Does ultrasound provide any added value in breast contouring for radiotherapy after conserving surgery for cancer?

Abstract

Background: Whole breast irradiation after conserving surgery for breast cancer requires precise definition of the target volume. The standard approach uses computed tomography (CT) images. However, since fatty breast and non-breast tissues have similar electronic densities, difficulties in differentiating between them hamper breast volume delineation. To overcome this limitation the breast contour is defined by palpation and then radio-opaque wire is put around it before the CT scan. To optimize assessment of breast margins in the cranial, caudal, medial, lateral and posterior directions, the present study evaluated palpation and CT and determined whether ultrasound (US) provided any added value.

Methods: Twenty consecutive patients were enrolled after they had provided informed consent to participating in this prospective study which was approved by the Regional Public Health Ethics Committee. Palpation and US defined breast margins and each contour was marked and outlined with a fine plastic wire. Breasts were then contoured on axial CT images using the breast window width (WW) and window level (WL) (401 and 750 Hounsfield Units -HU- respectively), at which setting the plastic wires were invisible. Then, the lung window function (WW 1601 HU; WL -300 HU) was inserted to visualize the plastic wires which were used as guidelines to contour the palpable and US breast volumes. As each wire had a different diameter, both volumes were easily defined on CT slices. Results were analyzed using descriptive statistics, percentage overlap and reproducibility measures (agreement and reliability).

Results: Volumes: US gave the largest and palpation the smallest. Agreement was best between palpation and CT. Reliability was almost perfect in all correlations. Extensions: Cranial and posterior were highest with US and smallest with palpation. Agreement was best between palpation and CT in all extensions except the cranial. Since strong to almost perfect agreement emerged for all comparisons, reliability was high.

Conclusions: US may be useful in defining the cranial and posterior extensions, mainly when tumours are localized there. This study demonstrates that the now standard radio-opaque wires around the palpable breast may not be needed in breast contouring.

Fig. 1

Patient position for palpation, US and CT scans. The figure shows a patient positioned supine on the breast board immobilization device, with two plastic wires in place (the wire used to define palpation margins was 4 mm in diameter, while the wire used to define US margins was 3.33 in diameter)

Fig. 2

Digitally reconstructed radiography showing: Panel a - Anterior field; Panel b - Lateral field. The figure shows the point of interest (POI), extensions from the POI (cranial, caudal, lateral, and medial in Panel a; posterior in Panel b). Breast volume and the encompassing field are also illustrated. Distances from the POI and the field edges were used to measure breast extension

Fig. 3

Percentage overlaps in contouring techniques. Percentage overlaps between pairs of contouring techniques (CT vs US, US vs Palpation, Palpation vs CT) according to breast volume quintiles. The percentage overlap variability is independent of breast size

Fig. 4

Bland-Altman agreement plots. Breast volume differences were plotted against the mean breast volume for each pair of contouring techniques, with each point representing one patient. The horizontal lines indicate the mean difference (middle line) and the 95 % limits of agreement. The smaller the range between these two limits the better agreement is

Fig. 5

Kendall’s τ correlation coefficients. Kendall’s τ correlation coefficient established whether measurement variability depended on measurement size assessing the interdependence of intra-measure mean and standard deviation (SD)