Surrogate molecular subtyping of breast carcinomas – A study on recent modications and their clinicopathological signicance

Context: Breast carcinoma is a heterogenous disease with varied clinicopathological features and response to therapy. Molecular classification through gene studies helps in planning therapy but has economic constraints. Hence immunohistochemical subtyping of breast carcinomas has been used as a surrogate method. Criteria for this subtyping has undergone many modifications since it was originally proposed. Objectives: To immunohistochemically subtype breast carcinomas based on St.Gallen 2017 guidelines and analyse the differences in clinicopathological parameters like age, tumour size, histopathological grade and lymph node staging between the various subtypes. Materials and methods: The study was done retrospectively at a tertiary care health centre in South India on breast carcinoma patients from January 2017 to June 2020. Immunohistochemistry was done with antibodies to the Estrogen receptor, Progesterone receptor, Human epidermal growth factor receptor-2 (HER-2) and Ki-67. Immunohistochemical Subtypes were correlated with Clinicopathological features. Results: The study had 107 cases. Hormone receptor (HR) positive HER-2 negative was the most common subtype (55 cases, 51.4%). This subtype frequently presented without nodal metastasis (58.2%) and in >50 years of age (56.4%). Triple-negative subtype frequently presented with grade III (69.2%), highest nodal metastasis stage (38.5%) and in < 50 years of age (69.2%). Conclusion: St.Gallen 2017 guidelines for immunohistochemical subtyping classified breast carcinomas into groups that differed significantly in their clinicopathological features. Further studies on differences in treatment response and survival rate differences between these different subtypes are needed.


Introduction
Breast cancer is one of the leading causes of cancer-related mortality and morbidity in females.
Various advancements have been made in the diagnosis and treatment of breast cancer in recent decades. Due to these advancements in breast cancer therapy, the current focus is on identifying the optimal treatment strategy for individual breast cancer patients. For this purpose, breast cancer patients can be classified into various subgroups based on their clinicopathological features and the ideal treatment for each subgroup is being studied [1]. These clinicopathological variables, otherwise called the "predictive markers" indicate the sensitivity or resistance to various forms of therapies in a particular patient thereby helping to choose the ideal therapy for them.
The molecular subtype is one such predictive marker that has got prognostic significance as well

Materials and methods
Place of study: Our study was done at a tertiary care health centre in South India from April 2019 to June 2020.

Results
Our study had 107 cases based on the inclusion and exclusion criteria. HR-positive HER-2 negative subtype (55 cases, 51.4%) was the most common subtype in our study with 38 "luminal Alike" cases ( 35.5%) and 17 "luminal B-like" cases ( 15.9%).
This was followed by HR negative HER-2 positive and triple-negative subtypes (Table I). HR-positive HER-2 positive subtype was the least common subtype in our study with 11 cases (10.3%). These subtypes showed significant differences in their clinicopathological features.
Triple-negative subtype cases more often presented with grade III ( 69.2%). HR-positive HER-2 positive (81.8%) cases more often presented with grade II.
Triple-negative, HR-positive HER-2 positive subtypes never presented with grade I (Table II). On the other hand Grade I tumours were more common in HR-positive HER-2 negative subtype when compared with other subtypes. The association between various subtypes and grade was found to be statistically significant (p-value <0.001).
Triple-negative subtype cases also frequently had the highest stage of lymph node metastasis (N3-38.4%), whereas HR-positive HER-2 negative subtype often did not show nodal metastasis (N0-58.2%) (Table III). In the latter subtype, "luminal Alike" cases often did not show nodal metastasis (76.3%) compared to "luminal B -like cases (17.6%) The association between various subtypes and lymph node staging was found to be statistically significant (p-value <0.001).  In this subtype also smaller tumours were more common in " luminal Alike" cases (42.1%) compared to "luminal B-like " cases. Triple-negative and HR negative HER -2 positive rarely presented with T1 tumours. The association between various subtypes and tumour size was also found to be statistically significant (p-value < 0.001).
With regards to age, triple-negative (69.2%) subtype often presented in women of the age group of fewer than 50 years (Table V). All the other subtypes were more common in women of more than 50 years. On the statistical analysis of data, the association between various subtypes and age was not significant (p value=0.401).     While "Luminal A-like" tumours were often of smaller tumour size without nodal metastasis in many cases, "Luminal-B" like tumours were often of a larger tumours size with nodal metastasis.

Discussion
One of the limitations of our study was we were not able to do a long term follow up of these patients to assess the difference in treatment response and survival rates among the different subtypes. This was because many patients were lost to follow-up. Another limitation was Ki-67 was not done in all cases due to economic constraints.
However this was not necessary in all cases as according to the St. Gallen guidelines, the grade of the tumour can be used instead of Ki-67 for classifying the "Luminal A-like" and "Luminal Blike" tumours. We followed the same guidelines. In cases where the grade was intermediate alone, we used Ki-67 for classification. Such an approach would be cost-effective for subtyping breast carcinomas in developing countries.

Conclusion
Breast cancer is a heterogeneous disease with different subtypes differing in their clinicopathological features. Since these subtypes are likely to differ in their treatment response as well, it is essential to identify the subtype in every individual patient.
Since molecular subtyping might not be affordable in developing and undeveloped countries, surrogate immunohistochemical subtyping can be used. We adopted the recent St. Gallen guidelines for this immunohistochemical subtyping and observed that the subtypes differed significantly in their clinicopathological features further validating the guidelines.
Further long term studies with a large sample size and adequate follow up will help in assessing the treatment response and the survival rates in these different subtypes. Such large scale studies will help in planning the treatment guidelines for each subtype particularly in the Indian population.