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This chapter defines the Suspicious of Malignancy category, discusses the reasons why suspicious of malignancy reports occur, and why a definitive diagnosis of malignancy cannot or should not be made in certain circumstances. The category allows for the maintenance of a high positive predictive value of a malignant diagnosis. Many of the reasons for an inability to make a definitive diagnosis of malignancy are similar to those seen in the atypical category and include the specimen quality, which is largely reliant on the skill of the FNAB operator, the experience of the cytopathologist interpreting the smears, and the inherent nature of the lesion. There are overlapping cytological criteria between some breast proliferative and in situ lesions, and between in situ lesions and some malignant tumours, and distinguishing carcinoma and lymphoma also may be difficult on occasion. The chapter details the cytological features of low-grade ductal carcinoma in situ (DCIS), and discusses this controversial area to provide an approach to the diagnosis and differential diagnoses for low-grade DCIS. The management of Suspicious of Malignancy FNAB cytology cases is presented and highlights the need for further investigation by core needle or excision biopsy in all cases. Sample reports are presented.

The International Academy of Cytology (IAC) Yokohama System for Reporting Breast Fine Needle Aspiration Biopsy (FNAB) Cytology reporting system is an initiative of the IAC that seeks to provide a comprehensive, standardized approach to breast FNAB reporting to facilitate reproducible cytopathology reporting and to establish best practice guidelines. The system is evidence-based, supports a structured reporting format and will aid in maximizing the usefulness of the FNAB technique and optimizing communication with the clinical team and the patient.

This chapter provides an outline of a pattern approach to the interpretation of breast fine needle aspiration biopsy direct smears. A number of different low and medium power patterns are described that suggest certain lesions. High power is then used to assess nuclear and other features, and low and high power key features are integrated into a diagnosis or a differential diagnosis. Details of the key diagnostic cytological features are provided in the various chapters of the atlas.