1 INTRODUCTION
Figure 1: Sectional view anatomy of nasopharynx
The nasopharynx is cuboidal in shape which locate at upper part of the pharynx that lies behind nose. Tumour most frequently in the lateral walls or roof (Hanna, Crosby, & Macbeth, 2008). Direct superior extension of tumour leads to destruction of the skull base with involvement of cranial nerve III to VI lying in the covernous sinus (Hansen & Roach, 2010).
2 PATIENT PRE-DIAGNOSIS
I. Presenting signs/ symptoms
Symptoms that related to this disease are hearing loss, otitis media, neck mass, nasal obstruction, epistaxis, headache, diplopia and trismus (Hanna et al., 2008; Hansen & Roach, 2010).
II. Diagnosis
Several procedures need to perform to diagnose this disease which are fiberoptic nasopharyngolaryngoscopy with biopsy, and neck nodes exam and liver via ultrasound (Abbasi et al., 2013). CT or MRI with contrast should be used to assess the extent of primary disease . Corresponding to the examination, staging of the tumour is done. As an example is T4N2Mx where T4 (tumor with intracranial extension and/or involvement of cranial nerves, hypopharynx, or orbit, or with extension to the infratemporal fossa/masticator space), N2 (bilateral metastasis in cervical lymph nodes = 6cm in greatest dimension, above the supraclavicular fossa) and Mx: presence of metastases cannot be assessed trismus (Hanna et al., 2008; Hansen & Roach, 2010; Barrett et al. , 2009; Taheri-kadkhoda, 2007).
3 TREATMENT DETAIL
Radiotherapy is the principal treatment modality in nasopharyngeal cancer (NPC) and is often combined with chemotherapy in advanced disease (stage IIb, III and IV)(Kristensen et al., 2007; Peponi et al., 2015). Dose prescribed was given within fraction and delivered using LINAC and treatment is calssified either radical or palliative. Surgery role to treat nasopharyngeal cancers at the primary disease site is very limited due to anatomy limitation. Metastasis or locally advanced nasopharyngeal carcinoma is not possible to treat by surgery.3D conformal radiotherapy is providing improvement in dose distribution conformity to the tumor volume while concomitantly declining dose to the surrounding normal structure(Wu et al., 2000; Abbasi et al., 2013). This protocol is usually carried out based on CT scan based planning (3D treatment planning) offers quantitative parameters for evaluation of dose distributions in target volumes and OARs (Taheri-kadkhoda, 2007). Some treatment planning based on fusion of MRI (Kristensen et al., 2007).
4 PRE-TREATMENT IMAGING AND SIMULATION
Process in patient data acquisition include:
CT Nasopharynx
Patient is positioned on the CT examination table, head near the scanner and hand at the body sides. Marker is put at the region of interest as for verification. Contrast media is injected via intravenous injection (IV). The CT scan slices measuring 3–5mm are taken from the vertex to the level of C6 (Wu et al., 2000; Barrett et al., 2009). Then, planning target volume (PTV) is validated by the oncologist.
CT image is used for treatment planning to determine number of field to cover the tumour volume, weighting and isodose distribution. The contour of CTV, GTV, PTV and critical organs include spinal cord, brain stem, optic chiasma and parotic gland are drawn in each axial CT slice (Chau et al., 2001).
Figure 2: Isodode distribution of NPC treatment plan using Plato RTS v2.5.2 system software
Treatment plan is created using Plato RTS v2.5.2 system software. Based on the isodose line in Figure 2, isodose lines of 20, 40, 60, 80, 100, 120, 140, 160, 180, 200, 210, 220 and 230 are form of 3 beam of radiation field. The dose is prescribed 200 cGy to 100% of isodose line.
Figure 3: Comparison of the mean DVH of PTV, temporal lobes, optic chiasma and brainstem between Ho’s technique and modified Ho’s technique using BEV in setting field and customized shielding block in phase II (Chau et al., 2001)
2D fluoroscopy simulator
Simulation of patients in radiotherapy is done to ensure that the radiation beams used for treatment are correctly chosen and properly aimed at the intended area. First step, head rest is put on the table. Patient is in supine position with head positioned on the head rest. The neck is extended to elevate chin but spinal cord is straight as possible if posterior neck nodes are present (Barrett et al., 2009). The choice of head rest depended on the patient. Shoulder rest, SR’1’ and SR’2’ might be used to ensure patient shoulder is flat and simulation is accurate.
Figure 4: Diagram showing lateral opposed and anterior field with shielding
The separation is measured. Landmark is drawn on patient’s facial skin. Patient is treated using SSD technique. Beam direct shell (BDS) is soaked in warm water. The fluoroscopic machine is used to stimulate beam angle identical to the treatment plan. Then, BDS is put on the patient’s facial and formed according facial shape. The marker is drawn on the BDS. The BDS is written patient’s name.
5 RADIOTHERAPY PARAMETER
Patients are treated with 6 MV photon beam using linear accelerator (Wu et al., 2000; Chau et al., 2001; Zheng et al., 2005; Abbasi et al., 2013; Peponi et al., 2015). The usual beam arrangement are lateral opposed field and anterior facial. In order to treat NPC cases, dose required is greater than tolerance dose of normal surrounding structure. Thus, treatment id divided into 3 phase.
In phase I, 40 Gy of dose is prescribed to the tumour and deliver within 20 fractions. The treatment time is 5 days per week and will finish within 4 weeks. Lateral parallel opposed field approach are used that extend 1cm superior to the cranial extent of NPC tumor to the as low as reasonably possible by keeping the fact of shoulder clearance in lateral fields (Hanna et al., 2008; Abbasi et al., 2013). Anterior field is used to treat lower cervical lymph nodes and SCF nodes (Hanna et al., 2008). OARs are brain stem, optic chiasm and oral cavity are shielded using the MLC (Barrett et al., 2009).
Figure 5: Treatment plant for lateral field for phase 1
Figure 6: Treatment plan for anterior facial
For phase II, 20 Gy of dose is prescribed to the tumour for 10 fractions and also three-beams which are lateral opposed and anterior field (Barrett et al., 2009). Thefore, treatment time is 5 days per week and will finish within 2 weeks. Posterior border of lateral fields moved anteriorly to shield the spinal cord (Abbasi et al., 2013) and reduce temporal lobe and TMJ dose (Barrett et al., 2009). The anterior field is enlarged to cover all cervical lymphatics. While anterior border of lateral field is located to bisect the maxillary antrum anteroposteriorly superior field border is located at 0.5 cm above anterior clinoid process. The posterior border is at the posterior surface of the odontoid process (Chau et al., 2001).
In phase III, 10 Gy in 5 fractions with additional shielding to reduce the treated volume (Barrett et al., 2009). Others practice is boosting the enlarged lymph nodes with 60 - 66Gy with electron and boost to the primary target volume that had minimal intracranial extension with 66 – 70 Gy (Abbasi et al., 2013).
Figure 7: Treatment plan for lateral field for phase 3
Figure 8: Treatment plant for anterior neck to treat lower cervical node
Tolerance dose for the brain is 50 Gy, spinal cord is 45 Gy, eye lens is 6 Gy and eye retina is 50 Gy. During treatment, patient setup same as during simulation which is supine, head and shoulder rest is used if necessary and immobilized using BDS (thermoplastic mask). For the linear accelerator not having MLC, wedge is using to compensate irregular body surface and then, providing uniform dose. Wedges are applied to the two lateral oblique fields with angles ranging from 30° to 60° (Wu et al., 2000).
6 SPECIAL CONSIDERATION
As we concern, nasopharyngeal tumors are located at the base of the skull in close proximity to several sensitive organs such as eyes, optic chiasma, brain stem and salivary gland. Nevertheless, dose to the OARs still high using 3D conformal radiotherapy. This limitation lead to increase using IMRT to treat this cancer. IMRT increased the conformity of the tumour dose and minimum dose to the OARs compared to 3D conformal. Thus, decrease in maximum dose to the spinal cord and brain stem and also enabled protection of a part of the parotid glands (Kristensen et al., 2007).
Figure 9: Comparison between isodose distribution between 2DRT, 3DRT and IMRT in T1M0N0 NPC (Teo, Ma, & Chan, 2004)
7 REFERENCES
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Barrett, A., Dobbs, J., Morris, S., & Roques, T. (2009). Practical Radiotherapy Planning. Scottish Medical Journal (4th ed., Vol. 55). http://doi.org/10.1258/rsmsmj.55.1.29
Chau, R. M. C., Teo, P. M. L., Choi, P. H. K., Cheung, K. Y., & Lee, W. Y. (2001). Three-dimensional dosimetric evaluation of a conventional radiotherapy technique for treatment of nasopharyngeal carcinoma. Radiotherapy and Oncology, 58(2), 143–153. http://doi.org/10.1016/S0167-8140(00)00336-4
Hanna, L., Crosby, T., & Macbeth, F. (2008). Practical Clinical Oncology (1st ed.). United State: Cambridge University Press.
Hansen, E. K., & Roach, M. (2010). Handbook of evidence-based radiation oncology (2nd ed.). California: Springer. http://doi.org/10.1007/978-0-387-92988-0
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Taheri-kadkhoda, Z. (2007). Nasopharyngeal carcinoma : past , present and future directions. Institute of Clinical Sciences Göteborg University SWEDEN.
Teo, P. M. L., Ma, B. B. Y., & Chan, A. T. C. (2004). Radiotherapy for nasopharyngeal carcinoma - Transition from two-dimensional to three-dimensional methods. Radiotherapy and Oncology, 73(2), 163–172. http://doi.org/10.1016/j.radonc.2004.06.005
Wu, V. W. C., Chan, Z. H. R., Kung, S. W. S., Chau, C. K. R., Fu, K. C. K., & Yip, K. K. Y. (2000). in Practice Dose analysis of three 3-D conformal radiotherapy techniques used in booster treatment of nasopharyngeal carcinoma. Journal of Radiotherapy in Practice, 27–36.
Zheng, X.-K., Ma, J., Chen, L.-H., Xia, Y.-F., & Shi, Y.-S. (2005). Dosimetric and clinical results of three-dimensional conformal radiotherapy for locally recurrent nasopharyngeal carcinoma. Radiotherapy and Oncology : Journal of the European Society for Therapeutic Radiology and Oncology, 75(2), 197–203. http://doi.org/10.1016/j.radonc.2005.03.008
