Evaluation of a software system for estimating planned dose error in patients, based on planar IMRT QA measurements

  • Mohammad Bakhtiari
  • A Parniani
  • F Lerma
  • S Reynolds
  • J Jordan
  • S Sedaghat
  • M Sarfaraz
  • J Rodgers

Abstract

Purpose: Intensity Modulated Radiation Therapy (IMRT) dosimetry verification is routinely conducted via integrated or individual field dosimetry using film or a matrix of detectors. Composite dose evaluation (whole plan delivered intact to dosimetry device) may more closely present relevant 3D dosimetry but such measurements must still be performed in a surrogate phantom  not in the patient’s anatomy at high sampling density enabling precise assessment of critical anatomy volumes.  Techniques and software systems are commercially available which use individual field dosimetry measurements as input into algorithms that estimate 3D patient dose distributions on CT scan derived target volumes and OARs, thus allowing direct DVH analysis vs. TPS DVH. The purpose of this work is to present a systematic benchmarking technique to evaluate the latter claim, utilizing a flat phantom as a simple patient where measurements at different depths are used to compare measured and reconstructed dose planes in the phantom.

Method and Materials: A MapCheck2 diode array and 3DVH™ software from Sun Nuclear were used for this study.  Delivered planar dose was measured with the diode array as an input to 3DVH™ software that was used to estimate the 3D dose matrix. The 3DVH™ software uses the Planned Dose Perturbation (PDP) algorithm to reconstruct a 3D dose in the patient as defined by the boundaries of the patient “skin” contour. Conventional planar dose measurements at fixed depths are input to generate a 3D dose matrix for DVH evaluation. In this study, benchmarking consists of taking IMRT beams created in actual patient treatment plans; re-computing these on a flat phantom stored in the RTPS, and exporting these calculations to 3DVH™ as individual patient plans. Accuracy of the output of 3DVH™ is tested by comparing measured planar doses over a range of depths to the same planes reconstructed by 3DVH™. Four complex IMRT cases were selected and examined in this study. Planar measurements were conducted at three depths, yielding three sets of analyses per patient plan evaluated. The sensitivity to depth of measurement was evaluated. Additionally, patient cases computed on the patients’ CT scans are imported into 3DVH™ and DVH analyses are evaluated. Deviations in the DVH analyses were evaluated by comparing DVHs obtained by 3DVH™’s  to the RTPS’ DVHs.

Results: The Gamma Index analysis, comparing calculated 3D dose with measured 3D dose with 2% and 2mm DTA criteria returned a pass rate of > 90% for all patient cases calculated by the treatment planning system and it returned a pass rate of > 96% in 9 out of 10 cases calculated by 3DVH™. Extracted computed dose planes with 3DVH™ software at different depths in the flat phantom passed all gamma evaluation analyses when compared to measured planes at different depths using MapCheck2.

Conclusion: Studying complex Head and Neck IMRT fields, it was shown that the 3D dose distribution predicted by the PDP algorithm is both accurate and consistent.
Published
2014-02-03
How to Cite
Bakhtiari, M., Parniani, A., Lerma, F., Reynolds, S., Jordan, J., Sedaghat, S., Sarfaraz, M., & Rodgers, J. (2014). Evaluation of a software system for estimating planned dose error in patients, based on planar IMRT QA measurements. Radiology and Oncology, 48(1). Retrieved from https://www.radioloncol.com/index.php/ro/article/view/1489
Section
Experimental oncology