Head and Neck Squamous Cell Carcinoma (HNSCC) is the 9th most common cancer worldwide, accounting for approximately 90% of all head and neck malignancies. HNSCC affects the squamous mucosa of the upper aerodigestive tract, including the lip and oral cavity, and constitutes ~90% of all head and neck cancers [1]. Lip and oral cavity cancers are the most frequent non-melanoma head and neck cancers, with around 350,000 new cases diagnosed annually [2]. Oral Squamous Cell Carcinoma (OSCC) can arise in various oral sites, most commonly the tongue, and major risk factors include tobacco use and alcohol consumption, particularly in individuals in the sixth and seventh decades of life [3][4]. Based on national and international guidelines, the most effective current treatments include surgery, radiotherapy, chemotherapy, and immunotherapy. Despite major advances in diagnoses and therapy since the 1970s, overall survival rates have remained unsatisfactory [5, 6]. In contrast, Lip Squamous Cell Carcinoma (Lip SCC) is generally less aggressive, with Ultraviolet (UV) light exposure, fair skin, older age, male sex, and the lower lateral vermillion border being recognized risk factors [7-9].

The TNM method, established by the American Joint Committee on Cancer (AJCC), evaluates tumor size (T), lymph node involvement (N), and distant metastases (M), which are among the most important characteristics used for therapy and prognostic management [10]. Clinical grading of the illness based on TNM classification and tumor location is frequently used as the main criterion for determining prognosis and therapy options[11]. However, even if the location and stage are the same, there are wide variations in treatment response and prognosis for oral squamous cell carcinoma; some patients live for a long time while others quickly metastasize [11, 12]. Furthermore, according to Almangush et al. [13], the BD histologic grading approach was used, which solely assesses tumor budding and depth of invasion in the invasive front region. There is also a strong correlation between these two factors and prognosis [11, 14, 15]. Tumor budding (B) denotes the presence of a solitary cancer cell or a small cluster of fewer than five cancer cells observed at the invasive front. Depth of invasion (D) was quantified from the tumor's surface to its farthest point of invasion, categorized as low (<4 mm) or high (≥4 mm). The histologic grading system entails the following criteria: a score of 0 corresponds to a tumor with <4 mm depth of invasion and fewer than five buds at the invasive front; a score of 1 aligns with a tumor that has ≥4 mm depth of invasion and fewer than five buds at the invasive front, or a superficial tumor with <4 mm depth of invasion but exhibiting high tumor budding activity at the invasive front (≥5 buds); a score of 2 designates a tumor with ≥4 mm depth of invasion and high tumor budding activity at the invasive front (≥5 buds). Tumors with a score of 0 are categorized as low risk, those with a score of 1 as intermediate risk, and those with a score of 2 as high risk [15].

Strieder et al. [15] compared some histologic grading methods for squamous cell carcinoma of the lip. They discovered that combining TNM staging with the BD model of histologic grading can aid in treatment planning, preventing mutilations and the resulting loss of patient quality of life. According to them, the BD model was the most objective, straightforward, and useful technique for determining the prognosis of lower lip SCC.

It is critical to select the right treatment technique since any flaw may harm the patient in the long run, whether in the form of recurrence, residual lesion, or irreversible morbidity. Many recent studies have demonstrated that if the initial therapy fails, the disease-related mortality rate skyrockets, making survival a distant possibility [15]. This retrospective study aims to compare Invasive Front (IF), Depth of Invasion (DOI), and other histomorphologic parameters between intraoral SCC and lip SCC.

A total of 80 patients were included, comprising 40 cases of Lip SCC and 40 cases of OSCC. The demographic characteristics are summarized in Table 1. There was no statistically significant difference between the two groups regarding age or gender distribution (p > 0.05).

OSCC and Lip SCC are among the malignant lesions of the head and neck region, and these two lesions, despite morphological similarities, differ in terms of prognosis and survival. Considering the unpredictable behavior of OSCC in the digestive and respiratory systems, the prognosis of lip SCC is much better than that of OSCC. Cancer prognosis is described by the invasive front of cancerous cells into the stroma and vessels, their depth of invasion, histopathological features, colonization, and proliferation in the lymph nodes. Recent studies show that the number of cells at the invasive front can be a prognostic factor in early-stage patients, with more than 5 cells at the edge of invasion indicating a poorer prognosis. According to studies, the depth of invasion of 4 mm or more is strongly related to the possibility of cervical metastasis, and its increase is strong along with the increase in mortality in patients with oral SCC [19]. Our study aimed to evaluate and compare invasive front, depth of invasion, and CAP grading indices in oral SCC and Lip SCC.

This study found that the relationship between the invasive front and histological differentiation is significant. This measure was also significant for the depth of invasion. It was found that there is a significant relationship between vascular invasion and histological differentiation, but this criterion was not significant for neural invasion. Also, according to the significant relationship between the lymphocyte response and the degree of the disease, it can be concluded that the lower the lymphocyte response, the higher the degree of the disease.

In addition to histopathological parameters, the oral mucosal microenvironment plays a crucial role in modulating tumor invasion and progression. Chronic irritation, mechanical trauma, and the presence of orthodontic appliances or other prosthetic devices can induce local inflammatory responses, leading to alterations in tissue architecture and stromal composition. These changes may promote epithelial-mesenchymal transition and facilitate cancer cell invasion, thereby influencing both the Invasive Front (IF) and Depth of Invasion (DOI) in OSCC and lip SCC. The inflammatory microenvironment, characterized by increased lymphocyte infiltration, cytokine release, and vascular changes, can modify the tumor-stroma interactions and affect histological patterns observed at the invasive edge. Therefore, considering the impact of local mucosal factors is essential for interpreting IF and DOI findings, as these microenvironmental influences may contribute to variability in histological differentiation, tumor aggressiveness, and ultimately patient prognosis.

In the 2021 study by Mohtsham et al., 87 OSCC cases were evaluated and classified into low-risk and high-risk groups based on the pattern of the invasive front. Overall, 50.6% of the cases were categorized as low-risk and 49.4% as high-risk. Notably, 75% of patients in the low-risk group presented with grade 1 tumors [20]. In the current study, in the examination of the edge of invasion, 62.5% of lip SCC samples were in the low-risk group, and 37.5% were in the high-risk group, and in the OSCC samples, 60% were low risk, and 40% were high risk, which is consistent with the results of the above study.

In a study by Chang Wang et al. in 2011, tumor buddings were counted. Those who had more than or equal to 5 buds were placed in the high intensity group. Their results show that among 230 patients, 111 samples were in the high intensity group. Their study showed that the number of buds is related to the size of the tumor and the degree of differentiation [21]. In our study, IF status was also significantly related to histological differentiation; So that the low-risk group is better differentiated, included grade 1 and 2.

In the study by Wing Yuen et al., which included 72 patients with tongue carcinoma, the invasive edge grading method introduced by Byrne and tumor thickness were used. The samples were divided into three groups by thickness: less than or equal to 3 millimeters, 3 to 9 millimeters, and greater than 9 millimeters. They found that 59.7% of patients were in the middle group in terms of thickness [22]. In our study, DOI was evaluated, which, according to the CAP protocol, has greater prognostic value than tumor thickness.

Almangush et al.'s study in 2020 [19] aimed to investigate tumor depth and the invasive front in 233 early-stage OSCC patients. They evaluated tumor depth from the tumor edge to the deepest point of invasion and divided it into two groups: greater than or equal to 4 mm or less than 4 mm. Among the patients, 153 people (65.6%) were placed in the high group. In the two groups of our study, 40% and 37.5% of the cases were in the high group. This difference may be due to sample size, racial and geographical differences, and oral habits.

P. Santos et al. investigated grading systems in squamous cell carcinoma of the lip in 2014. In this study, which included 59 patients with lip SCC, the samples were also examined for invasion pattern. In terms of invasive front grading, patients were divided into low and high groups. Sixty-one percent of patients were in the high-grade group of invasive edge. In terms of invasion pattern, the patients were divided into five groups: (1) pushing, (2) large separated islands, (3) small tumor islands, (4) tumor satellite, and (5) finger. Overall, 52.5% of patients were placed in groups 1, 2, and 3 in terms of invasion pattern [23]. Despite differences in the classification of the invasion pattern, 65% of the samples we studied in the PUSH and STRAND groups were almost similar to the above study.

It can be concluded that the significant relationship between histological differentiation and IF status indicates that this is the best way to evaluate the invasion rate in lip SCC and OSCC. Parameters related to IF and DOI are cheaper and more accessible tools than biological markers for evaluation the new grading of SCC. The lack of statistically significant difference between the new grading indices in lip SCC and OSCC indicates that the location of lip SCC and early detection of the lesion compared to oral SCC, the difference in the biological conditions of the oral environment compared to vermilion, oral habits, anatomical proximity such as the distance from glands Lymphatic and epigenetic factors effective in saliva, can be one of the main factors affecting prognosis and response to lesion treatment in these two adjacent places.

The grading of the studied samples was done based on the criteria of the CAP protocol, and for the first time, lip SCC and OSCC were compared from the perspective of new grading methods. Our most important limitation in conducting this study was that some low-quality samples were excluded, and it was difficult to obtain replacement samples due to the limited number available. Incompleteness of patients' files in medical-educational and research centers caused the withdrawal of a significant number of samples from the study.

Considering the significant relationship between histological differentiation and both IF and DOI, these parameters are valuable for evaluating the invasion pattern and prognosis of oral and lip SCC. Moreover, IF and DOI can directly inform clinical decision-making by guiding the determination of surgical margins, risk stratification for recurrence, and the design of postoperative follow-up protocols. These findings highlight the practical relevance of histopathological assessment in patient management, providing accessible and cost-effective tools compared to molecular markers. Further studies integrating IF and DOI with patient outcomes could refine surgical and therapeutic strategies in OSCC and lip SCC.

This retrospective, cross-sectional comparative study provides valuable insights into the prognostic significance of Invasive Front (IF) and Depth of Invasion (DOI) in Oral and Lip Squamous Cell Carcinoma (OSCC and Lip SCC). However, several limitations should be considered. The strict inclusion criteria and limited availability of high-quality biopsy samples resulted in a total of 80 patients, which may restrict the generalizability of the findings. Incomplete medical records led to the exclusion of some patients, potentially affecting the accuracy of correlations between histological parameters and clinical outcomes. Moreover, variability in tissue handling and slide quality could introduce minor biases in pathological assessment.

Future research should focus on prospective, multicenter studies with larger and more diverse patient populations to reduce selection bias and improve generalizability. Standardized protocols for sample collection, slide preparation, histopathological evaluation, and data management are recommended to minimize variability and ensure reproducibility of IF and DOI measurements. These studies could further clarify the clinical implications of IF and DOI in surgical decision-making and postoperative follow-up strategies for OSCC and Lip SCC.