The velocity of ultrasound in human primary melanoma tissue - implications for the clinical use of high resolution sonography

Abstract

Background: Ultrasonography with 20 MHz frequency can be used to estimate tumour thickness preoperatively in malignant melanoma (MM) of the skin. The vertical invasion depth is the single most important prognostic factor for localised MM, and its preoperative knowledge would be very useful for the planning of surgical procedures. Since ultrasonographic distance measurements directly depend upon the tissue specific ultrasound velocity, we determined the ultrasound velocity in primary melanoma.

Results: Ultrasound velocity was calculated from runtime differences of a 20 MHz ultrasound signal along a known distance either through water alone or through thick specimens of primary MM. The ultrasound velocities varied between 1553 m/s and 1588 m/s with a mean of 1564 m/s in four different MM specimens. The analysis of different parts of the specimens showed that the variation of the calculated velocities was larger between different specimens than within one individual specimen.

Conclusions: The ultrasound velocity in MM tissue may be slightly lower than normally assumed, thereby explaining a part of the overestimation usually found in sonographic measurement of melanoma invasion depth. Additionally, the variation of ultrasound velocity between individual tumours may contribute to the impairment of the correlation found between sonometry and Breslow's measurement of MM invasion depth. For practical reasons, a setting of 1580 m/s will be appropriate for ultrasonography of primary malignant melanoma.

Figure 1

Schematic representation of the sonographic device. A 20-MHz transducer (T) is fixed to a basin filled with water. The 12-millimeter-distance (so) between the transducer and the plexiglas bottom can be measured with or without interposing tissue samples. The melanoma tissue (M) can be fixed onto the bottom with a flexible clip in different positions.

Figure 2

Scope signals of the measurements. The signal runtimes were determined by electronic cursors. The time the ultrasound pulse needs to pass a given distance changes whether the melanoma tissue (tumour) is interposed (A) or whether it goes through water alone (B). From the runtime difference with or without the tissue sample in the signal way, the ultrasound velocity within the tissue can be calculated.

Figure 3

Calculated ultrasound velocities in different parts of 4 individual primary melanomas. The 16 measurements gave a mean value of 1564 m/s (solid line). The dashed line represents the ultrasound velocity normally assumed in human skin (1580 m/s).

Figure 4

Correlation between sonometric and histometric measurement of tumour invasion depth in 400 patients with primary malignant melanoma. Simple regression analysis revealed a correlation coefficient R = 0.92 (p < 0.0001), while the slope of the regression line (b = 0.82) was substantially differing from perfect correlation between both measures, in which case b should equal 1.0. As can be seen from the graph, sonometry was systematically overestimating tumour depth. After recalculation of the estimations using a different ultrasound velocity for melanoma tissue of 1564 m/s instead, the value of slope only rose to b = 0.84, which still accounted for a substantial overestimation compared to histological measurement.