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A practical method for the estimation of vitiligo surface area: a comparison between the point counting and digital planimetry techniques

European Journal of Dermatology. Volume 17, Number 1, 30-2, January-February 2007, Investigative report

DOI : 10.1684/ejd.2007.0186

Summary

Author(s) : Fatma Aydin, Nilgun Senturk, Bunyamin Sahin, Yuksel Bek, Esra Pancar Yuksel, Ahmet Yasar Turanli , Department of Dermatology, Ondokuz Mayis University, Medical Faculty of Ondokuz Mayıs University 55139 Kurupelit, Samsun, Turkey, Department of Anatomy, Ondokuz Mayis University, Samsun, Turkey, Department of Biostatistics, Ondokuz Mayis University, Samsun, Turkey.

Summary : The accurate measurement of vitiligo surface area is fundamental to the evaluation of treatment efficacy. Digital photography, planimetry and clinical evaluation by the clinician have been used for assessment. Although digital photography with computerized planimetry is a reliable and accurate method, it is labour intensive and expensive. The aim of this study was to evaluate the accuracy and reliability of the point counting method and to compare it with digital planimetry techniques for the estimation of vitiligo surface area. Surface areas of 31 vitiligo lesions of five volunteers were estimated using the point counting and digital planimetry methods. Three independent observers evaluated the outlined areas twice using the point counting technique with an interval of 2 weeks. The same lesions were also measured by one observer applying digital planimetry. The estimation results of three observers were compared using inter and intra-observer correlation analysis test. There was significant inter- and intra-observer agreement for all measurements. There was also significant coincidence between each observer’s estimation of the point counting and digital planimetry method. No significant differences on the results of surface area obtained using the two methods were found (p > 0.05). The point counting method may be used as a direct and reliable technique to measure the vitiligo surface area.

Keywords : vitiligo, planimetry, point counting

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ARTICLE

Auteur(s) : Fatma Aydin¹, Nilgun Senturk¹, Bunyamin Sahin², Yuksel Bek³, Esra Pancar Yuksel¹, Ahmet Yasar Turanli¹

¹Department of Dermatology, Ondokuz Mayis University, Medical Faculty of Ondokuz Mayıs University 55139 Kurupelit, Samsun, Turkey
²Department of Anatomy, Ondokuz Mayis University, Samsun, Turkey
³Department of Biostatistics, Ondokuz Mayis University, Samsun, Turkey

accepté le 2 Septembre 2006

It is important to determine the size of vitiligo surface areas and the degree of treatment response in vitiligo patients. In studies on vitiligo, several methods are described for assessing the surface area of the lesion. In some, a repigmentation percentage had been presumed empirically by the clinician during the examination and a score given [1]. However, rough assessments without any objective measurements might lead to incorrect evaluations. In other studies, the response to treatment was compared using digital photography and planimetry [2, 3]. It is also stated however, that this approach is time consuming and inaccurate and does not facilitate clinical decision-making. On the other hand, evaluation with digital photography measurements is not accurate when the lesion is three-dimensional [4].Digital photography with computerized planimetry is a reliable and accurate method and provides a digital image record. This process is known as image digitization. Each pixel in the image is individually sampled, and its brightness measured and quantified. This integer value is stored in the corresponding pixel of the computer’s bitmap image. The area that defines the object’s outline is reported as a polygon area and in order to measure this, certain computer programmes using this digitization technique can be employed, one of which is the image-pro plus 4.5 analysis processing programme [5, 6].Point counting is a simple, accurate and practical technique, which is widely used in the estimation of the irregularly shaped sectional surface area to obtain the volumes of organs or structures using macroscopic, microscopic or radiological images [7-9]. We have not, however, found a study describing the estimation of lesion surface area in vitiligo using a point counting approach. The present study was carried out to describe a simple, accurate and practical technique for estimating the sizes of vitiligo areas by the point counting method and to compare this method with digital planimetry measurements as the gold standard of the study.

Methods

A total of 31 vitiligo areas from five patients (mean age ± SD, 31.6 ± 17.7 years) were evaluated.

Point counting procedure

The borders of the lesions were marked with an ordinary ballpoint pen and a piece of paper was immediately placed over the lesion as described before [10]. For each lesion, the copied borders of projection areas were enhanced by redrawing the contours with a pen. To estimate the number of points, transparent sheet that has points (+) on it was randomly superimposed on lesion projection area. The numbers of intersections (i.e. upper right corner of the crosses) hitting the area of interest were counted ( (figure 1) ). The total area of each lesion was estimated by multiplying the representative area of a point on grid by total number of points counted for the lesion. The representative area of the points (a (p)) in grid was 0.1 cm².

Three independent observers estimated the surface area of the same projection drawings using the same point counting grid with the representative area given above. Surface area estimations were repeated after two weeks by each observer, in order to analyse intra-observer variability.

Image pro plus-4.5 procedure

Since the reliability of the point-counting method had been investigated using the figures drawn on paper the same figures were scanned to compare the surface area estimations using the digital planimetry method. All drawings were scanned and stored in “jpeg” format with true colour class and 300 × 300 pixel resolution then converted to black and white format because the frame of areas in our study were important. In order to convert the area from pixel to cm², we attached a square with 1 cm² area and counted the number of pixels in this area to calibrate the area measurement of the vitiligo lesion. Surface areas of the lesion and a 1 cm² drawn area were measured by image-pro plus 4.5 analysis processing programme on the basis of planimetry.

Statistical analyses

Statistical analyses were carried out using SPSS 13.0 for Windows (Release 13.01, license code 9071653). The correlation and regression analyses were performed to evaluate the data obtained by all measurement techniques. The regression coefficient was also expressed as 95% confidence limits.

The estimation results of three observers were compared statistically to evaluate inter-observer variations using inter-observer correlation analysis test. The intra-observer reliability was assessed by comparing coefficients of intra-observer correlation. The estimation results of the point counting method and image-pro plus 4.5-analysis processing, which was accepted as a gold standard, were compared for each observer separately. The mean areas obtained by different measurement techniques were compared using Wilcoxon Signed Ranks Test.

Results

The mean vitiligo surface areas obtained using point counting techniques by three observers in the two point counting sessions (with a two-week interval) was 4.70 ± 0.76 (min-max: 0.69-16.26) cm². The mean vitiligo surface area obtained by the planimetry technique was 6.26 ± 1.71 (min-max: 0.50-16.01) cm² (mean ± SEM).

The estimation results of three observers were compared to assess inter-observer variation using correlation analysis test. There was significant agreement between each observer’s estimates using the point counting method.

The estimation results of the same observers at a 2-week interval were analyzed statistically to check intra-observer variation using correlation analysis test. Using intra-observer correlations, a high degree of agreement was found for the results estimated using the point counting method.

The estimation results obtained by the point counting method and measuring results obtained by image-pro plus 4.5-analysis processing were compared to assess the accuracy of the point counting method. There was a significant agreement between the observers’ estimates. Inter-observer, intra-observer and methodological agreements for the first and second area measurements are summarized in table 1( Table 1 ).

The comparisons among the estimates of the point counting approach and between the results obtained from point counting and planimetry techniques were performed using Willcoxon Signed Rank Test. No statistical difference was found between the estimated surface areas using the two methods (p > 0.05).
Table 1 Inter-observer, intra-observer and methodological agreements for the first and second area measurements

Agreements	Relations Between	r
Inter-observer agreement for 1^st Measurement	O1-O2	0.999**
O1-O3	0.999**
O2-O3	0.997**
Inter-observer agreement for 2^nd measurement	O1-O2	0.999**
O1-O3	0.992**
O2-O3	0.998**
Intra-observer agreement between 1^st and 2^nd measurements	M1₁-M1₂	0.999**
M2₁-M2₂	0.999**
M3₁-M3₂	0.997**
Agreement between point counting and image-pro plus for 1^st measurement	O1-C	0.994**
O2-C	0.993**
O3-C	0.993**
Agreement between point counting and image-pro plus for 2^nd measurement	O1-C	0.996**
O2-C	0.994**
O3-C	0.995**

Discussion

Several methods such as digital photography, planimetry, and clinical evaluation by the clinician have been used to measure vitiligo surface areas [1-3]. Photographic measurement of size involves the use of a camera equipped with a macro lens. A planimeter or digitizing tablet is used to calculate the surface area. A major advantage of photographic measurement is the provision of a permanent visual record of the size. However, a number of technical limitations may reduce the effectiveness of digital photography in vitiligo. For example, measurement precision is compromised when the distance between the camera and the surface is inconsistent. Failure to place the camera at exactly the same distance for each photograph may create the impression that the size of the lesion has changed when it has not. Similarly, the camera angle in relation to the size may affect the precision of the photographic measurement. If an instant camera is not used, the need to develop and process the film before measurements can be made and the uncertainty about the success of the photograph (e.g., possible overexposure or underexposure) contributes to challenges in obtaining data. On the other hand, the cost and amount of equipment and time required to use computer-assisted planimetry restricts its clinical usefulness in daily practice [11-13].

Our results showed that there was significant agreement between the each observer’s estimates using the point counting method and image-pro plus 4.5-analysis processing. The point counting method showed statistically significant reliability for measurement of surface areas. Moreover, the method described in this study represents a simple, practical, and inexpensive technique to evaluate the vitiligo surface area. Good agreement was found between the results obtained by three independent observers for the point counting method. The estimates of each observer were closely correlated to the others’ estimates.

This study was also designed to evaluate inter- and intra-observer reliability and accuracy of point counting method for vitiligo lesion surface area measurement. For practical reasons, we prefer to analyse the drawings with point counting and image pro-plus analysis. This is the only study in which the outcome of both methods could be compared to the true size of the lesion. Moreover, the design allowed a valid comparison of the outcome of measurements in different body sites.

The estimation problem which was encountered when measuring small areas, can be resolved by decreasing the scales of the grids. Since the application of the small grid increases sensitivity, it might be advantageous, especially in the case of regular geometric figures with indefinite borders. Sensitivity increases when the scales of the grids decrease; it does, however, cause an increase in point counting time. Appropriate scale choice can be made according to the size of the lesion. Empirically, we may suggest the researcher to choose a point counting grid that is suitable for counting at least 20-25 points per lesion.

Our purpose was to develop an easy way to assess the total surface area of vitiligo lesions without changing the routine procedure in daily practice. This method can also be used for measuring the size of pathological skin lesions such as psoriasis, ulcers, alopecia, burn wounds, or scars in which accurate evaluation is essential for clinical follow-up and research. The values obtained using the method described in this study are notably independent from observers. Moreover, the method is inexpensive and fast, since point counting can be carried out within 1-2 minutes per subject. However, the need for well-delineated lesions of a measurable size may restrict the application of this method. This handicap can be overcome by choosing reference lesions to evaluate patients. The described method here could be used as a criterion for the evaluation of treatment or the follow-up process of vitiligo lesions.

Acknowledgements

Financial support: None. Conflict of interest: None.

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