Validation of the geometric equivalent field concept in total scatter factor calculations, for half-, quarter- and off-isocenter asymmetric square fields.

OBJECTIVE: Monitor unit (MU) verification for any symmetric or asymmetric field is performed using a total scatter factor (Scp ), that is calculated based on the geometric equivalent square field (GESF) concept. In this study, we measured the Scp of various asymmetric square fields (ASFs ) and their respective GESFs. METHODS: Square half-fields (SHFs ), square quarter-fields (SQFs ) and square off-isocenter fields (SOFs ), with sizes ranging from 3×3 cm2 to 20×20 cm2 were created, by varying the collimator jaws of two Varian iX Linacs (6/18 and 6/23 MV). A semi-flex ion chamber was used to measure Scp at a depth of 10 cm within a water phantom, at the effective field center (EFC) of all ASFs , and at the isocenter (IC) of their respective GESFs. The later Scp values were corrected by the off-axis ratio [OAR(r)] of the 40×40 cm2 field size, where r is the distance between EFC and IC. RESULTS: The results show that the Scp (EFC) is independent of the type of the ASF (SHF, SQF, or SOF) and no significant difference exists between the 18 and 23 MV beams. Compared with the Scp (IC), the Scp (EFC) increased with increasing r, by up to 2% and 4% for 18/23 and 6 MV, respectively. CONCLUSIONS: The GESF concept provides acceptable accuracy (< 2%) for the calculation of Scp of the ASFs used in most clinical situations (except from SOF with EFC at large r), and thus can be used in MU verification calculations.

Poorly differentiated thyroid carcinoma arising from substernal goiter: a case report.

Introduction and importance: Poorly differentiated thyroid carcinomas represent a rare heterogeneous group of malignant tumors that constitute ~2-4% of all thyroid neoplasms. Substernal goiter (SG) is defined as an enlargement of the thyroid gland that is located below the thoracic inlet. Malignant neoplasms arising from a SG were reported in only 2-3% of cases.This case report has been reported in line with the Surgical CAse REport (SCARE) Criteria.21. Case presentation: This article presents a 54-year-old Syrian female who presented at our institution due to dysphagia, dyspnea, cervical swelling, and loss of appetite. Following clinical and radiological examinations, total thyroidectomy with lymph node dissection was performed. Microscopic examination revealed an infiltrative growth pattern of insular, trabecular, and solid formations of epithelial cells with scant eosinophilic cytoplasm, hyperchromatic nuclei, and bizarre mitotic figures with areas of necrosis. Subsequently, the final diagnosis was confirmed as a multifocal poorly differentiated thyroid carcinoma arising from a SG. Clinical discussion: The heterogeneity of histologic features of poorly differentiated thyroid carcinoma represents a diagnostic challenge. Diagnosis of poorly differentiated thyroid carcinomas is based on the Turin Criteria, which highlights histopathological features. Computed tomography plays a major role in SG for further evaluation. Conclusion: In this manuscript, the authors aimed to present a unique case report with challenging diagnostic features including the rapid development of an infiltrative poorly differentiated thyroid carcinoma from a SG highlighting the importance of a detailed histopathological examination of thyroid nodules in the absence of significant medical history.