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Minimum Detectable Change in Lung Nodule Volume in a Phantom CT Study

Rationale and Objectives

The change in volume of lung nodules is being examined as a measure of response to treatment. The aim of this study was to determine the minimum detectable change in nodule volume with the use of computed tomography.

Materials and Methods

Four different layouts of synthetic nodules with different shapes but with the same size (5, 8, 9, or 10 mm) for each layout were placed within an anthropomorphic phantom and scanned with a 16-detector-row computed tomography scanner using multiple imaging parameters. Nodule volume estimates were determined using a previously developed matched-filter estimator. Analysis of volume change was then conducted as a detection problem. For each nodule size, the pooled distribution of volume estimates was shifted by a percentage c to simulate a changing nodule, while accounting for standard deviation. The value of c resulting in a prespecified area under the receiver operating characteristic curve (AUC) was deemed the minimum detectable change for that AUC value.

Results

Both nodule size at baseline and choice of slice collimation protocol had an effect on the value of minimum detectable growth. For AUC = 0.95, the minimum detectable nodule growth in volume when using the thin-slice collimation protocol (16 × 0.75 mm) was 17%, 19%, and 15% for nodule sizes of 5, 8, and 9 mm, respectively.

Conclusions

Our results indicate that an approximate bound for detectable nodule growth in subcentimeter nodules may be relatively small, on the order of 20% or less in volume for a thin-slice CT acquisition protocol.

It is widely accepted in the scientific community that personalized medicine, the customization of health care with decisions and practices being tailored to the individual patient, is transforming patient care . Personalized medicine is being enabled by recently obtained genetic information as well as advances in quantitative science and emerging technologies such as nanotechnology . Quantifiable features, such as the expression of molecular or image-based biomarkers, are currently being used in many aspects of clinical practice, with still more potential uses being examined in research. The success and adoption of such biomarkers in clinical practice depend on the precision and accuracy of the techniques used to make the measurements.

As a case in point, the change in size of lung nodules as determined by computed tomography (CT) is being used as a measure of treatment response for some cancer treatments. Clinicians need to know whether a nodule’s change in size following treatment is due to the effectiveness of that treatment or simply related to measurement error. In one of the earliest attempts to incorporate quantitative data into clinical practice, Moertel and Henley recommended a 50% reduction in the product of perpendicular tumor diameters as a cut-off for determining partial response to treatment . That cut-off was based on an experiment of repeat measurements with calipers of synthetic tumor-mimicking objects placed under plastic foam, where 7.8% of the measurements of the same investigator and 6.8% of the measurements of different investigators varied by at least 50%. The 50% cut-off was the basis for the first standardized response criteria, the World Health Organization criteria , and subsequently influenced the partial response category of the Response Evaluation Criteria in Solid Tumours (RECIST) criteria, since a 50% reduction in nodule area (product of perpendicular diameters) corresponds to approximately a 30% reduction in maximum diameter for a sphere .

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Materials and Methods

Anthropomorphic Phantom and Synthetic Nodules

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Figure 1, (a) Photograph of the anthropomorphic phantom and vasculature insert used in this study. Shown are the foam receptacles holding the synthetic nodules in specific locations. (b) Photograph of the synthetic nodules used in this study. From left to right : lobulated, elliptical, spiculated, and spherical nodules. Nodule sizes shown from top to bottom are 20 mm (not used in this study), 10 mm, and 5 mm. (Color version of figure is available online).

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Imaging Protocol

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Volume Estimation

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Prediction of Minimum Detectable Change in Nodule Volume

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pci,c=(pbi−pbi¯¯¯¯¯)∗σi,cσi+pbi¯¯¯¯∗(1+c), p

c

i

,

c

=

(

p

b

i

p

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)

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i

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σ

i

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b

i

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where pc__i,c is the change distribution by a relative change c of the baseline volume estimates of nodules with nominal diameter i , and σ__i,c is the standard deviation of pc__i,c , defined as:

σi,c=(pbi¯¯¯¯∗(1+c))∗CVi,c, σ

i

,

c

=

(

p

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(

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+

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C

V

i

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where CV__i,c is the interpolated coefficient of variation of pc__i,c at relative change c and for a baseline nominal diameter i , derived from a spline curve fitted across the coefficient of variation (standard deviation over mean) values of the four volume distributions. Figure 3 shows the interpolated coefficient of variation curve derived using pooled measurements from all imaging protocols.

Figure 2, Volume estimate distributions across all protocols, nodule shapes, and sizes. (Color version of figure is available online).

Figure 3, Coefficient of variation as a function of nodule volume. A spline curve was fitted across coefficient of variation values derived from the volume estimate distributions for each size category (5, 8, 9, and 10 mm) shown in Figure 2 . (Color version of figure is available online).

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Validation Step

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Results

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Figure 4, Minimum detectable percent increase in volume as a function of the area under the receiver operating characteristic curve derived using volume estimates from all imaging protocols ( top plot ), volume estimates using a thin-slice collimation protocol (16 × 0.75 mm, middle plot ), and volume estimates using a thick-slice collimation protocol (16 × 1.5 mm, bottom plot ). Results are presented for baseline nodule nominal diameters of 5, 8, and 9 mm. Error bars represent 95% confidence intervals. (Color version of figure is available online).

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Figure 5, Minimum detectable percent decrease in volume as a function of the area under the receiver operating characteristic curve derived using volume estimates from all imaging protocols ( top plot ), volume estimates using a thin-slice collimation protocol (16 × 0.75 mm, middle plot ), and volume estimates using a thick-slice collimation protocol (16 × 1.5 mm, bottom plot ). Results are presented for baseline nodule nominal diameters of 8, 9, and 10 mm. Error bars represent 95% confidence intervals. (Color version of figure is available online).

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Figure 6, Minimum detectable percent increase in volume as a function of the area under the receiver operating characteristic curve derived using volume estimates across specific nodule shapes: spherical, elliptical, lobulated, and spiculated, shown from top to bottom . Results are presented for baseline nodule nominal diameters of 5, 8, and 9 mm. Error bars represent 95% confidence intervals. (Color version of figure is available online).

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Discussion

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) 1A study focusing on interreader variability and a comparison among one-, two-, and three-dimensional nodule size metrics , the QIBA 1C study focusing on interscanner variability , and the recently completed QIBA 3A study, which examined interalgorithm variability in volume estimation through a public algorithm challenge.

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Conclusions

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Acknowledgments

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