Reduction of normal lung irradiation in locally advanced non-small-cell lung cancer patients, using ventilation images for functional avoidance

Abstract

Purpose: To investigate the ability of four-dimensional computed tomography (4D-CT)-derived ventilation images to identify regions of highly functional lung for avoidance in intensity-modulated radiotherapy (IMRT) planning in locally advanced non-small-cell lung cancer (NSCLC).

Methods and materials: The treatment-planning records from 21 patients with Stage III NSCLC were selected. Ventilation images were generated from the 4D-CT sets, and each was imported into the treatment-planning system. Ninetieth percentile functional volumes (PFV90), constituting the 10% of the lung volume where the highest ventilation occurs, were generated. Baseline IMRT plans were generated using the lung volume constraint on V20 (<35%), and two additional plans were generated using constraints on the PFV90 without a volume constraint. Dose-volume and dose-function histograms (DVH, DFH) were generated and used to evaluate the planning target volume coverage, lung volume, and functional parameters for comparison of the plans.

Results: The mean dose to the PFV90 was reduced by 2.9 Gy, and the DFH at 5 Gy (F5) was reduced by 9.6% (SE = 2.03%). The F5, F10, V5, and V10 were all significantly reduced from the baseline values. We identified a favorable subset of patients for whom there was a further significant improvement in the mean lung dose.

Conclusions: Four-dimensional computed tomography-derived ventilation regions were successfully used as avoidance structures to reduce the DVH and DFH at 5 Gy in all cases. In a subset, there was also a reduction in the F10 and V10 without a change in the V20, suggesting that this technique could be safely used.

Figure 1. Ventilation Volumes of Interest (VOIs)

A coronal section through the treatment planning free-breathing CT image volume with VOIs generated from the ventilation image (90, 70, 50, and 30 percentile ventilation) is shown. The gross tumor volume (GTV) is shown in red and the planning tumor volume (PTV) in blue. There is a ventilation defect in the right upper lobe adjacent to the GTV seen as a lack of contours in that region. The defect fills in progressively with the lower percentile contours. The 90 percentile lines show a clear separation between the highly functional lung and the remaining lung.

Figure 2. Effect on isodose distribution

a) A coronal section through a 4D CT derived ventilation image is shown with 90 percentile volume of interest (PTV90) contours (light green), the GTV (red), and the PTV (blue). b) The same PFV90, GTV, and PTV contours are shown on the corresponding treatment planning CT coronal section. c) Same treatment planning CT coronal section shown in a & b with isodose distribution from the volume-constrained baseline plan. d) treatment planning CT coronal section with the isodose distribution from the ventilation-constrained plan, there is shifting of the 1000, 2000, and 3000 cGy isodose lines away from the PFV90 lung, sparing the regions of the normal lung with the highest function (PFV90).

Figure 3. Example case DVH and DFH

a) Example case dose volume histogram (DVH) of the resultant dosimetry for the baseline volume-constrained plan (broken lines) and the ventilation-constrained plan using PFV90 avoidance structures (solid lines). The DVH changes in the Total Lung, PFV90, and PTV are shown in this graph for the baseline and ventilation constrained plans. b) The change in Total Lung dose function histogram (DFH) is shown in this graph for the baseline and ventilation contrained plans.

Figure 4. Percentage improvements

a) The average improvement (± SE) for the entire set (n=21) in the total lung DVH, total lung DFH, and PFV90 DVH. The improvement in the ventilation contrained plan versus the volume constrained plan was calculated on a case by case basis at each dose value then averaged. b) The average improvement (± SE) for the favorable subset (n=10) in the total lung DVH, total lung DFH, and PFV90 DVH.

Figure 5. Effect of ventilation-constrained functional planning

There is a statistically significant difference in dosimetry between baseline and ventilation-constrained functional treatment plans for the favorable subset (n=10). The Boxplots show distribution of parameter values for each planning category.