VMAT Treatment Evaluation in Patients with Head-And-Neck Cancer (HNC): Analysis of Systematic, Random Errors and Evaluation of CTV-PTV Margin.

Abstract

This study focuses on refining the accuracy of treatment setup and delivery for patients with advanced head-and-neck cancer (HNC) undergoing Volumetric Modulated Arc Therapy (VMAT). VMAT allows rapid, dynamic delivery of highly targeted radiation doses, making precise error control essential to avoid unnecessary radiation exposure to healthy tissue. We evaluated 40 patients with stage III or IV HNC who received VMAT at our center, delivering a simultaneous integrated boost (SIB) of 70 Gy to the primary tumor and 57.75 Gy to the nodal regions. This research specifically assessed variations in patient setup along the vertical, longitudinal, and lateral axes.  The study relied on data from the Monaco Treatment Planning System (Version 5.11.02, Elekta) and the Mosaiq software (Version 2.0), integrated with iViewGT and XVI onboard imaging systems on the Elekta Versa-HD machine. Before treatment, we conducted quality assurance and quality controls (QA/QC) and dosimetric verification using the MyQA platform and a high-resolution Matrixx Universal detector array. This allowed us to create a database capturing treatment setup variations to ensure consistency and reliability across all treatment fractions.  The results emphasize the importance of understanding both systematic and random errors to enhance treatment precision. Following the International Commission on Radiation Units (ICRU-51 and 62) recommendations, we calculated systematic errors in the X, Y, and Z directions to be 0.93 mm, 0.9 mm, and 0.88 mm, respectively, while random errors were 0.98 mm, 0.82 mm, and 0.92 mm in these same directions. Using Van Herk’s margin formula, we determined the Clinical Target Volume to Planning Target Volume (CTV-PTV) margins as 3.011 mm, 2.824 mm, and 2.844 mm for each axis. QA/QC results also confirmed a high dose accuracy with a gamma index average of 99.5% across treatments.  In conclusion, managing systematic and random errors in external beam therapy is critical, especially for complex regions like the head and neck, where anatomy can vary significantly. Despite advancements in VMAT, errors remain inherently unpredictable, underscoring the need for precise margin calculations to maintain treatment quality. Accurate CTV-PTV margins are crucial for minimizing radiation exposure to healthy tissue while ensuring that the tumor receives the prescribed dose, ultimately improving treatment outcomes for patients with large tumor volumes in radiosensitive areas.