Commissioning and clinical implementation of the first commercial independent Monte Carlo 3D dose calculation to replace CyberKnife M6™ patient-specific QA measurements.

Maaike T W Milder,Mischa S Hoogeman,Markus Alber,Matthias Söhn

doi:10.1002/acm2.13046

Abstract

PurposeTo report on the commissioning and clinical validation of the first commercially available independent Monte Carlo (MC) three‐dimensional (3D) dose calculation for CyberKnife robotic radiosurgery system® (Accuray, Sunnyvale, CA).MethodsThe independent dose calculation (IDC) by SciMoCa® (Scientific RT, Munich, Germany) was validated based on water measurements of output factors and dose profiles (unshielded diode, field‐size dependent corrections). A set of 84 patient‐specific quality assurance (QA) measurements for multi‐leaf collimator (MLC) plans, using an Octavius two‐dimensional SRS1000 array (PTW, Freiburg, Germany), was compared to results of respective calculations. Statistical process control (SPC) was used to detect plans outside action levels.ResultsOf all output factors for the three collimator systems of the CyberKnife, 99% agreed within 2% and 81% within 1%, with a maximum deviation of 3.2% for a 5‐mm fixed cone. The profiles were compared using a one‐dimensional gamma evaluation with 2% dose difference and 0.5 mm distance‐to‐agreement (Γ(2,0.5)). The off‐centre ratios showed an average pass rate >99% (92–100%). The agreement of the depth dose profiles depended on field size, with lowest pass rates for the smallest MLC field sizes. The average depth dose pass rate was 88% (35–99%). The IDCs showed a Γ(2,1) pass rate of 98%. Statistical process control detected six plans outside tolerance levels in the measurements, all of which could be attributed the measurement setup. Independent dose calculations showed problems in five plans, all due to differences in the algorithm between TPS and IDC. Based on these results changes were made in the class solution for treatment plans.ConclusionThe first commercially available MC 3D dose IDC was successfully commissioned and validated for the CyberKnife and replaced all routine patient‐specific QA measurements in our clinic.

Highlights

To ensure safe dose delivery in stereotactic radiotherapy, uncertainties and errors in dose delivery must be minimized by an extended and strict quality assurance (QA) protocol
This paper describes the commissioning and clinical implementation of the first commercially available 3D Monte Carlo dose engine (SciMoCa RT, Munich) for two CyberKnife® M6TM robotic radiosurgery system (Accuray Inc., Sunnyvale, CA) in order to replace patient-specific QA measurements
Absolute depth dose curve (DDC) and output factor (OF), and relative cross profiles calculated by SciMoCa were compared to measurements in a homogenous water phantom using a Γ(2,0.5) and Γ(1,0.5), respectively.[18]

Summary

Introduction

To ensure safe dose delivery in stereotactic radiotherapy, uncertainties and errors in dose delivery must be minimized by an extended and strict quality assurance (QA) protocol. An established part of a QA protocol is the patient-specific pre-treatment verification of the calculated dose, either by measurements or by checking the monitor units with an independent dose calculation (IDC).[1]. Patient-specific QA measurements can be performed using chambers, film, or diode arrays.[2,3,4,5,6,7] Especially for stereotactic CyberKnife treatment plans, both measurement equipment and analysis require stringent quality criteria. Acknowledged gamma criteria of 2% dose difference and 2 mm distanceto-agreement Γ(2,2) are insufficient to detect possible errors relevant during CyberKnife dose delivery.[2,4,8] measurements are costly by consuming valuable personnel and machine time. Despite fulfilling the strict criteria, the relevant errors that can be picked up are limited and mainly refer to problems regarding the delivery system.[9,10,11] It is more efficient that these types of issues are addressed by proper commissioning and machine QA.[8]

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Conclusion