Impact of different IMRT techniques to improve conformity and normal tissue sparing in upper esophageal cancer
Purpose: Intensity modulated radiotherapy (IMRT) for cervical esophageal cancer is challenging. Although IMRT techniques using inverse planning algorithms are facilitating the treatment planning process, the irradiation dose to the normal tissues can be a critical issue. This study was performed to investigate the effect of beam numbers and their directions and local optimization on: (1) dose conformity and homogeneity to the planning target volume (PTV) and (2) dose to the organ at risks (OARs).
Methods: Four upper esophageal cancer cases were randomly selected for this treatment planning study. Eight IMRT plans were generated for each case with the same dose-volume constraints but with different beam numbers and arrangements. Local optimization using regular structures drawn automatically around the PTV with margins from 0.5-1.5 cm was performed. IMRT plans were evaluated with respect to isodose distributions, dose-volume histograms (DVHs) parameters, homogeneity index (HI), and conformity index (CI). The statistical comparison between the types of plans was done using the One Way ANOVA test.
Results: The results showed that IMRT using three or five beams was not sufficient to obtain good dose optimization. The seven field plans showed the best coverage for the PTV with tolerable doses for the OARs, and the beam orientation was very critical. Increasing beams (Bs) number from 7 to 13 did not show significant differences in the PTV coverage, while the mean lung dose was increased. The PTV coverage were 95.1, 95.1, 98.1, 97.3, 97.3, 97.3, 97.0, and 97.0% for 3Bs, 5Bs, 7Bs, 9Bs, 13Bs, 7Bs(30), 7Bs(60) (beam angles were changed from 0o to 30o and 60o), and 7Bs(R) (seven IMRT plans with ring), respectively. The mean heart dose did not exceed 0.36 Gy with p < 0.05. For lung doses, the best plan was the one with 9Bs which reduced lung volume doses V20Gy (%) and V30Gy (%), and reduced mean lung dose from 5.4 to 4.5 Gy with p < 0.05 for 7Bs(R) plans. IMRT improved the homogeneity indices (p > 0.05), yet conformity was better with 9Bs and 7Bs(R) IMRT plans with p < 0.05.
Conclusion: Seven equispaced coplanar intensity-modulated beams with an addition of a ring structure can produce desirable dose distributions to the PTV. Moreover, dose-volume of exposed normal lung can be reduced with 9Bs and 7Bs(R) IMRT plans.
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