Independent secondary dose calculations are a well-established and highly effective verification method which complements measurement-based patient-specific QA. They help to streamline patient QA workflows and play a crucial role in identifying errors that may slip through primary checks during treatment planning.  

In our recent interview, Dr Aurora Vicedo González, medical physicist at Consortium General University Hospital of Valencia, talks about the advantages of using a Monte Carlo-based 3D dose calculation system for both standard and advanced treatment techniques. She also shares insights on key factors to consider when selecting a secondary dose check software.

Why opt for a secondary dose calculation system in your facility?

There were multiple reasons why we decided to implement a secondary dose calculation system in our patient-specific QA workflow.

Firstly, a secondary dose calculation system is vital in detecting potential errors that may have been overlooked during primary checks when commissioning a new treatment planning system (TPS) or a new TPS release. It is imperative that the secondary dose calculation system relies on Golden Beam Data or completely independent measurements to ensure its accuracy.

Moreover, a redundant calculation system allows us to enhance dose calculation accuracy through continuous evaluation of data and systematic discrepancies between the two systems. This leads to ongoing improvements in treatment quality.

A secondary dose calculation software also aids in identifying specific errors or discrepancies during the patient planning process, such as wrongly overwritten contours, the use of inappropriate calculation algorithms, or inaccuracies in calculating heterogeneous materials.

While we conduct VMAT/IMRT phantom measurements as part of our patient-specific QA, we are well aware that this method does not account for patient geometry and heterogeneities. Integrating a 3D secondary dose verification system alongside experimental measurements helps us to bridge this gap. By calculating the dose within the same patient as the primary TPS, the independent dose calculation system enables us to detect systematic errors arising from inaccurate TPS dose calculations in heterogeneous regions.

Numerous publications, like the AAPM TG-219 Report, recommend the use of secondary systems. Furthermore, compliance with Spanish legislation mandates having a redundant system in clinics.

In summary, implementing a secondary dose calculation system enables us to detect and prevent errors, enhance treatment quality, and meet institutional requirements.

What are the advantages of incorporating Monte Carlo dose recalculations in your patient QA program?

In the past, dosimetric verification involved checking dose calculations at a single point within a water phantom. However, with the emergence of complex, non-uniform, and highly modulated treatments like VMAT, IMRT, and stereotactic treatments with multiple metastases, relying solely on single-point calculations is no longer sufficient. As treatment calculations become more precise and complex, there is a growing need for more sophisticated methods to validate dose calculation accuracy. Volumetric calculations that take into account patient geometry and heterogeneities are now essential.  

The accuracy of dose calculations depends on the algorithm used. While most treatment planning systems calculate dose-to-water, clinical studies recommend using dose-to-medium calculation algorithms in clinical settings for better accuracy. Comparing TPS outcomes to Monte Carlo calculations is beneficial as Monte Carlo calculations precisely simulate radiation transport through the patient and its heterogeneities, allowing us to compare our treatment to what is considered the ground truth. This comparison not only helps us to identify calculation errors but also highlights TPS limitations, especially for treatments involving lung, bone, or high-density materials.

A Monte Carlo-based secondary check system is particularly valuable for SBRT lung treatments, where small fields and low-density tissues can pose challenges for dose calculation.

What factors should clinical users consider when selecting an independent dose calculation software?

When selecting an independent dose calculation system, clinical users should consider various factors. As the system is used frequently throughout the day, it is essential that it seamlessly integrates into daily operations without disrupting the workflow in your department. The software should be user-friendly, time-efficient, and ideally fully automated. It should also be easily accessible for any user (e.g., through server/web access), with customizable and comprehensive reports displaying key information.

Independence from TPS data is crucial, with the software utilizing Golden Beam Data or other TPS-independent measurements.

The algorithm implemented in the software is equally important, as well as a comprehensive 3D verification that includes patient geometry, heterogeneities, and dose-to-medium, allowing users to identify errors or discrepancies in complex treatment plans. Tools like Dose Volume Histogram comparison and Gamma Index analysis aid medical physicists in pinpointing the source of discrepancies.