Systematic end-to-end testing of multiple target treatments using the modular RUBY phantom

Leonie Brodbek,Hui Khee Looe,Björn Poppe,Jana Kretschmer,Katrin Büsing,Daniela Poppinga

doi:10.1088/2057-1976/ac3e37

Abstract

The RUBY head phantom in combination with the System QA insert MultiMet can be used for simultaneous point dose measurements at an isocentric and two off-axis positions. This study investigates the suitability of the system for systematic integral end-to-end testing of single-isocenter multiple target stereotactic treatments. Several volumetric modulated arc therapy plans were optimized on a planning CT of the phantom positioned in a stereotactic mask on the stereotactic treatment board. The plans were created for three artificial spherical target volumes centred around the measurement positions in the MultiMet insert. Target diameters between 5 and 40 mm were investigated. Coplanar and non-coplanar plans were optimized using the collapsed cone algorithm of the Oncentra Masterplan treatment planning system and recalculated with the Monte Carlo algorithm of the Monaco treatment planning system. Measurements were performed at an Elekta Synergy linear accelerator. The head phantom was positioned according to clinical workflow comprising immobilization and CBCT imaging. Simultaneous point dose measurements at all target positions were performed with three PinPoint 3D chambers (type 31022) as well as three microDiamond detectors (type 60019) and compared to the treatment planning system calculations. Furthermore, the angular dependence of the detector response was investigated to estimate the associated impact on the measured point dose values. Considering all investigated plans, PTV diameters and positions, the point doses calculated with the Monaco treatment planning system and the microDiamond measurements differed within 3.5%, whereas the PinPoint 3D showed differences of up to 6.9%. Point dose differences determined in comparison to the Oncentra Masterplan dose calculations were larger. The RUBY system was shown to be suitable for end-to-end testing of complex treatment scenarios such as single-isocenter multiple target plans.

Highlights

For intracranial lesion irradiations, several publications [1,2,3] indicate that the stereotactic treatment of multiple lesions is associated with better neurological outcomes and less toxicity compared to whole brain radiation therapy
Since only positive angles relative to the detector orientation have been used in the non-coplanar volumetric modulated arc therapy (VMAT) treatment plans delivered within this study, the uncertainty associated with the angular response in the dose measurements is smaller
Our results show discrepancies between the treatment planning system (TPS) calculations at off-axis positions, whereas better agreement was obtained between measurements and Monaco calculations

Summary

Introduction

Several publications [1,2,3] indicate that the stereotactic treatment of multiple lesions is associated with better neurological outcomes and less toxicity compared to whole brain radiation therapy. In the past years, dedicated protocols have been published providing recommendations for QA of geometric and dosimetric accuracy required for stereotactic treatments [9,10,11]. These QA measurements include an integral system specific end-to-end (E2E) test of the complete treatment chain generally performed with a dedicated phantom. While there are several approaches to perform an E2E test for the case of isocentric single lesion irradiations[12], an E2E test for the irradiation of multiple and/or off-axis targets requires a specialized phantom suitable for such complex scenarios

Methods

Results

Discussion

Conclusion