Treatment planning comparison between coplanar and non-coplanar intensity modulated radiation therapy for late-stage nasopharyngeal carcinoma

Abstract

Purpose/Objective(s): The purpose of this study was to compare the dosimetric difference between coplanar and non-coplanar intensity-modulated radiation therapy (IMRT) for late-stage nasopharyngeal carcinoma (NPC). The potential benefits of non-coplanar IMRT in treatment of late-stage NPC would be highlighted so as to provide radiation oncologist with an alternative option for individual clinical case.

Methods: Ten late-stage (Stage III and IV) NPC patients were planned by both 1) non-coplanar IMRT and 2) coplanar IMRT using the 〖Philips Pinnacle〗^3 version 9.6 treatment planning system. Standard 9-field IMRT with each beam separated by 40 degrees would be used for coplanar IMRT. For non-coplanar IMRT, 9 coplanar beams which orientations are the same as that used in coplanar IMRT would be applied and 2 extra non-coplanar beams of 90 degree couch angle would be added. The dose levels of planning target volumes (PTVs) with 70/60/54 Gy in 33 fractions were prescribed and delivered as a simultaneous integrated boost. The homogeneity index, conformity index of PTVs, dose to organs at risk (OARs), such as, brainstem, spinal cord, optic chiasm, optic nerves, lens and temporal lobe and monitor units (MU) would be analyzed.

Results: All plans fulfilled the departmental planning criteria. Non-coplanar IMRT showed better homogeneity and PTV coverage than coplanar IMRT. Significant reduction of doses for optic chiasm and right lens were observed in coplanar IMRT when comparing with non-coplanar IMRT. A reduction of maximum dose of optic chiasm by 4.29±4.02 Gy was measured. A reduction of maximum dose of right lens by 0.10±0.11 Gy was obtained. Monitor unit was also found reducing by 131.7±93.1 MU.

Conclusion: In late-stage NPC cases, non-coplanar IMRT provided better results in terms of PTV coverage while coplanar IMRT provided better sparing of OARs. Non-coplanar technique could be a feasible choice for cases in which coplanar IMRT could not achieve satisfactory dosimetric results.