The oral cavity is located adjacent to vital organs, and hard and soft tissues are mixed in it to form a complex shape. Thus, determining the surgical margin of oral cancer is not simple. There are many reports concerning the establishment of the surgical margin [16–23]. With a safety margin of 5 mm or less from the tumor, the postoperative local recurrence rate is no different from cases with a positive margin [23]. However, the recurrence rate for a margin of 5 mm or less has been observed to be five times higher than a margin of over 5 mm (10 %) [20], and samples show that local tumor recurrence rate drops reliably when there is a distance of 5 mm or more from the tumor margin. From these reports, a safety margin of 10 mm or more at the time of resection can be considered reasonable [21, 24–26]. However, in the oral cavity, local recurrence occurs at a higher frequency for the endophytic and superficial types of clinical growth patterns compared with the exophytic type [1]. Additionally, since recurrence tendencies are high for cases where the histopathological mode of tumor invasion is bad, it is necessary to consider various factors when establishing the safety margin [14–16]. Furthermore, it is important to perform a suitable mandibulectomy in accordance with the invasion into the bone in cases where the oral cancer is adjacent to the mandible [17, 18].

Vital staining with iodine solution has become an indispensable auxiliary diagnostic method for the detection of early-stage cancer and it is widely used in the upper gastrointestinal region. In the oral cavity, iodine staining allows visualization of epithelial dysplasia and can identify malignant and potentially malignant lesions [27–30] (Figs. 7.6 and 7.7). Further, the accuracy of iodine staining compared with visual examination in discriminating between normal and epithelial dysplastic oral mucosa has been demonstrated [28–31] (Fig. 7.8). In an examination of primary lesion recurrence rate in cases of partial resection that considered the presence or absence of iodine staining, the group that utilized iodine staining was shown to have a significantly lower primary lesion recurrence rate. Consistent with these results, it has also been reported that, for superficial T1 or early T2 lesions, medical cases for which the surgical margin was determined by performing vital staining with iodine solution had a clearly lower local recurrence rate compared with cases where it was not performed (Fig. 7.9). Hence, determining the surgical margin by performing vital staining with iodine solution is useful for superficial lesions. However, there are also cases where the unstained area covers a large range, and so judgments of where to resect to can be difficult for such cases [32, 33] (Figs. 7.10 and 7.11).

Looking at the tongue anatomically, it is organized into the intrinsic and extrinsic muscles of the tongue. The intrinsic muscles are the superior longitudinal muscle, the inferior longitudinal muscle, the transverse muscle, and the vertical muscle; the extrinsic muscles are the genioglossus muscle, the hyoglossus muscle, and the styloglossus muscle. For surgery, the practitioner judges the invasion status of these muscle groups and the tumor to determine the surgical margin. Also, resection at a shallow depth is not problematic for tumors with superficial or exophytic clinical growth patterns, but performing vital staining with iodine solution is useful to judge the extent of dysplastic epithelium in the tumor periphery. When resecting endophytic tumors, careful attention should be paid to the deep parts of the surgical margin [17]. It is commonly necessary to resect not only the intrinsic but also the extrinsic muscles of the tongue. Furthermore, the tongue is adjacent to the floor of the mouth, the lower gingiva and alveolus, and the buccal mucosa; because a tumor can easily invade these tissues, accurate diagnosis of the extent of tumor progression at the time of surgery is important [34–38]. The histopathological malignancy of such tumors must also be considered (Fig. 7.12). It is unnecessary to take a wide safety margin for tumors with low malignancy, but a safety margin of 1 cm or more must be taken for those with high malignancy or invasiveness. Therefore, the surgical margin for a tongue carcinoma will change depending on the primary lesion’s size, the depth of invasion [17], and the invasive state and histopathological malignancy with regard to the floor of the mouth [34], the base of the tongue, and the mandible [35–38]. Furthermore, it may be necessary to perform neck dissection concurrently via a pull-through operation because tongue carcinomas result in a high frequency of cervical lymph node metastasis.

Lower gingival carcinomas invade the mandible relatively quickly in many cases, causing bone destruction. In the establishment of surgical margins, because the tumor progression into the bone cannot be directly probed physically, it is diagnosed via diagnostic imaging techniques such as dental X-ray, panoramic X-ray, CT, and MRI scans. It is important to have a complete picture of the depth and type of bone resorption and invasion to adjacent structures and to establish a treatment plan [49–51].