerostomia can be observed after 1–2 weeks of conventionalradiation therapy with a minimal dose of20 Gy or less (Wescott et al. 1978). And permanentsalivary dysfunction is common after a mean dose of26–30 Gy if the entire gland is encompassed in thetreatment volume (Eisbruch et al. 2001). Therefore,the dose constraints of the parotid glands should belimited to 26 Gy or less, which should be achieved in atleast one parotid gland. Alternatively, at least 20 cc ofthe combined volume of both parotid glands shouldreceive 20 Gy or less, or at least 50% of one gland shouldreceive 30 Gy or less.While submental glands are usually spared in mostportals of IMRT definitive treatment of NPC, the submandibularglands are at risk in patients with positivecervical lymph adenopathy. Submandibular glands arein close proximity to the level II neck nodes, which arethe most frequently involved neck nodal group. Forpatients with involved level II neck nodes, encompassingthe submandibular gland(s) are recommended toensure sufficient treatment margins to the gross diseasesin the neck. However, it is recommended that irradiationto the submandibular glands are kept at minimalif possible to reduce the symptoms of xerostomia.by dose calculation, and lastly the display, evaluation,and modification of the dose distributions (Lee et al.2004). A thoroughly prepared “forward planning”treatment plan could provide a relative optimal dosedistribution for the treatment of NPC (Fig. 16.2).However, owing to the complexity of the anatomy inthe head and neck area, particularly adjacent to thenasopharynx, the workload associated with an optimaltreatment planning is usually insurmountable.To fully use the potential of IMRT in nasopharyngealcancer, IP is required (Chui et al. 2001a, b;Nutting 2003). The process of IP initiates with clinicalobjectives that are specified mathematically. It is aresult-oriented paradigm, which is based on thedesired dose distributions to the tumor targets, subclinicalregions, as well as normal organs and tissuesat risk. According to the defined targets and thedesired dose distribution, the computerized optimizationalgorithm subdivides each of the radiationbeams into a number of segments with differentintensity (thus modulates the intensity of radiation).The combined radiation beams composed of segmentswith various intensities then produce a 3Ddose distribution that tailors the irregular shapes ofthe targets (Fig. 16.3 and 16.4).Although forward planning IMRT could provideacceptable dose coverage and normal tissue sparing in16.4Treatment Planning and Delivery16.4.1Treatment PlanningVarious methods of beam intensity modulation havebeen used in an attempt to differentiate the dosebetween tumor targets and the adjacent normal tissuesand organs. Prior to the introduction of inverseplanning (IP), beam modulation using wedge filtersto variably attenuate the radiation beam was used. Inaddition, the use of field-in-field technique to delivermultiple levels of intensity has also been tried. Thesetwo samples represent the simple forms of beam modulationand “forward planning” intensity-modulatedradiation techniques. The planning initiates withactively defining the beam directions and shapes,beam weights, wedges, blocks, and margins, followedFig. 16.2. A representative slice of a forward plan for a patientwith NPC. (From Poon et al. [2007]. Used with permissionfrom Elsevier)