A dosimetric comparison of split and open x- jaw methods in volumetric modulated arc therapy for extended target volumes of head and neck cancer

Abstract

Author(s): M. Boopathi, D Khanna*, P. Mohandass and P. Venkatraman

The Treatment Planning System (TPS) in radiotherapy often faces challenges in optimizing techniques for large tumor volumes due to mechanical limitations, such as the over-travel distance of the Multileaf Collimator (MLC). This study compares the dosimetric outcomes of split field and open field techniques in Volumetric Modulated Arc Therapy (VMAT) for carcinoma tongue patients with large target volumes. Twenty-one patients treated with VMAT were evaluated using Eclipse™ TPS. The dosimetric parameters including conformity index, homogeneity index, and dose to Organs at Risk (OAR) were compared between split and open field techniques.

Methods: VMAT plans utilizing two arcs were generated in Eclipse™ TPS with 6MV photon beams on a Truebeam STx Linear accelerator for all patients using both split and open field techniques. In the split field technique, fields were split along the X-Jaw direction. Optimization objectives and collimator angles were kept consistent for both techniques. Dosimetric data were collected and analyzed for conformity index, homogeneity index, and OAR doses.

Results: The study revealed a 3% and 4% difference in homogeneity index, with conformity index values averaging 95% and 97% for split and open fields, respectively. Split field technique demonstrated reduced doses to OARs including Brainstem Dmax (9%), Spinal cord Dmax (6%), PORV cord Dmax (5%), Right Parotid Mean Dose (3%), Left Parotid Mean Dose (3%), Right Parotid (D50%) (5%), and Left Parotid (D50%) (2%), compared to the open field technique. The 50% isodose volume was also 4% lower in the split field technique. While the maximum dose remained identical, differences were observed in homogeneity index, conformity index, maximum dose, and mean dose to OARs.

Conclusion: The split field technique demonstrates superior dosimetry outcomes with reduced doses to OARs compared to the open field technique, albeit with increased treatment time. Thus, the split field technique presents as a favorable option for planning large target volumes in carcinoma tongue cases.

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