This study clearly disclosed that the TD-assisted FDG PET/CT protocol is easy and effective in analyzing the localization and extent of the tumor and detecting tumor involvement in adjacent structures when evaluating oral cancer.
In recent years, the incidence of oral cancer has increased significantly. Rebecca et al. reported in 2017 that oral cancer will rank tenth among new cases of malignant tumors [19]. Evaluating the extent of lesions is very important for the treatment and prognosis of oral cancer. CT or MRI is commonly used for routine examinations for oral cancer [20]. Studies have shown that CT or MRI can provide high-quality morphological assessment of oral cancer. However, the examinations are challenged by anatomical complexity of the head and neck and the presence of imaging artifacts [5]. Moreover, after oral cancer is treated, the complex anatomical structure of the area becomes more complicated due to changes after surgery or radiation. The imaging landmarks and symmetry are lost, and the normal anatomical structure is significantly deformed, which distinguishes the changes after treatment from recurrence or residual tumors are challenging.
Instead, PET/CT could provide both accurate anatomical and useful metabolic information of oral cancer. F-18 FDG PET/CT is particularly useful for staging, restaging and radiotherapy planning as well as for assessment of treatment response in head and neck squamous cell carcinoma patients, due to its superior accuracy over clinical examination and conventional anatomic imaging such as MRI and CT examinations [21–23].
F-18 FDG PET/CT as compared with the MRI and CT image, FDG PET/CT has a high sensitivity in tumor detection and anatomic location [24]. The main limitations, especially in the post-treatment setting, are possible false positive results due to inflammation and the inability to detect microscopic disease [4].
However, the oral cavity is a small cavity with complex components including lips, buccal mucosa, tooth, gingiva, tongue, hard palate, floor of mouth, maxilla, mandible bones and some people with dental amalgam, etc. An invasive oral cavity cancer frequently disrupts the anatomical barrier in this region and causes difficulty in the interpretation of an FDG PET/CT study as a metabolic anatomical scan for disease extent evaluation. Some non pathological FDG uptake such as salivary glands’ activity or muscle activity also interfere the interpretation of the tumor mapping [25–26]. Therefore, for a long time, the images of oral cancer patients have puzzled nuclear medicine physicians.
An open-mouth protocol in the staging of oral cancer had been developed at CT and F-18 FDG PET/CT examinations [14, 27]. Cistaro et al. reported that the open-mouth scan improved the anatomic tumor localization and extent and detection of tumor involvement in adjacent anatomic structures achieved by the standard F-18 PET/CT procedure. In addition, time of the examination (mid morning), relaxation of muscles before the compound was administered, and an upright position while the patient waited caused a reduction of the frequent equivocal physiologic uptake in the head and neck region. The open-mouth method does not influence the nodal staging. [14].
However, some patients with oral cancers suffered from severe limited mouth opening. Trismus is a significant complication of oral malignancies or its surgical and radiotherapy treatment, or both. Consideration must be given to its early diagnosis, to help in timely intervention and planning of preventive strategies [15].
An ideal examination technique for the assessment the oral cancers is the main issue for nuclear medicine doctor in the F-18 FDG PET CT scan. In this study, we designed a new TD-assisted method to resolve the aforementioned problem by using a wooden tongue depressor to place either between buccal mucosa and teeth or between tongue border and teeth when performing the F-18 FDG PET/CT scan on the delayed 3-hour image. In both groups of patients with tongue and buccal mucosal cancers, the discrimination ratio of tumors [(patients with being able to be clearly distinguished the characteristics of tumors /all patients) × 100%] were significantly higher on images with TD placed, compared with images without TD placed (p < 0.001). In the group of patients with other cancers (lip, palate, gingiva, oropharynx, mouth floor & tonsil), the discrimination ratio of tumors were also higher on images with TD placed, compared with images without TD placed. The feasible, inexpensive and noninvasive TD-assisted FDG PET/CT method introduced in this study provided great benefits in diagnosing the tumor’s border with separation from the adjacent anatomical structure (such as gum, tooth, palate, palatine tonsil, metal artifacts and dental amalgum). In addition, the TD-assisted FDG PET/CT method also has several advantages for patients with oral cancers:
1. easily performed by nuclear medicine physicians without spending longer time,
2. no harm and without painful sensation to patients,
3. increasing oral cavity space without artifact on the images due to low density of the wood material (about 0.35–0.5 g/cm
3),
4. easily reach a consensus between radiologists and otolaryngologists,
5. reproducibility.