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Pre-treatment Verification of Carcinoma Breast VMAT plan based on Mono-isocentric technique : Assessment of the combined fields feature of new 2D MatriXX arrays

Raoui Yasser

Objective: The aim of our work is to verify Mono-Isocentric technique based Volumetric Modulated Arc Therapy (VMAT) plan for carcinoma breast and regional nodes employing the new 2D arrays Matrix Resolution from IBA dosimetry systems, Schwarzenbruck, Germany loaded with the combined field feature.

Methods: This study included 12 Mono Iso-centric VMAT plans for breast cancer with supraclavicular and axillary nodes. The radiotherapy planning was performed by the Monaco TPS (5.51 Elekta Limited, Crawley, UK) following the departmental planning protocols employing 6 MV photons using XVMC algorithm for Dose calculation. 

The plans were optimized using an arc geometry with 25 increments in gantry angle spacing between control points with a 3 mm resolution dose grid size and 1% per calculation dose to medium, minimum segment width 0.5 cm and high fluence smoothing.

These plans were delivered clinically by an Elekta Infinity linear accelerator equipped with Agility 160 leaf MLC (Elekta Limited, Crawley, UK). Two CT scans of the Matrix resolution inserted in the Mini Phantom R were acquired using CT simulator (GE discovery (General Electricals, USA). Out of these two scans, the first one taken as the default CT and the second one as the extended CT, in order to use it for large fields combination.

In this study, normal and combined fields were compared using my QA patients’ software (IBA Dosimetry, Germany) based on the gamma index analysis and point dose measurements with the ion chamber cc04 according to IAEA Protocol TRS398.

Results: The new 2D array detector provided good agreement for dose maps without combined field feature over the field lengths ranging from 22 cm to 24 cm and excellent agreement for maps with combined fields for lengths ranging from 24 cm to 28 cm. VMAT Clinical cases passed with more than 95% for the set criteria of 3% DD & 3 mm. The absolute point dose measurement agreement was found to be more than 98%.

Conclusion: The Matrix Resolution is a convenient, fast, robust, and practical tool for routine large-field pre-treatment verification in IMRT, VMAT and other advanced techniques.