Max Hanlon
RMIT University
PhD Candidate

Ryan Smith
Senior Medical Physicist
Alfred Health Radiation Oncology

Rick Franich
Professor of Medical Physics
RMIT University

Background and Purpose

An aspect of treatment verification in HDR brachytherapy involves calculating the dose from the measured source dwell positions. Current TPSs cannot reconstruct delivered dose measured by our source tracking verification system, as they can only utilise dwell positions that reside within previously defined catheter paths. Due to this, a fast dose calculation engine (DCE) that can accept the input of arbitrary dwell positions from the source tracking system is required. Here we present a TG-43 based DCE that computes 3D dose grids for measured dwell positions and allow comparison with the treatment plan.

Methods

The DCE, dubbed MaxiCalc, calculates dose grids using the input of measured dwell positions and times, allowing direct comparison to the treatment plan. MaxiCalc was validated against Oncentra Brachy (OCB v4.3) at 27 single dose points, as per OCB commissioning, as well as a 3D dose grid of 13 dwells.

Dwell positions and times delivered in a phantom were measured by our source tracking system, as previously published.[1] The measured dwell positions were then used as input to MaxiCalc and the resultant dose grid compared to that from OCB. Observed dose differences due to source position measurement uncertainties were investigated.

Results

For the 27 dose points, MaxiCalc differed from OCB by a mean of 0.08% (σ=0.07%, max 0.41%) demonstrating differences that are similar to those between published values [2] and OCB. In a multi-source plan for doses between 50-200% of the prescription dose, MaxiCalc has a mean agreement of 0.2% with 98% of points agree by <1%. For doses above 50% of the prescription, the 3D gamma pass rate (1%/1mm) is >99%.

A dose grid was generated from the measured positions from a delivered plan of 25 dwells, with a mean deviation between planned and measured positions of 0.6 mm. Comparing to the planned dose, the dose difference due to difference in measured positions can be clearly seen in figure 1. Results from measured delivery errors such as these will also be presented.

Conclusion

MaxiCalc provides a tool to perform real-time dosimetric treatment verification in conjunction with our source tracking system. It can overcome limitations of clinical TPSs by providing fast dose calculations for arbitrary measured dwell positions.

References

1. Smith, R L., et al. Medical physics 43.5 (2016): 2435-2442.

2. Daskalov, G M., Löffler E, and Williamson J. Medical physics 25.11 (1998): 2200-2208.


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