Dosimetric Comparison Study On Three Therapy Plans For Nasopharynx Carcinoma

Purpose1. To investigate the features of dosage for nasopharyngeal carcinoma (State Ⅰ-Ⅲ) using intensity-modulated radiation therapy (IMRT) in7and9radiation fields and volumetric modulated arc therapy (VMAT).2. To compare therapy time for nasopharyngeal carcinoma (State Ⅰ-Ⅲ) using intensity-modulated radiation therapy (IMRT) in7and9radiation fields and volumetric modulated arc therapy (VMAT).Methods and Materials1. PatientsDuring the period from March2013to September2013,12patients with nasopharyngeal carcinoma (NPC)(four females and eight male; age range,23-70years; mean,48years) were enrolled in the study. All the primary tumors were attributed to T1to T3stage according to the American Joint Commission on Cancer (AJCC)2010staging system. There was one patient (8.3%) with T1-stage,8(66.6%) of T2-stage,3(25%) of T3-stage. All of them were without distant metastases. 2. Methods2.1Immobilization and simulationPatients were immobilized in the supine position with a thermoplastic head and shoulder mask. Intravenous contrast-enhanced CT examination using slice thickness of3mm was performed for planning. The CT data were imported to treatment planning system for treatment design.2.2Target and OAR definitionThe primary gross tumor volume (GTV_P) and involved lymph nodes (GTV_N) included all gross tumors was determined by imaging, clinical, and endoscopic findings. The enlarged retropharyngeal nodes were outlined together with primary GTV, as the GTV_P on the IMRT plans. Two clinical target volumes (CTVs) were defined in our radiotherapy:CTV1and CTV2. TheCTV1should also encompass the entire nasopharynx, skull base, parapharyngeal space, etropharyngeal lymph nodal regions, clivus, inferior sphenoid sinus, pterygoid fossae, the posterior third of the nasal cavity and maxillary sinuses, and any high risk nodal regions, including the bilateral upper deep jugular nodes, and then ear station of the positive lymph nodes. CTV2was defined aslymph nodal regions at low risk including the lymph nodal regions of the neck which were not encompassed in the CTV1. The PTV_C would encompass the CTV with a3-mm margin in all directions. The organs at risk (OAR) include the spinal cord, brain stem, optic chiasm, optic nerves, eyeballs, lens, temporal lobes, parotid glands, oral mucosa, larynx and temporomandibular joints. A3-mm margin was added to the spinal cord and brainstem during optimization to form the planning organ-at-risk volume (PRV)2.3Treatment plansAll patients were treated with external-beam radiation therapy using6-MV photons,7-9radiation fields and VMAT. The treatment technique was simultaneous integrated boost (SIB) technique. The prescribed dose was69.96Gy to PTV_GTV,60Gy to PTV1and54Gy to PTV-2. All patients were treated one fraction per day,5days per week.The dose received by each organ at risk was limited to tolerance as followed: the maximum dose was45Gy to spinal cord,50Gy to spinal cord+3mm,54Gy to brain stem,60Gy to brain stem+3mm,6Gy to lens,50Gy to optic nerves,28Gy to parotid glands.2.4Treatment plan evaluationAccording to the International commission on Radiation Units and Measurements Reports82, Conformity Index (CI) and Homogeneity Index (HI) were calculated. With a ranged of0to1.0, a CI value close to1.0implies that the two volumes closely coincide spatially. Smaller values of HI correspond to more homogenous target volume irradiation, with a value of0indicating absolute homogeneity of dose within the target.Dose volume histograms (DVH) were generated for each treatment plan, and the PTV dose coverage and OAR dosimetry were both used for treatment plan evaluation. The DVH parameters for PTV70, PTV59.4and the OARs were calculated. For the OARs, we recorded the mean dose (Dmean) for the parotid glands and maximum dose (Dmax) for the spinal cord, brain stem, lens, and optic nerves. The homogeneity index, conformity index, and coverage index for PTV70and PTV59.4were also reported.3. Statistical analysisStatistical analysis was performed with the Statistical Package for Social Sciences (SPSS, Chicago, IL) software package, version13.0, for Windows. All data are expressed as mean±standard deviation. Paired-sample comparisons were performed, and the one-way ANOVA and the paired t test were used. A two-tailed P-Value less than0.05was considered as statistical significance in this study.Result1. PTV analysisAll IMRT plans and VMAT achieved the protocol dose criteria. The area of the maximum dose in IMRT plan up to70Gy (PTV70) was0.991±0.003in7beam, and0.993±0.002in9beam; and0.994±0.003in VMAT. PTV60was0.993±0.002in7beam and0.993±0.001in9beam; and0.991±0.003in VMAT. PTV54was0.981±0.003in7beam and in0.985±0.004in9beam; and0.986±0.006in VMAT. There was no significant difference in the three different kinds of Intensity Modulated Radiotherapy0>0.05).All HI and CI achieved the protocol dose criteria. The mean values of HI was shown below:1.03to PTV-GTV in7beam and1.02in9beam;1.11to PTV-1in7fraction and1.10in9fraction;1.14to VMAT;1.26to PTV-2in7fraction and1.24in9fraction;1.23to VMAT. The mean values of HI and CI showed no significant difference (p>0.05).2. OAR dose analysisIn general, OAR doses achieved the protocol dose criteria and were similar in patients. The mean Dmax and Dmean received by brain-stem and optic nerves showed no significant difference. Compared with the IMRT, the mean DmaX received spinal cord was significant higher than that in VMAT. Furthermore, the mean Dmax received spinal cord was significant higher of IMRT in7beam than in9beam.3.Time of treatment and average number of hopsThe MU was significantly smaller in VMAT than IMRT of7and9beam (p<0.001). The mean treatment time is3.3±0.30min for VMAT,11.4±0.49min for IMRT in7beam and12.7±0.76min.Conclusion 1. The target coverage,CI and HI of VMAT, IMRT in7beam and9beam was achieved the protocol dose criteria. There was no significant different among the three group (p>0.05).2. OAR doses of VMAT, IMRT in7beam and9beam was achieved the protocol dose criteria. the VMAT plans did show an increase in some dosimetric parameters for certain OARs than IMRT. The OAR doses of VMAT is slightly higher than that of IMRT group.3. There was no significant difference in the OAR dose of the IMRT in the7and9beam.4. For the aspect of treatment time, the treatment of VMAT is much shorter than that of IMRT(P<0.001),which can markedly improve the accuracy of treatment.5. If there was no significant difference in OARs, the IMRT in7beam was superior to in9beam; otherwise,9beam was superior to7beam on PTV.