Abstract PO5-26-09: Digital image analysis and a novel set of cell line samples as aids in the development of a quantitative external quality assessment programme for Ki-67
Authors: Dodson Andrew , Berisha Fitim , Wilkingson Dawn , Zabaglo Lila , Parry Suzanne 
Published: May 2024

Background and Aims

Ki-67 is a well-established biomarker of proliferation in breast cancer (BC). However, its value in treatment decision making is hampered by a lack of analytical reproducibility.

Regular participation in external quality assessment (EQA) substantially improves inter-laboratory concordance.

Pre-requisites in establishing a fit-for-purpose EQA for Ki-67 are a well-validated testing substrate and a reproducible method of assessing Ki-67 scores in participants’ submitted materials. We report here on the results of work using cell line controls analysed by digital image analysis (DIA) as a first step towards providing such an EQA.

Materials and Methods

A formalin-fixed paraffin embedded (FFPE) cell line microarray (CLMA) was designed and produced in conjunction with Array Sciences LLC (Sausalito, USA). It was comprised of cores taken from a pure population of Sf9 caterpillar cells, which have been shown to be completely unreactive with most commercial antibodies to human Ki-67, together with cores of Sf9 cells mixed with four different human BC cell lines. These were BT-20 (85% BC cells), ZR-75-1 (75%), BT-474 (65%) and BT-483 (55%). Sections from the CLMA were mounted onto glass microscope slides together with sections from a FFPE tonsil sample and two BC samples; one BC showed high (~30%, BC-high) and the second, low proliferation (~5%, BC-low). In both BC samples proliferating cancer cells were distributed homogeneously throughout the the block.

Unstained sections were distributed to laboratories participating in the Scheme’s Ki-67 BC programme; after routine IHC-staining for Ki-67 returned slides were centrally visually assessed for stain quality and subjected to DIA using an in-house application developed on Visiopharm software (Visiopharm A/S, Hoersholm, Denmark).

Results

Slides were returned by 37 laboratories.

Analysis of Ki-67 scores obtained on the Sf9 core identified two distinct groups. The first (n = 27) were negative or showed low Ki-67 scores (mean = 1.1%, 95% CIs: 0.2-1.9%), the second (n = 10) displayed a step-change in scores (mean = 49.9%, 95% CIs: 33.2-66.7%); the means of the two groups were significantly different (P&lt; 0.0001). When Ki-67 scores for each of the tissue samples were dichotomized into similar groups, the means of those groups differed significantly for the two BC samples (P&lt; 0.001), but not for tonsil.

Quality scores generated by visual assessment did not differ significantly between the two groups. However, when slides bearing Sf9 cores demonstrating aberrant Ki-67 scores were visually examined nuclear staining was clearly visible, and non-specific nuclear staining could also be identified in the matched BC tissue samples, but not in the tonsil sections.

Correlation of Ki-67 scores between the four BC cell line cores and each of the BC tissue samples was examined using Pearson’s correlation statistics. The r statistic range was 0.57-0.71 in comparisons between BC cell line cores and the BC-high sample, and 0.53-0.70 for those with BC-low; in each case BT-483 showed the highest correlation score and BT-20 the lowest. A similar analysis was undertaken between the tonsil and the two BC tissues. The r statistic for correlation between tonsil and BC-high scores was 0.72; it was 0.64 for those between tonsil and BC-low.

Conclusions

By using a pure population of Sf9 cells we have developed a sensitive indicator of non-specific nuclear staining in IHC preparations stained for Ki-67 which identifies the presence of the artefact quantifiably.

Cores made from Sf9/BC cell line mixtures (especially BT-483) produce Ki-67 scores which correlate with those obtained in breast cancer samples at a similar level to those achieved between tonsil and BC samples; this is true for both high and the low proliferation ranges. Cell line mixtures can be adjusted to show Ki-67 scores in the clinically relevant ranges, and they do not show the inherent biological variations seen in tissue controls such as tonsil.

Citation Format: Andrew Dodson, Fitim Berisha, Dawn Wilkingson, Lila Zabaglo, Suzanne Parry. Digital image analysis and a novel set of cell line samples as aids in the development of a quantitative external quality assessment programme for Ki-67 [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-26-09.