Retrospective analysis of portal dosimetry pre-treatment quality assurance of intracranial SRS/SRT VMAT treatment plans

Abstract

Background: The complexity associated with the treatment planning and delivery of stereotactic radiosurgery (SRS) or stereotactic radiotherapy (SRT) volumetric modulated arc therapy (VMAT) plans which employs continuous dynamic modulation of dose rate, field aperture and gantry speed necessitates diligent pre-treatment patient-specific quality assurance (QA). Numerous techniques for pre-treatment VMAT treatment plans QA are currently available with the aid of several different devices including the electronic portal imager (EPID). Although several studies have provided recommendations for gamma criteria for VMAT pre-treatment QA, there are no specifics for SRS/SRT VMAT QA. Thus, we conducted a study to evaluate intracranial SRS/SRT VMAT QA to determine clinical action levels for gamma criteria based on the institutional estimated means and standard deviations. Materials and methods: We conducted a retrospective analysis of 118 EPID patient-specific pre-treatment QA dosimetric measurements of 47 brain SRS/SRT VMAT treatment plans using the integrated Varian solution (RapidArcTM planning, EPID and Portal dosimetry system) for planning, delivery and EPID QA analysis. We evaluated the maximum gamma (γmax), average gamma (γave) and percentage gamma passing rate (%GP) for different distance-to-agreement/dose difference (DTA/DD) criteria and low-dose thresholds. Results: The gamma index analysis shows that for patient-specific SRS/SRT VMAT QA with the portal dosimetry, the mean %GP is ≥98% for 2–3 mm/1–3% and Field+0%, +5% and +10% low-dose thresholds. When applying stricter spatial criteria of 1 mm, the mean %GP is >90% for DD of 2–3% and ≥88% for DD of 1%. The mean γmax ranges: 1·32 ± 1·33–2·63 ± 2·35 for 3 mm/1–3%, 1·57 ± 1·36–2·87 ± 2·29 for 2 mm/1–3% and 2·36 ± 1·83–3·58 ± 2·23 for 1 mm/1–3%. Similarly the mean γave ranges: 0·16 ± 0·06–0·19 ± 0·07 for 3 mm/1–3%, 0·21 ± 0·08–0·27 ± 0·10 for 2 mm/1–3% and 0·34 ± 0·14–0·49 ± 0·17 for 1 mm/1–3%. The mean γmax and mean γave increase with increased DTA and increased DD for all low-dose thresholds. Conclusions: The establishment of gamma criteria local action levels for SRS/SRT VMAT pre-treatment QA based on institutional resources is imperative as a useful tool for standardising the evaluation of EPID-based patient-specific SRS/SRT VMAT QA. Our data suggest that for intracranial SRS/SRT VMAT QA measured with the EPID, a stricter gamma criterion of 1 mm/2% or 1 mm/3% with ≥90% %GP could be used while still maintaining an in-control QA process with no extra burden on resources and time constraints.

A hybrid method to improve efficiency of patient specific SRS and SBRT QA using 3D secondary dose verification

“…Popular measurement-based methods for PSQA of lung SBRT plans include film, [7][8][9] multi-dimensional diode arrays, [10][11][12] high-resolution diode arrays, 10,13,14 and electronic portal imaging device (EPID) based QA. 12,[15][16][17][18] Due to the use of small field segments and steep dose gradients that are associated with lung SBRT plans, previous studies have demonstrated the need to use high resolution measurement methods to be able to detect potential errors through the use of stricter gamma criteria. 10,12,17 However, even with high resolution measurement devices, the sensitivity of measurement based QA to detect errors has been shown to be limited.…”

“…12,[15][16][17][18] Due to the use of small field segments and steep dose gradients that are associated with lung SBRT plans, previous studies have demonstrated the need to use high resolution measurement methods to be able to detect potential errors through the use of stricter gamma criteria. 10,12,17 However, even with high resolution measurement devices, the sensitivity of measurement based QA to detect errors has been shown to be limited. [19][20][21][22] Similar to lung SBRT, single isocentre multi-lesion SRS plans frequently make use of small field segments and high dose gradients.…”

Portal dosimetry correction method for validation of single isocenter VMAT plans for multiple brain metastases

“…4,5 One challenge in treating patients with large numbers of mets in a single isocenter setup is performing robust plan quality assurance (QA) that will measure the dose delivered to all targets to the high resolution necessary for cranial SRS. Portal dosimetry 6 is able to verify plan delivery for intensity-modulated radiation therapy (IMRT), VMAT, and SRS plans 7,8 for both individual and composite fields. However, due to the high Z material in the phosphor layer and the relatively shallow depth of measurement, the portal imager is known to overrespond to low-energy radiation.…”