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Five Years of Clinical Application of independent Monte Carlo-Based Patient-Specific Quality Assurance at the Maastro Proton Therapy Center

Ilaria Rinaldi, Giorgio Cartechini, Angelo Schiavi, Jan Gajewski, Nils Krah, Antoni Rucinski, Gloria Vilches Freixas, Vincenzo Patera, Sebastiaan Nijsten

TL;DR

This paper reports five years of fully automated, GPU-accelerated Monte Carlo PSQA for pencil beam scanning proton therapy at Maastro. By integrating Fred into the DGRT framework, the authors implemented two workflows—pre-treatment plan recalculation and log-file-based delivered-dose reconstruction—to replace conventional measurement-based QA. Across 6000+ pre-treatment recalculations and 3513 log-file verifications, they achieved highly reliable 3D dosimetric verification with significant time savings (~4090 QA hours) and detected two planning-related failures that measurements would have missed. The study demonstrates that measurement-less MC-based PSQA is feasible, accurate, and clinically advantageous, offering a practical blueprint for other centers. The results also show that log-file PSQA provides limited added value beyond robust pre-treatment verification in a mature, stable delivery system, while maintaining strong safety margins and expanding 3D, heterogeneous-geometry verification capabilities. Overall, the work supports adopting MC-based PSQA as a superior, efficient alternative to traditional measurement-based QA in proton therapy.

Abstract

At the Maastro Proton Therapy Center in Maastricht, patient-specific quality assurance (PSQA) using an independent GPU-accelerated Monte Carlo (MC) calculation has fully replaced conventional measurements, which are time-consuming and have limited sensitivity to clinically relevant errors. A fully automated and robust pipeline was developed, integrating two clinical workflows based on the fast MC code Fred. The system is fully operational, and automatic verification reports are part of daily clinical practice. The first workflow performs a pre-treatment dose recalculation in Fred using the planning CT and clinical plan. The second uses Fred with machine log files to verify the actually delivered dose. Both generate automatic reports for clinical review. Over five years, this workflow has become part of routine clinical operations, providing robust 3D dosimetric verification in heterogeneous anatomies. So far, Fred has recalculated more than 6000 pre-treatment plans and 3513 log file-based PSQA cases, saving an estimated 4090 hours of QA work. The pipeline identified true negatives and detected two planning-related failures that would have been missed by conventional measurements. No false positives or negatives were observed, confirming high accuracy and reliability. The MC-based PSQA pipeline offers an efficient, sensitive, and clinically meaningful alternative to measurement-based QA in pencil beam scanning proton therapy. By eliminating routine measurements, it saves resources while improving patient safety and treatment quality. Five years of experience confirm that measurement-less MC-based PSQA is a viable and superior approach, providing full 3D verification and early error detection - a practical blueprint for other proton therapy centres.

Five Years of Clinical Application of independent Monte Carlo-Based Patient-Specific Quality Assurance at the Maastro Proton Therapy Center

TL;DR

This paper reports five years of fully automated, GPU-accelerated Monte Carlo PSQA for pencil beam scanning proton therapy at Maastro. By integrating Fred into the DGRT framework, the authors implemented two workflows—pre-treatment plan recalculation and log-file-based delivered-dose reconstruction—to replace conventional measurement-based QA. Across 6000+ pre-treatment recalculations and 3513 log-file verifications, they achieved highly reliable 3D dosimetric verification with significant time savings (~4090 QA hours) and detected two planning-related failures that measurements would have missed. The study demonstrates that measurement-less MC-based PSQA is feasible, accurate, and clinically advantageous, offering a practical blueprint for other centers. The results also show that log-file PSQA provides limited added value beyond robust pre-treatment verification in a mature, stable delivery system, while maintaining strong safety margins and expanding 3D, heterogeneous-geometry verification capabilities. Overall, the work supports adopting MC-based PSQA as a superior, efficient alternative to traditional measurement-based QA in proton therapy.

Abstract

At the Maastro Proton Therapy Center in Maastricht, patient-specific quality assurance (PSQA) using an independent GPU-accelerated Monte Carlo (MC) calculation has fully replaced conventional measurements, which are time-consuming and have limited sensitivity to clinically relevant errors. A fully automated and robust pipeline was developed, integrating two clinical workflows based on the fast MC code Fred. The system is fully operational, and automatic verification reports are part of daily clinical practice. The first workflow performs a pre-treatment dose recalculation in Fred using the planning CT and clinical plan. The second uses Fred with machine log files to verify the actually delivered dose. Both generate automatic reports for clinical review. Over five years, this workflow has become part of routine clinical operations, providing robust 3D dosimetric verification in heterogeneous anatomies. So far, Fred has recalculated more than 6000 pre-treatment plans and 3513 log file-based PSQA cases, saving an estimated 4090 hours of QA work. The pipeline identified true negatives and detected two planning-related failures that would have been missed by conventional measurements. No false positives or negatives were observed, confirming high accuracy and reliability. The MC-based PSQA pipeline offers an efficient, sensitive, and clinically meaningful alternative to measurement-based QA in pencil beam scanning proton therapy. By eliminating routine measurements, it saves resources while improving patient safety and treatment quality. Five years of experience confirm that measurement-less MC-based PSQA is a viable and superior approach, providing full 3D verification and early error detection - a practical blueprint for other proton therapy centres.
Paper Structure (14 sections, 10 figures, 1 table)

This paper contains 14 sections, 10 figures, 1 table.

Table of Contents

  1. Introduction
  2. MATERIALS AND METHODS
  3. Improvements to the MC code Fred
  4. PSQA clinical workflows
  5. PSQA validation and evaluation
  6. Clinical timeline
  7. RESULTS
  8. Fred improvements for PSQA
  9. PSQA validation and evaluation
  10. Five years of MC-based PSQA in Maastro
  11. Plans Failing Pre-Treatment PSQA
  12. Beam Delivery Accuracy and Stability Assessment
  13. DISCUSSION
  14. CONCLUSION

Figures (10)

  • Figure 1: Schematic of the two clinical PSQA workflows implemented with independent MC Fred. (1) Pre-treatment workflow based on the clinical RTplan. (2) Log file-based workflow.
  • Figure 2: Difference in the gamma index passing rate between the new and old Fred versions for different patient indications. In the case of breast patients there is an improvement between 2.5 and 5$\%$.
  • Figure 3: The left panel shows an axial view of the 2D dose map calculated with the clinical TPS. The central and right panels show the relative percentage difference between the clinical TPS dose and the Fred doses calculated with the old and new versions, respectively. Black lines highlight the CTV and the left breast contours.
  • Figure 4: The speed-up factor between the new and old Fred versions for different patient indications. The average speed-up factor is 3.6.
  • Figure 5: Gamma index pass rate (%) as a function of time for the two clinical Fred workflows. (a) Pre-treatment workflow. (b) Log file-based workflow.
  • ...and 5 more figures