Classified by the WHO, benign salivary gland tumors comprise 11 different types, among which BCA is characterized as a benign unspecified entity, exhibiting an overall incidence rate ranging from 1.1–3.7%. [4] BCAC, a rare entity comprising only 2.9% of all salivary gland carcinomas, has resulted in limited research on this specific subtype. [7] There has been a recent surge in studies focused on BCA and BCAC conducted at a single center. Rito et al. reported a single-institution-based study, presenting 41 cases of BCA and 27 cases of BCAC. [18] Tereda et al. reported on the clinical features of nine patients with BCAC, comparing them to 45 patients with BCA treated during the same period. [7] Our study, spanning over two decades, primarily focused on the demographics, tumor characteristics, and clinical outcomes of patients diagnosed with BCA and its malignant counterpart, BCAC. This study reports the largest number of BCA cases among studies conducted at a single institution, aiming to provide insights into the clinical characteristics and surgical outcomes of both BCA and BCAC.
Demographics and clinical characteristics
BCA and BCAC are both reported that there is a near equal proportion of gender balance in each, and both occur over a wide age range, most often in the sixth to seventh decade. [10, 18, 19] In this study, what notably differs from previous reports is the diverse age distribution of BCA, ranging from the 20s to the 80s, with a peak in the 50s. For BCAC, all 7 cases involved female patients. BCAC is known to originate in the parotid glands in 88% of cases, and there is no gender preference.[8] This suggests a potential difference in demographics concerning occurrences in Asians, indicating the need for further research in this direction. Subsequent studies are warranted for a more comprehensive understanding of these demographic variations.
Limited research has been conducted to date on the specific locations of occurrence within the parotid gland for BCA and BCAC. In our study, 47 out of 81 cases of BCA were located exclusively in the superficial lobe, whereas no BCAC cases were confined solely to the superficial lobe; instead, all cases involved the deep lobes. In terms of tumor size, 76 out of 81 BCA cases and all 7 BCAC cases were confirmed to be at T2 stage or below, indicating sizes of 4 cm or smaller. In contrast to previous reports highlighting larger sizes for BCAC, our study showed relatively smaller sizes, suggesting a potential association with the observed favorable prognosis. [10, 16]
Differential diagnosis
Fine needle aspiration (FNA) has emerged as a pivotal diagnostic tool in evaluating parotid masses, providing preliminary insights before surgical intervention.[9] Studies have reported the diagnostic utility of FNA in BCA or BCAC as well. [20–22] In a recent study discussing the imaging features of 43 cases of parotid BCA, it was noted that BCAs with widespread cribriform structures and of the membranous type may undergo malignant transformation, requiring careful consideration in clinical management. [17] The study emphasized FNA's importance in differentiating between benign and malignant conditions, identifying malignancy or suspicion of malignancy in 6 out of 9 BCAC patients. However, even in all studies highlighting the usefulness of FNA, there is an emphasis on the potential diagnostic pitfalls and the need to consistently consider the possibility of misdiagnoses. The low prevalence of BCA and BCAC further attributes to such errors. In addition, BCA and BCAC have common cellular characteristics, including trabecular, tubular, solid, and membranous patterns, with frequent occurrences of mixed patterns. [10] In our study, reflecting on the cases of BCAC, four out of seven were initially approached without suspicion of malignancy, and surgery proceeded presuming BCA. This suggests that even when diagnosed as BCA through FNA, surgeons should be mindful of the potential for malignancy during the surgical procedure. Therefore, it is essential to explicitly explain this possibility to the patient before surgery.
The malignant transformation of benign tumors of the salivary gland is predominantly seen in the pleomorphic adenoma, with a range of 4.5–8.5% of cases. [23] However, the occurrence of carcinomas arising in BCA is rare, with few reported cases presenting various malignant components such as adenocarcinoma not otherwise specified, salivary duct carcinoma, and basaloid carcinoma [23, 24] In Muller et al.'s study, a literature review of previous 65 cases of BCAC revealed that 77% developed de novo, while 23% originated from preexisting BCAs. [16] The occurrence of malignant transformations of BCA in the parotid gland is a rarely documented event.[25] In cases where such transformations do occur, their manifestation is predominantly observed as malignant basaloid tumors.
Surgical treatments and prognosis
In the case of BCA, tumor excision including the capsule was usually performed, given its typical encapsulated nature. No instances of recurrence were observed during the follow-up period when such excision, including the capsule, was undertaken. As mentioned above, BCAC may present challenges in FNA diagnosis, and surgical findings can be similar to those of encapsulated BCA. Interestingly, even in cases where malignancy was not initially suspected, and only marginal excision was performed through partial parotidectomy, our institution observed no long-term occurrences of local recurrence or metastasis. Taking into consideration findings from other studies, when parotidectomy is performed thoroughly with complete extracapsular dissection, a positive long-term outcome can be expected. [26, 27] In light of this, for BCAC, even without wide excision, appropriate resection including the capsule, particularly in cases with a relatively low T stage, can result in favorable outcomes. However, as there is still limited reporting with few studies addressing the invasive features of BCAC, future research is essential to solidify these conclusions. It can be inferred that in cases where excision is performed suspecting BCA, including the capsule in a safe manner is necessary for a thorough resection.
This study has several limitations. In the case of BCAC, the limited number of cases remains a constraint for comprehensive characterization of clinical features. Second, the retrospective nature of the study introduces the possibility of selection bias. A substantial number of prospective studies are required to gain a thorough understanding of the prognosis and treatment of BCA and BCAC. However, considering the rarity of reported cases for both BCA and BCAC, our study is believed to contribute as a valuable resource in furthering the understanding of the clinical characteristics of these two tumors.