Comparison of Dosimetric Benefits of Three Precise Radiotherapy Techniques in Nasopharyngeal Carcinoma Patients Using a Priority-Classified Plan Optimization Model

Abstract

Introduction: Although intensity-modulated radiotherapy (IMRT), volumetric-modulated arc therapy (VMAT) and tomotherapy (TOMO) are broadly applied for nasopharyngeal carcinoma (NPC), the best technique remains unclear. Therefore, this study was conducted to address this issue.

Methods: The priority-classified plan optimization model was applied to IMRT, VMAT and TOMO plans in forty NPC patients according to the latest international guidelines. And the dosimetric parameters of planning target volumes (PTVs) and organs at risk (OARs) were compared among these three techniques. The Friedman M test in SPSS software was applied to assess significant differences.

Results: The median PGTVnx coverage of IMRT was the lowest (93.5%, P < 0.001) for all T categories. VMAT was comparable to TOMO in OARs clarified as priority I and II, and both satisfied the prescribed requirement. IMRT resulted in a relatively high dose for V25 and V30. Interestingly, subgroup analysis showed that the median PTV coverage of the three techniques was no less than 95% in the early T stage. The heterogeneity index (HI) of PGTVnx in VMAT was better than that in IMRT (P = 0.028). Compared to TOMO, VMAT showed a strong ability to protect eyesight and decrease low-dose radiation volumes. In the advanced T stage subgroup, TOMO numerically achieved the highest median PGTVnx coverage volume compared with VMAT and IMRT (93.61%, 91% and 90%, respectively). The best CI and HI of PCTV-1 were observed in TOMO. Furthermore, TOMO was better than VMAT for sparing the brain stem, spinal cord and temporal lobes (all P < 0.05). However, the median V5, V10, V15, V20 and V25 were significantly higher with TOMO than with VMAT (all P < 0.05).

Conclusion: In the early T stage, VMAT provides a similar dose coverage and protection of OARs to IMRT, and there are no obvious advantages to choosing TOMO for NPC patients in the early T stage. TOMO may be recommended for patients in the advanced T stage due as it provides the largest dose coverage of PGTVnx and the best protection of the brain stem, spinal cord and temporal lobes. Additionally, more randomized clinical trials are needed for further clarification.

Keywords: dosimetry; intensity-modulated radiotherapy; nasopharyngeal carcinoma; tomotherapy; volumetric-modulated arc therapy.

Figure 1

Isodose distributions and dose-volume histograms (DVHs) for a representative T₁-stage NPC patient with IMRT (left), VMAT (middle) and TOMO (right) planning. Maroon, forest, lavender, yellow-green, light-blue, red, green and blue lines in three DVH are optic nerve in left, optic nerve in right, optic chiasma, brain stem, spinal cord, PGTVnx, PCTV-1 and PCTV-2, respectively. For IMRT and VMAT planning, color-wash areas: PGTVnx (red), PGTVnd-left (purple), PGTVnd-right (yellow), PCTV-1 (green), PCTV-2 (blue); and the red, purple and sky-blue lines are isodose curves of 69.96Gy, 59.4Gy and 54Gy. For TOMO planning, isodose curves of 69.96Gy, 59.4Gy and 54Gy are shaded in the red, purple and sky-blue, respectively; targets are represented by lines: PGTVnx (red), PGTVnd-left (purple), PGTVnd-right (yellow), CTV-1 (green), PCTV-2 (blue).

Figure 2

Isodose distributions and dose-volume histograms (DVHs) for a representative T₄-stage NPC patient with IMRT (left), VMAT (middle) and TOMO (right) planning. Maroon, forest, lavender, yellow-green, light-blue, red, green and blue lines in three DVH are optic nerve in left, optic nerve in right, optic chiasma, brain stem, spinal cord, PGTVnx, PCTV-1 and PCTV-2, respectively. For IMRT and VMAT planning, color-wash areas: PGTVnx (red), PGTVnd-left (purple), PGTVnd-right (yellow), PCTV-1 (green), PCTV-2 (blue); and the red, purple and sky-blue lines are isodose curves of 69.96Gy, 59.4Gy and 54Gy. For TOMO planning, isodose curves of 69.96Gy, 59.4Gy and 54Gy are shaded in the red, purple and sky-blue, respectively; targets are represented by lines: PGTVnx (red), PGTVnd-left (purple), PGTVnd-right (yellow), PCTV-1 (green), PCTV-2 (blue).