Does a lesion in the fat anterior to the psoas muscle, caudal to the lower pole of the kidney, which decreased in visibility with slice thicknesses of 3.75mm and 3.00mm, but increased in visibility with a slice thickness of 2.50mm, confirm growth?

From the Guidelines

The lesion in question does not confirm growth based on the provided information, as the change in visibility is likely due to the variation in slice thickness rather than actual growth. The apparent increased visibility of the lesion with a slice thickness of 2.50mm, compared to 3.75mm and 3.00mm, can be attributed to the improved resolution and detail provided by thinner slices in CT scans 1. This technical variation, rather than actual growth, is the most plausible explanation for the observed change in visibility.

Key Considerations

• The American Society of Clinical Oncology clinical practice guideline for the management of small renal masses emphasizes the importance of consistent imaging techniques for accurate assessment 1.
• The American College of Radiology's appropriateness criteria for indeterminate renal masses highlight the role of MRI in characterizing renal lesions, including the use of contrast-enhanced MRI for predicting RCC 1.
• The change in slice thickness can significantly affect the appearance of lesions on CT scans, making it essential to consider technical parameters when evaluating growth.

Recommendations

• To accurately assess growth, scans should be performed with consistent technical parameters, including slice thickness.
• A radiologist should compare the images directly, taking into account the different slice thicknesses, to determine whether the lesion has truly grown or if its changed appearance is due to technical variations.
• Consideration should be given to using MRI, particularly contrast-enhanced MRI, for further characterization of the lesion, as it has been shown to have higher sensitivity and specificity for diagnosing RCC compared to CT scans 1.

From the Research

Lesion Visibility and Slice Thickness

• The visibility of a lesion can be affected by the slice thickness used in CT scans, as seen in the case of a lesion in the fat anterior to the psoas muscle, caudal to the lower pole of the kidney 2.
• A study on the influence of slice thickness on CT histogram analysis in indeterminate adrenal masses found that thinner slice thicknesses can increase image noise, which may affect the visibility of lesions 2.
• In the case of the lesion in question, the decrease in visibility with slice thicknesses of 3.75mm and 3.00mm, but increase in visibility with a slice thickness of 2.50mm, may be due to the reduced image noise and improved resolution at the thinner slice thickness.

Lesion Measurement and Growth Assessment

• Accurate measurement of lesions is crucial for assessing growth or regression, and slice thickness can affect the accuracy of these measurements 3.
• A study on frameless stereotaxy for pre-treatment planning and post-treatment evaluation of radiosurgery found that lesion measurements may be duplicated and growth or regression accurately established using a virtual frame and transformation matrix 3.
• However, the uncertainty in this system of re-application resides in possible patient movement, CT scan slice thickness, and inter-observer error in the identification of natural fiducial markers 3.

Diagnostic Accuracy and Sampling Error

• The diagnostic accuracy of CT-guided fine-needle aspiration (FNA) of lung lesions can be improved by using metabolic PET/CT guidance, which can decrease the false-negative rate and improve the accuracy of FNA results 4.
• A study on metabolic PET/CT-guided lung lesion biopsies found that the distance between the tip of the aspirating needle and the area with the highest SUV within the lesion was significantly greater in the false-negative group than in the true-positive group 4.
• However, it is unclear whether these findings can be applied to the diagnosis of lesions in the fat anterior to the psoas muscle, caudal to the lower pole of the kidney.

Conclusion on Lesion Growth

• Based on the available evidence, it is unclear whether the change in visibility of the lesion with different slice thicknesses confirms growth or regression 3, 2, 5, 4.
• Further studies would be needed to determine the optimal slice thickness and diagnostic approach for assessing lesion growth in this specific location.