RESUMEN

PURPOSE: To compare the effect of two different intensity-modulated radiation therapy (IMRT) planning techniques on parotid gland doses in patients with nasopharyngeal carcinoma (NPC). PATIENTS AND METHODS: Radiotherapy for 10 NPC patients referred to the University of Istanbul Cerrahpasa Medical School was planned with arc- and static seven-field IMRT. The simultaneous integrated boost (SIB) technique was used to deliver 70 Gy (2.12 Gy per fraction) to the primary tumor and involved nodes; 60 Gy (1.81 Gy per fraction) to the entire nasopharynx and 54 Gy (1.63 Gy per fraction) to elective lymph nodes in 33 fractions. Plans also aimed to keep the mean parotid dose below 26 Gy and limit the maximum doses to the spinal cord and brain stem to 45 and 54 Gy, respectively. Mean parotid gland doses for the two planning techniques were compared using a paired t-test. Target coverage and dose inhomogeneity were evaluated by calculating conformity- (CI) and homogeneity index (HI) values. RESULTS: Target coverage and dose homogeneity were identical and good for both planning techniques: CI = 1.05 ± 0.08 and 1.05 ± 0.08; HI = 1.08 ± 0.02 and 1.07 ± 0.01 for arc- and static field IMRT, respectively. Mean doses to contralateral parotid glands were 25.73 ± 4.27 and 27.73 ± 3.5 Gy(p = 0.008) for arc- and static field IMRT plans, respectively, whereas mean ipsilateral parotid doses were 30.65 ± 6.25 and 32.55 ± 5.93 Gy (non-significant p-value), respectively. Mean monitor units (MU) per fraction for the 10 patients were considerably lower for arc- than for static field treatments-540.5 ± 130.39 versus 1288.4 ± 197.28 (p < 0.001). CONCLUSION: Normal tissues--particularly the parotid glands--are better spared with the arc technique in patients with NPC. MU and treatment times are considerably reduced in arc IMRT plans.

Asunto(s)

Tomografía Computarizada de Haz Cónico/métodos , Neoplasias Nasofaríngeas/radioterapia , Glándula Parótida/efectos de la radiación , Traumatismos por Radiación/diagnóstico , Traumatismos por Radiación/etiología , Planificación de la Radioterapia Asistida por Computador/métodos , Radioterapia de Intensidad Modulada/métodos , Tronco Encefálico/efectos de la radiación , Simulación por Computador , Fraccionamiento de la Dosis de Radiación , Humanos , Ganglios Linfáticos/efectos de la radiación , Metástasis Linfática/radioterapia , Estudios Prospectivos , Dosis de Radiación , Traumatismos por Radiación/prevención & control , Médula Espinal/efectos de la radiación , Xerostomía/diagnóstico , Xerostomía/etiología

RESUMEN

PURPOSE: This study reports the feasibility of the symmetric fields for the usage of asymmetric fields, and checks the accuracy of treatment planning system (TPS). MATERIALS AND METHODS: A GE Saturne-42 linear accelerator, which produces dual photon energies of 6 and 15 MV, and an ISIS-3D TPS were used in this study. Percent depth doses (PDDs), dose profiles, and isodose distributions for symmetric and asymmetric fields were generated in a water phantom and these data were compared to those calculated by the TPS. RESULTS: When the results were compared it was seen that the PDDs decreased with increasing off-axis distance and depth. The PDDs comparison demonstrated that for 15 MV, asymmetric and symmetric field PDDs can differ by as much as 4%, and the differences between the calculated by the TPS and the measured PDDs for asymmetric fields were less than 2%. It was also seen that the penumbra of symmetric and asymmetric fields were very close to each other and the difference between the calculated and the measured ones were less than 2 mm. The comparison between the measured and calculated isodose distributions for 6 and 15 MV photon beams for asymmetric field showed that a good agreement has been achieved in general, and the differences between the field edges and near the centre of the fields were found within 2 mm. CONCLUSION: This study shows that the measured and calculated data using water phantom and TPS (ISIS-3D), respectively, were in good agreement. Therefore TPS can be used safely for asymmetric fields in routine practice.