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Comparison of a lung fitting algorithm with CT data for tangential fields in radiotherapy of the breast

¹ Medical Physics Department, Torbay Hospital, Newton Road, Torquay, Devon TQ2 7AA and ² Medical Physics Department, Royal Marsden NHS Trust, Downs Road, Sutton, Surrey SM2 5PT, UK

View larger version (14K): [in a new window]   Figure 1. Illustration of circular lung fit criteria: tangent (FG) from portal image defines p, which generates lung path length, L. Arc centred at O varies in position along MN. Tissue thickness, t, is constrained not to be less than a pre-determined minimum over the whole of the arc anterior to MN. An example of tissue thickness, t, is shown for the horizontal extension used for joining the lung shape to the centre of the patient.

View larger version (21K): [in a new window]   Figure 2. Lung path, L, along medial–lateral line plotted against lung in tangential fields, p, as measured from CT images. Linear relationship: L=41+2.75p is used for lung fitting purposes. Points are contained within indicated lines 20 mm above and below the central line.

View larger version (18K): [in a new window]   Figure 3. Error, dL, in lung path length, L, required to produce an error of 2% in the dose to the breast either side of the lung. s is the separation (mm) of the medial and lateral marks, and is the density of lung relative to breast tissue.

View larger version (85K): [in a new window]   Figure 4. Example of lung fit using arc of circle superimposed on original CT image. Only the fit above the medial–lateral line is relevant.

View larger version (9K): [in a new window]   Figure 5. Worst lung fit patient in dataset from both centres, caused by flattened lung shape. The fitted lung is shown as a dotted line. The dose either side of the lung is underestimated by about 2% using the fitted lung, and by about 8% if no lung correction is used. The medial–lateral line is given by ML.

View larger version (16K): [in a new window]   Figure 6. Histograms of dose errors to the breast either side of the lung (7 mm from back edge): no lung correction (black), Torbay data (mean=–8.4%, SD=1.8%, N=46); using fitted lung (white), combined data from both centres (mean=–0.2%, SD=1.0%, N=64). In both cases, the comparison is with lungs defined by CT data.