PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s): Excessive toxicities have been reported among patients treated with stereotactic body radiotherapy (SBRT) for central lung tumors in or near the hilar regions. Respiratory motion affects the dose distribution of not only tumors but also organs at risk (OARs) in that region. Some dose constraints are advocated for OARs of the hilar region. However, they are based on planning CT for dose calculation. The purpose of this study was to analyze changes in dose distribution due to respiration in SBRT for central lung tumors abutting the hilar regions using 4D-CT, and also evaluate the changes in relation to the long-term clinical outcomes.
Materials/Methods: Five patients with lung tumors in or adjacent to the right hilar region were included. The prescribed dose was 60 Gy at the isocenter under shallow breathing in two, 57.6 Gy at the isocenter under respiratory gating in one, 57.6Gy at the isocenter under exhalation breath hold in one, and 48.9 Gy at the PTV D95 under exhalation breath hold in one. All doses were delivered in 6 fractions twice a week. Planning CT was performed under exhalation breath hold. 4D-CT images were reconstructed into 10 phases of the respiratory cycle. PTVs, fields and beam arrangements were determined on the planning CT, and were copied onto each 4D-CT image with the same isocenter. GTVs and OARs were delineated on each 4D-CT image. Dose distribution was recomputed on each 4D-CT using the monitor units identical to those on the planning CT. Dose-volume parameters of the GTVs and OARs on each 4D-CT were compared with those on planning CT in each patient.
Results: The ratios of GTV Dmax and GTV D95 to those of planning CT in all patients were 0.99-1.03 and 0.97-1.01, respectively. The ratios of the ipsilateral bronchus Dmax and D4cc in all patients were 0.94-1.03 and 0.78-1.25, respectively. The ratio of the ipsilateral bronchus D4cc of Patient 2 was 0.78-1.25 (mean: 1.06). The ratios of the hilar major vessels Dmax and D10cc in all patients were 0.97-1.01 and 0.55-2.33, respectively. The ratio of the hilar major vessels D10cc of Patient 2 was 1.00-2.33 (mean: 1.55). The ratios of the lung V20 in all patients were 0.96-1.21. The ratio of Patient 1 was 0.98-1.21 (mean: 1.08). The follow-up period for living patients was 24-57 months. Patient 2 died of radiation pneumonitis 5 months after treatment. Patient 1 had grade 2 chronic coughing. Patient 5 had grade 2 radiation pneumonitis 3 months after treatment.
Conclusion: The dose distributions varied with changes of the respiratory phase. The differences may occur between planning CT and 4D-CT. The doses to the GTV were maintained within an acceptable range. However, the doses to the OARs of the hilar region increased compared to the doses on planning CT in some cases. This might be one of the causes of toxicities.
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