Background: To evaluate the pros and cons of 9-field intensity modulated radiotherapy (IMRT) compared to 7-field IMRT in the treatment of nasopharyngeal carcinoma (NPC).
Methods: Ten NPC patients were treated with 7F-IMRT and 9F-IMRT. A dose prescription of 70 Gy was delivered in 35 fractions to gross planning target volume (PTV1). Plan verification was performed via 2D-array and film dosimetry. Dose-Volume Histogram (DVH) parameters were used to evaluate the quality of IMRT plans.
Results: Dose data for the investigated planning techniques obeyed the Radiation Therapy Oncology Group (RTOG) protocol no. 0615. The dose delivered to PTV1 and organs-at-risk (OARs) for 9F-IMRT was significantly better than 7F-IMRT, except for OARs which were at a distance from PTV1, such as eyes, optical nerves, and chiasma. Ninety five percent of PTV1 was covered by more than 95% of the prescribed dose (67.75 ± 1.1 Gy and 68.57 ± 1.2 Gy for 7F-IMRT and 9F-IMRT, respectively). The maximum dose to 1% of brainstem was 50.06 ± 2.7 Gy and 47.75 ± 2.6 Gy for 7F-IMRT and 9F-IMRT, respectively. Dose verification showed good agreement with treatment planning system with a maximum deviation for 2D-array of 2.16% ± 0.86 and 1.73% ± 0.33 for 7F-IMRT and 9F-IMRT, respectively. Similarly, radiochromic film reported maximum dose deviations of 3.38% ± 1.68 and 2.77% ± 1.3, respectively.
Conclusion: 9F-IMRT provides better homogenous dose to PTV1 and more sparing of OARs over 7F-IMRT for NPC patients, except for OARs which are are a distance from PTV1.
Keywords: Intensity modulated radiotherapy; nasopharyngeal carcinoma; radiation dosimetry; radiation therapy oncology group.