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. 2018 Apr 4;13(1):62.
doi: 10.1186/s13014-018-1012-3.

Efficacy of virtual block objects in reducing the lung dose in helical tomotherapy planning for cervical oesophageal cancer: a planning study

Makoto Ito  1   2 , Hidetoshi Shimizu  3 , Takahiro Aoyama  3 , Hiroyuki Tachibana  3 , Natsuo Tomita  3 , Chiyoko Makita  3 , Yutaro Koide  3 , Daiki Kato  3 , Tsuneo Ishiguchi  4 , Takeshi Kodaira  3
Affiliations

Efficacy of virtual block objects in reducing the lung dose in helical tomotherapy planning for cervical oesophageal cancer: a planning study

Makoto Ito et al. Radiat Oncol. .

Abstract

Background: Intensity-modulated radiotherapy is useful for cervical oesophageal carcinoma (CEC); however, increasing low-dose exposure to the lung may lead to radiation pneumonitis. Nevertheless, an irradiation technique that avoids the lungs has never been examined due to the high difficulty of dose optimization. In this study, we examined the efficacy of helical tomotherapy that can restrict beamlets passing virtual blocks during dose optimization computing (block plan) in reducing the lung dose.

Methods: Fifteen patients with CEC were analysed. The primary/nodal lesion and prophylactic nodal region with adequate margins were defined as the planning target volume (PTV)-60 Gy and PTV-48 Gy, respectively. Nineteen plans per patient were made and compared (total: 285 plans), including non-block and block plans with several shapes and sizes.

Results: The most appropriate block model was semi-circular, 8 cm outside of the tracheal bifurcation, with a significantly lower lung dose compared to that of non-block plans; the mean lung volumes receiving 5 Gy, 10 Gy, 20 Gy, and the mean lung dose were 31.3% vs. 48.0% (p < 0.001), 22.4% vs. 39.4% (p < 0.001), 13.2% vs. 16.0% (p = 0.028), and 7.1 Gy vs. 9.6 Gy (p < 0.001), respectively. Both the block and non-block plans were comparable in terms of the homogeneity and conformity indexes of PTV-60 Gy: 0.05 vs. 0.04 (p = 0.100) and 0.82 vs. 0.85 (p = 0.616), respectively. The maximum dose of the spinal cord planning risk volume increased slightly (49.4 Gy vs. 47.9 Gy, p = 0.002). There was no significant difference in the mean doses to the heart and the thyroid gland. Prolongation of the delivery time was less than 1 min (5.6 min vs. 4.9 min, p = 0.010).

Conclusions: The block plan for CEC could significantly reduce the lung dose, with acceptable increment in the spinal dose and a slightly prolonged delivery time.

Keywords: Cervical oesophageal cancer; Complete block; Directional block; Helical tomotherapy; Intensity-modulated radiation therapy; Lung dose.

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Conflict of interest statement

Ethics approval and consent to participate

All procedures involving human participants were in accordance with the ethical standards of the institutional research committee, and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Consent for publication

All patients gave written informed consent.

Competing interests

The authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures

Fig. 1
Fig. 1
Axial image of virtual blocks. a Five fan-shaped blocks in increments of 10 degrees between 40 and 80 degrees, which set the tracheal bifurcation as a central point. b Four semi-circular blocks in increments of 1 cm between 5 and 8 cm from the tracheal bifurcation
Fig. 2
Fig. 2
Comparison of the dose distribution of non-block and block planning. a Non-block plan; (b) semi-circular plan for block 7. Each plan was made using the same target volume pertaining to a single patient. The dose gradient of block 7 was steeper than that of the non-block plan. Block 7: semi-circular virtual block that contoured the lungs outside a distance of 7 cm from the tracheal bifurcation
Fig. 3
Fig. 3
Comparison of the dose-volume histogram of non-block and block planning. Although the V20 values of the lungs were comparable, with approximately 15% between the plans, V5 and V10 of the block plans were much lower than those in the non-block plan (black arrow).V20, the lung volume receiving 20 Gy; V5, the lung volume receiving 5 Gy; V10, the lung volume receiving 10 Gy
See this image and copyright information in PMC

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