What is the best way to contour lung tumors on PET scans? Multiobserver validation of a gradient-based method using a NSCLC digital PET phantom

Abstract

Purpose: To evaluate the accuracy and consistency of a gradient-based positron emission tomography (PET) segmentation method, GRADIENT, compared with manual (MANUAL) and constant threshold (THRESHOLD) methods.

Methods and materials: Contouring accuracy was evaluated with sphere phantoms and clinically realistic Monte Carlo PET phantoms of the thorax. The sphere phantoms were 10-37 mm in diameter and were acquired at five institutions emulating clinical conditions. One institution also acquired a sphere phantom with multiple source-to-background ratios of 2:1, 5:1, 10:1, 20:1, and 70:1. One observer segmented (contoured) each sphere with GRADIENT and THRESHOLD from 25% to 50% at 5% increments. Subsequently, seven physicians segmented 31 lesions (7-264 mL) from 25 digital thorax phantoms using GRADIENT, THRESHOLD, and MANUAL.

Results: For spheres <20 mm in diameter, GRADIENT was the most accurate with a mean absolute % error in diameter of 8.15% (10.2% SD) compared with 49.2% (51.1% SD) for 45% THRESHOLD (p < 0.005). For larger spheres, the methods were statistically equivalent. For varying source-to-background ratios, GRADIENT was the most accurate for spheres >20 mm (p < 0.065) and <20 mm (p < 0.015). For digital thorax phantoms, GRADIENT was the most accurate (p < 0.01), with a mean absolute % error in volume of 10.99% (11.9% SD), followed by 25% THRESHOLD at 17.5% (29.4% SD), and MANUAL at 19.5% (17.2% SD). GRADIENT had the least systematic bias, with a mean % error in volume of -0.05% (16.2% SD) compared with 25% THRESHOLD at -2.1% (34.2% SD) and MANUAL at -16.3% (20.2% SD; p value <0.01). Interobserver variability was reduced using GRADIENT compared with both 25% THRESHOLD and MANUAL (p value <0.01, Levene's test).

Conclusion: GRADIENT was the most accurate and consistent technique for target volume contouring. GRADIENT was also the most robust for varying imaging conditions. GRADIENT has the potential to play an important role for tumor delineation in radiation therapy planning and response assessment.

FIGURE 1

Monte Carlo thorax phantom with simulated NSCLC lesion and mediastinal lymph node. Radii are created semi-automatically during GRADIENT segmentation and provide limits for boundary definition.

FIGURE 2

Sphere phantom cumulative volume segmentation errors for five PET cameras at seven different threshold levels. Errors are displayed for spheres with diameters > 20 mm and < 20 mm

FIGURE 3

Sphere phantom results comparing the optimum 45% THRESHOLD method to GRADIENT. The y-axis of all four graphs is the percent error between the actual sphere diameter and the measured sphere diameter determined from the volume of the segmented sphere. The 0% y-axis represents perfect accuracy. The top row compares similar source to background results in five cameras and the bottom row evaluates the effect of varying source to background ratio in one camera.

FIGURE 4

Thorax Monte Carlo phantom volume segmentation results for seven Radiologists/Radiation Oncologists using MANUAL, 25% THRESHOLD and GRADIENT contouring methods. Linear regression line for 31 segmentations with each method is displayed for each observer.

FIGURE 5

Comparison of GRADIENT and 25% THRESHOLD segmentations of a NSCLC and a mediastinal lesion. The GRADIENT contours are magenta and the 25% THRESHOLD contours are blue.