Abhishek Mahajan ^1,2,3* Shivam Rastogi ^3,4 SHREYA SHUKLA ^3,4,5 Ujjwal Agarwal ^3,4 Richa Vaish ^3,4 Nivedita Chakrabarty ^3,4 Renuka Ashtekar ³ Shonal Deokar ^3,4 Atif Shaikh ^3,4 Prathamesh Pai ^3,4 Pankaj Chaturvedi ^3,4 Sarbani Ghosh Laskar ^3,4 Vasundhara Smriti ^3,4 Swapnil U. Rane ^3,4 Munita Bal ^3,4 Asawari Patil ^3,4 Neha Mittal ^3,4 Vanita Noronha ^3,4 VIJAY PATIL ^3,4 Kumar Prabhash ^3,4 Anil K. Dcruz ^3,4

• ¹ The Clatterbridge Cancer Centre, Liverpool, United Kingdom
• ² Faculty of Health and Life Sciences, University of Liverpool, Liverpool, North West England, United Kingdom
• ³ Tata Memorial Hospital, Mumbai, Maharashtra, India
• ⁴ Homi Bhabha National Institute, Mumbai, Maharashtra, India
• ⁵ Homi Bhabha Cancer Hospital and Mahamana Pandit Madan Mohan Malaviya Cancer Centre, Varanasi, Uttar Pradesh, India

Background and objective: Extrathyroidal extension (ETE) is the term used to describe the growth of the primary thyroid tumour beyond the thyroid capsule. ETE is a critical prognostic marker for thyroid tumors, necessitating accurate preoperative assessment. The purpose of this study was to evaluate the diagnostic performance of computed tomography (CT)-based grading for ETE and tracheal invasion (TI) for preoperative prediction in patients with differentiated papillary thyroid carcinoma (PTC) and compare the diagnostic accuracy with ultrasound (US).This retrospective study was approved by our institutional review board.Preoperative US and CT were performed for 83 patients who underwent surgery for PTC between 1 st January 2010 to 31 st December 2020. The US and CT features of ETE and TI of each case were retrospectively and independently investigated by two radiologists. The diagnostic performances of US and CT, including their specificity, sensitivity, positive predictive values (PPV), and negative predictive values (NPV) for ETE, and their accuracy in predicting ETE and TI were analysed. As per the grading for ETE on USG and CT, lesions were graded into three grades and XXX grading was also devised on CT to predict the TI and graded into four grades.Results: Accuracy and specificity of CT are relatively good for identifying tumour infiltration into the adjacent structures, and ranges from 82-87% and 95%-98% respectively. It however, has a low sensitivity, between 14.3% and 77.78% when compared to US, which suggests, that in case of any doubt regarding CT evidence of tumor infiltration into surrounding structures, additional clinical examination must be performed. CT showed better sensitivity (78%) and specificity (75%) in detecting TI compared to the previous studies. The diagnostic accuracy of CT XXX grading was 91.5% with p<0.005 in prediction of TI.Preoperative US should be regarded as first line imaging modality for predicting minimal ETE, and CT should be additionally performed for evaluation of maximal ETE. Specificity and PPV of CT is higher than US in detecting overall ETE and TI of PTC. The US and CT based grading systems have the potential to optimize the pre-operative surgical planning.