A 58-year-old woman with a history of oral submucous fibrosis presents with a 4-month history of dysphagia and a palpable mass in the floor of mouth. Intraoral examination reveals a submucosal swelling with restricted mobility. Which investigation is most appropriate to assess the depth of invasion and involvement of the mylohyoid muscle?

## Investigation of Choice for Depth Assessment in Oral Cavity Carcinoma ### Clinical Scenario A floor-of-mouth mass with restricted mobility and dysphagia suggests advanced oral carcinoma with potential involvement of deep structures (mylohyoid muscle, hyoglossus muscle, submandibular gland). The question specifically asks for assessment of **depth of invasion** and **muscle involvement**. ### Why MRI is Superior for Soft Tissue Assessment **Key Point:** MRI is the investigation of choice for assessing depth of invasion and involvement of deep soft tissue structures in oral cavity carcinoma because it provides: - Superior soft tissue contrast resolution - Clear delineation of tumor margins - Assessment of muscle invasion (mylohyoid, hyoglossus, genioglossus) - Evaluation of perineural spread - No radiation exposure **High-Yield:** MRI is particularly superior to CT for: - Distinguishing tumor from edema - Assessing involvement of intrinsic tongue muscles - Detecting perineural invasion (associated with poor prognosis) - Evaluating mandibular marrow involvement (T4b disease) ### Comparative Imaging Modalities | Modality | Soft Tissue Contrast | Bone Cortex | Muscle Invasion | Perineural Spread | Radiation | |----------|----------------------|-------------|-----------------|-------------------|----------| | **MRI** | Excellent | Moderate | Excellent | Excellent | None | | **CT** | Moderate | Excellent | Moderate | Moderate | Yes | | **Ultrasound** | Good (operator-dependent) | Poor | Moderate | Poor | None | | **OPG** | Poor | Good | Poor | Poor | Yes | **Clinical Pearl:** In floor-of-mouth carcinomas, MRI can detect involvement of the mylohyoid muscle, which is critical for surgical planning and determines whether the tumor is resectable or requires composite resection. ### Role of CT in Oral Cavity Carcinoma While CT is excellent for: - Detecting bone cortical erosion and mandibular involvement - Assessing cervical lymph nodes - Pulmonary staging It is inferior to MRI for assessing soft tissue depth and muscle invasion. **Mnemonic:** **SOFT** = **S**oft tissue → **MRI** (Excellent); **O**sseous cortex → **CT** (Excellent); **F**low/vascularity → **CT angiography**; **T**issue depth → **MRI** (Best) [cite:Robbins 10e Ch 7]