METHODS
Thirty treatment plans each from Head and Neck (H&N) and pelvis sites were analyzed. FractionLab (Varian/
Mobius Medical System) was used for phantom-free gamma analysis of delivered and planned fluences
based on log files. PD was performed using an aS1200 EPID, and 3D gamma analysis was conducted using
the Octavius 4D 1500 2D detector array. Gamma evaluation in FractionLab was performed using log files
from 0.1%/0.1 mm to 1%/1 mm in increments of 0.1%/0.1 mm and compared with global gamma criteria.
RESULTS
The average gamma passing rates for H&N and pelvis sites using portal dosimetry, the 2D array (3%/2
mm), and FractionLab were 98.68% and 98.17%; 96.79% and 98.79%; 98.31% and 98.02% at 0.5%/0.5
mm, respectively. The portal dosimetry results (3%/2 mm) were statistically comparable with Fraction-
Lab (0.4%/0.4 mm-0.7%/0.7 mm).
CONCLUSION
This study demonstrated the performance and suitability of gamma criteria for FractionLab in a phantom-
free PSQA settingand it can serve as a reliable second check for PSQA.
Keywords: 2D-array; FractionLab; log files; portal dosimetry; PSQA; radiotherapy
Point dosimeters, typically cylindrical ionization
chambers, possess desirable dosimetric properties such
as dose and dose rate linearity, stability, directional independence,
and energy independence.[
PSQA ensures the accuracy and safety of the radiotherapy
treatment process. The most common method
for quantitative comparison is gamma analysis, which
combines dose difference (DD) and distance to agreement
(DTA) criteria.[
Adaptive radiotherapy, while a promising advancement
in personalized cancer treatment, faces
significant challenges with traditional phantom-based
quality assurance (QA).[
Recently, FractionLab (Varian/Mobius Medical System,
Houston, TX, USA) introduced a gamma index
analysis method for comparing planned and delivered
fluences using trajectory log files, eliminating the need for a phantom.[
This study aims to elucidate the dosimetric performance
of FractionLab-based logfile analysis and
compare it with the QA results of the EPID (aS1200;
Varian Medical Systems, Palo Alto, CA, USA) and
Octavius 4D (PTW Freiburg GmbH, Freiburg, Germany),
which are conventionally used for patientspecific
quality assurance.
Treatment Planning and Delivery Techniques
All RapidArc plans were created using the Eclipse treatment
planning system (v15.6; Varian Medical Systems,
Palo Alto, CA, USA) with dual-arc with jaw tracking
using a 6MV flattened photon beam. A photon optimizer
(PO; Version 15.6.06, Varian Medical Systems)
was selected for inverse optimization based on physical
and biological objectives. Hence, the physical constraints
of the upper, lower, and mean objectives were
used to limit the dose level in a defined portion of the
structure volume, to define the minimum dose level that a particular target volume should receive, and to
define the mean dose that should not be exceeded for
the structure, respectively. Dose computations for each
planned dose set were computed using the AAA algorithm
(Version 15.6.06, Varian Medical System) with
a 2.5-mm dose grid resolution. All RapidArc plans
were delivered using a Varian TrueBeam accelerator
equipped with a 120-leaf Millennium multi-leaf collimator
(MLC), capable of delivering 6MV FF photon
beams with a maximum dose rate of 600 MU/min.
MLC Log Files
The log file recording modes are active in TrueBeam,
unlike previous C-series accelerators (such as Trilogy,
EX, and iX). Therefore, there is no delay in positioning
the leaves due to the efficient design of the active
MLC controller, which distinguished TrueBeam from
its predecessors. Therefore, the leaves move promptly
to their planned positions without any delay. The log
files generated by TrueBeam, known as Trajectory logs,
are binary files that record both the planned and actual
positions of the MLCs. These logs are captured at a
sampling rate of 50 Hz (20 ms).
FractionLab
The FractionLab software analyzes MLC positioning errors,
beam shutoff speed, and planned and delivered
gamma agreement using the machine log files, hence the
trajectory log files generated by linear accelerators. The
trajectory log files include the delivered MLC position
information as a function of the fractional dose, which
FractionLab uses to create fluence maps magnified on the
iso-centre plane. These fluence maps are generated at a
fixed resolution of 0.5 mm per pixel. Two files ("A" bank
and "B" bank) were created for the trajectory log files of
a field. The trajectory log files were used in this study using
FractionLab software for analysis. The general parameter
specifications are as follows: sampling time=0.05 sec,
MLC position=0.01 mm, jaw position=0.1 cm, and gantry
angle=0.1°; the couch angle is not reflected in the log files.
Therefore, the trajectory log files are used in FractionLab to perform the gamma evaluation between the automatically calculated 2D fluence and the 2D fluence generated using the log files after irradiation for the first treatment fraction. The gamma criteria were used in FractionLab by varying the DD/ DTA values from 0.1%/0.1 mm to 1%/1 mm.
Electronic Portal Imaging Device
Octavius 4D with 2D Detector Array
The γ index metric was computed using Octavius 4D
phantom and VeriSoft software.[
Analysis of the Gamma Index Using Fraction-
Lab, 2D-array and EPID Dosimetry
Statistical Analysis
Portal dosimetry (PD) was used to evaluate the measured
fluence using the EPID attached to the Varian Truebeam
(Varian Medical Systems, Palo Alto, CA, United States) linear accelerator, which is equipped with an amorphas-
Si 1200 EPID. The A-Si EPID has a maximum irradiation
area measuring 43×43 cm², accompanied by a pixel
dimension of 1280×1280 pixels and detects a size of
40×40 cm2, yielding a pixel size of 0.34mm.[
Each plan was recalculated on the OCTAVIUS phantom
with the same parameters and AAA algorithm
to generate the patient-specific verification plan. The
Verisoft (version 7.1, PTW Dosimetry, Freiburg, Germany)
software was then used to evaluate the QA and
completed plans.
The comparison of the gamma passing rate of Octavius
and EPID was done for 2 mm/3% gamma criteria
and with FractionLab for various gamma criteria
(0.1%/0.1 mm to 1%/1 mm) in RapidArc Delivery in
H&N and pelvis sites.
We conducted a paired t-test on the portal dosimetry
and Fraction Lab QA results to determine an appropriate
gamma index when using FractionLab-based
patient-specific QA, as a 3%/3 mm gamma index was
considered when performing QA using portal dosimetry.
Statistical analysis was performed using IBM SPSS
version 22.0 (IBM Corp., Armonk, NY, USA). The
gamma passing rates of portal dosimetry (3%/3mm) and FractionLab at various gamma criteria (0.1%/0.1
mm-1%/1 mm) were analyzed, where p≤0.05 was considered
statistically significant.
EPID: Electronic portal imaging device.
When the global gamma criterion was 2%/2 mm, the average pass rates were 97.79±1.74% for EPID and 91.97±2.85% for Octavius. The average pass rates for the 2 mm/3% criterion were 98.68±1.11% for EPID and 96.79±1.47% for Octavius. For the 3 mm/2% criterion, the average pass rates were 99.22±0.82% for EPID and 96.56±1.50% for Octavius. Finally, the average pass rates for the 3%/3 mm criterion were 99.48±0.67% for EPID and 98.81±0.66% for Octavius.
When using local gamma criteria, the passing rates decreased. The average passing rates for the 2%/2 mm criterion were 93.95±3.48% for EPID and 76.78±5.56% for Octavius. The average pass rates for the 2 mm/3% criterion were 96.22±2.56% for EPID and 81.02±5.17% for Octavius. For the 3 mm/2% criterion, the average pass rates were 96.59±2.34% for EPID and 90.31±3.28% for Octavius. Finally, the average pass rates for the 3%/3 mm criterion were 98.00±1.79% for EPID and 92.01±2.98% for Octavius.
Furthermore, with local gamma criteria, none of the passing rates for the Octavius system met the ≥95% threshold for any gamma criterion. However, the EPID system achieved a passing rate of ≥95% with the 3%/3 mm, 3 mm/2%, and 2 mm/3% gamma criteria.
The comparison between Octavius and EPID showed a statistically significant difference in the gamma criteria for both global and local gamma (p≤0.05).
Figures
EPID: Electronic portal imaging device.
When the global gamma criterion was 2%/2 mm, the average pass rates were 94.20±3.20% for EPID and 95.11±2.85% for Octavius. The average pass rates for the 2 mm/3% criterion were 98.18±1.25% for EPID and 98.79±0.994% for Octavius. For the 3 mm/2% criterion, the average pass rates were 96.44±2.13% for EPID and 98.19±1.56% for Octavius. Finally, the average pass rates for the 3%/3 mm criterion were 98.89±0.82% for EPID and 99.65±0.34% for Octavius.
When using local gamma criteria, the passing rates decreased. The average passing rates for the 2%/2 mm criterion were 89.70±5.83% for EPID and 82.57±8.44% for Octavius. The average pass rates for the 2 mm/3% criterion were 94.71±4.31% for EPID and 87.65±7.05% for Octavius. For the 3 mm/2% criterion, the average pass rates were 93.90±4.17% for EPID and 92.74±4.83% for Octavius. Finally, the average pass rates for the 3%/3 mm criterion were 96.76±3.11% for EPID and 94.90±3.76% for Octavius.
A statistically significant difference in gamma pass rate was found for 3 mm/3% (p=0.001) and 3 mm/2% (0.007) global gamma criterion for Octavius and EPID. However, with local gamma criteria except for 3 mm/2% (p=0.254), all criteria showed a significant difference in passing rate.
Tables
For the H&N site, the average gamma passing rate for EPID using the 3%/3 mm global gamma criteria was 99.48% (range: 97.1%-100%). Under the 2 mm/3% criteria, the rate was 98.68% (95.8-99.2%). In the pelvis site, the gamma passing rates for EPID were 98.81% (range: 96.8-99.9%) and 98.17% (range: 95.0-99.90%) for the 3%/3 mm and 2 mm/3% criteria, respectively. These results were compared to those obtained using FractionLab at various gamma criteria (0.1%/0.1 mm-1.0%/1 mm).
EPID (2 mm/3%) and FractionLab also demonstrated statistically significant differences for gamma indices below 0.5%/0.5 mm and above 0.7%/0.7 mm for the H&N site and 0.6%/0.6 mm for the pelvis site. With the 3%/3 mm criteria, only the 0.7%/0.7 mm and 0.8%/0.8 mm indices showed comparable results for the H&N site, while the 0.6%/0.6 mm and 0.7%/0.7 mm indices were comparable for the pelvis site.
In Octavius, for the H&N site, the average gamma passing rate with the 3%/3 mm global gamma criteria was 98.81% (range: 97.0- 99.9%), and with the 2 mm/3% criteria, it was 96.79% (range: 92.7-97.0%). For the pelvis site, the rates were 99.65% (range: 98.5- 100.0%) and 98.79% (range: 96.4-99.7%), respectively. These results were compared to those obtained using FractionLab at various gamma criteria (0.1%/0.1 mm-1.0%/1 mm).
Furthermore, Octavius (2 mm/3%) and FractionLab exhibited statistically significant differences for all gamma criteria except for 0.4%/0.4 mm for the H&N site and 0.5%/0.5 mm to 0.7%/0.7 mm for the pelvis site. Similarly, with the 3%/3 mm criteria, the 0.5%/0.5 mm and 0.6%/0.6 mm criteria showed comparable results for the H&N site, and the 0.6%/0.6 mm and 0.7%/0.7 mm criteria showed comparable results for the pelvis site.
Figure
In a study by Lim et al.,[
Therefore, we compared the gamma
passing rates for H&N and pelvis sites using
different gamma criteria. We found a significant
difference in gamma passing rates
with 2 mm/3% gamma criteria for the H&N
site, and a drastic decrease in gamma passing
rates was observed with more stringent
local gamma criteria. However, the gamma
passing rate was ≥95% with 2 mm/3% global
gamma criteria. A comparable gamma passing
rate in the pelvis site was observed with
≥95% passing rate using the 2 mm/3% global
gamma criteria. The complexity of H&N
plans has been reported in previous studies.
[
The results showed a significant increase in gamma
passing rate with Octavius compared to portal dosimetry
in the pelvis site. However, the H&N site showed
a different, interesting result, where Octavius passing
rates were lower than EPID and decreased drastically
with stringent gamma criteria. The same result was
found by Urso et al.[
We analyzed the PSQA results using 2 mm/3% and
3%3mm criteria for comparison with trajectory log
file results, which were analyzed in FractionLab with
various gamma indices for H&N and pelvis sites. The
results showed that performing gamma index analysis
in the range of 0.4%/0.4 mm to 0.7%/0.7 mm is appropriate
when using FractionLab for patient-specific
QA in RA. This implies the clinical performance of
FractionLab by comparing its QA results using EPID
and Octavius for various gamma indices with the
results of patient-specific QA in RA treatment. The proposed method can present the appropriate gamma
index when performing patient-specific QA with
FractionLab. Recent studies have corroborated these
findings, suggesting that lower gamma index thresholds
provide a more stringent and potentially more
accurate assessment of treatment delivery accuracy.
[
Therefore, PSQA practices can be significantly improved
by log-file-based evaluation. The small sample
size in our current study could be a limitation regarding
the generalizability of our findings. Additionally,
variations in accelerators and equipment used across
different institutions could impact the universal relevance
of our results. We recommend a comprehensive
study across various institutions, including the equipment
and methodological differences, to improve the
wider applicability of future findings.
Conflict of Interest: All authors declared no conflict of interest.
Financial Support: None declared.
Use of AI for Writing Assistance: No AI technologies utilized.
Authorship Contributions: Concept - S.M., B.K.S., K.J.M.D.; Design - S.M., B.K.S., K.J.M.D.; Supervision - B.K.S., K.J.M.D.; Data collection and/or processing - S.M., B.K.S., K.J.M.D.; Data analysis and/or interpretation - S.M., B.K.S., K.J.M.D.; Literature search - S.M., B.K.S., K.J.M.D.; Writing - S.M., B.K.S., K.J.M.D.; Critical review - K.J.M.D., B.K.S., S.M.
Peer-review: Externally peer-reviewed.