On the value of an EM tracking quality assurance system for pretreatment verification of needle digitization accuracy in ultrasound-based prostate HDR brachytherapy

Abstract

PURPOSE

To develop and deploy an electromagnetic (EM) tracking-based platform for pretreatment, patient-specific quality assurance of brachytherapy applicator digitization, with the goals of intercepting human errors and quantifying treatment planning dose uncertainties.

METHODS

An EM tracking platform, EMQA, was developed for patient-specific quality assurance of HDR treatment plan needle reconstructions. EMQA compared needle geometry between a manually digitized clinical HDR treatment plan and EM-tracked positions. Pretreatment quality assurance (QA) was performed for 25 implants (20 patients). Distance and dose metric differences were reported. Factors influencing accuracy were analyzed, including radial distance from the ultrasound probe and EM field generator (EFG), needle ‘shadowing’ artifacts, needle depth adjustments using exposed needle length protruding from the implant template, and TRUS calibrated speed of sound, $v_{\text{Sound}\text{Effective}}$.

RESULTS

Needle digitization differences between the clinical plan and EM tracking had a magnitude (mean ± standard deviation [minimum, maximum]) of 0.46 ± 0.36 (0.002, 2.19) millimeters for the needle depths and 0.62 ± 0.44 (0.01, 3.26) millimeters for needle shafts. Dose metric differences (% of Rx) for PTV, CTV, bladder, rectum, and urethra were <1.7% on average, but differences >5% were observed in two patients. Accuracy was notably worse for locations shadowed by more than one needle, decreased with distance from the probe and EFG, and was optimal for $v_{\text{Sound}\text{Effective}}$=1570 m/s.

CONCLUSION

Clinical evaluation of an EMQA platform demonstrated the potential to intercept errors in the digital reconstruction of ultrasound-based prostate HDR brachytherapy needles prior to radiation delivery, which may be due to poor image quality or human error. Manual needle digitization accuracy was typically submillimeter, however errors as great as 3 mm were observed. The adoption of EMQA as standard of care is expected to reduce the potential for mistreatment.