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

Abstract

BackgroundIntensity-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.MethodsFifteen 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.ResultsThe 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).ConclusionsThe block plan for CEC could significantly reduce the lung dose, with acceptable increment in the spinal dose and a slightly prolonged delivery time.

Impact of treatment planning using a structure block function on the target and organ doses related to patient movement in cervical esophageal cancer: A phantom study

“…Thus, using the virtual contour shape in this study for a treatment plan in the DB mode for cervical esophageal cancer may keep the dose distribution changes that result from patient movement within a clinically acceptable level. In our previous study, we evaluated the dose reduction to the OARs and dose concentration to the targets using various virtual contour shapes; the virtual contour shape used in the present study was the one that scored the highest in the previous study, which was shown to be the most clinically useful for 20 patients . This virtual contour shape, a semicircle that follows the shape of the lung at a distance of 8 cm from the tracheal bifurcation, can effectively reduce the dose to OARs and provide safe delivery, even with patient movement, with the same PTV margin as the NB mode.…”

“…Figure (b) illustrates the DB mode, showing how beamlet A, which reached the virtual contour before passing through the PTV, is not included in the dose optimization process, whereas beamlet B, which reached the contour after passing through the PTV, is included. In our previous study, the DB mode with the virtual contour shape used in the present study could create treatment plans for 20 patients without any clinical problems . We therefore used the DB mode in the present study.…”

“…They used fan‐shaped virtual contours in the lungs, assessed with a virtual esophageal tumor delineated on the CT image of a phantom . Building on that study, our research group evaluated dose reductions to the organs at risk (OARs) and dose concentrations at the target using various virtual contour shapes for cervical esophageal cancers in 20 patients . This showed that a semicircular contour following the shape of the lung at a distance of 8 cm from the tracheal bifurcation was the most clinically useful, when the dose reduction to the OARs and the concentration of the dose at the target were considered as a single index .…”

“…Building on that study, our research group evaluated dose reductions to the organs at risk (OARs) and dose concentrations at the target using various virtual contour shapes for cervical esophageal cancers in 20 patients . This showed that a semicircular contour following the shape of the lung at a distance of 8 cm from the tracheal bifurcation was the most clinically useful, when the dose reduction to the OARs and the concentration of the dose at the target were considered as a single index . The restriction of beamlets in dose optimization of HT has been applied to reduce the dose to OARs in various other conditions as well as esophageal cancers .…”

Planning evaluation of a novel volume-based algorithm for personalized optimization of lung dose in VMAT for esophageal cancer

“…Many methods for reducing the lung dose have been reported in dynamic arc radiotherapy 9 , 10 . However, the selection of gantry arc angle and dose constraints are the key factors in radiotherapy treatment planning (RTP).…”