Page 83 - ARNM-2-4

Advances in Radiotherapy
& Nuclear Medicine Dose-volume histogram and gamma analysis

The dose prescription followed the usual goal of 95% A self-constructed Matlab programming software
of the PTV volume receiving at least 7560 cGy while 5% algorithm was used to compute the three-dimensional
receiving no more than 8300 cGy. In the critical structures, gamma index (3DGI). Dosimetric evaluations were
the limits of 30% of the volume were adhered to, receiving performed by comparing the volumetric fluence map of the
a maximum of 7000 cGy and 10% received up to 7500 cGy, dose of each plan with an error in relation to the original
both for the rectum and bladder. plan without deviations. The evaluation criteria were at
The plan used 6MV X-ray beams of a Varian Unique least 95% of approved points, considering the maximum
linear accelerator (Unique, Varian Medical Systems, USA difference of 3% in the dose and 2 mm DTA, using a 10%
equipped with an 80-leaf Millennium MLC (Millennium threshold (3%/2 mm/10%), as the AAPM TG-218 protocol
MLC, Varian Medical Systems, USA) using the IMRT recommends.
sliding window technique. Seven fields with a 50° angle Quantitative analysis related to the DVH metrics was
between them were used, following the guidelines of the performed from plotted graphs of the percentage of dose
TG-119. Such planning was carried out using Eclipse versus structure volume values. Based on these data, it was
software version 15.6 (Varian Medical Systems, Palo Alto, possible to evaluate the percentage difference in the dose of the
California), using the analytical anisotropic algorithm plans with an inserted error in relation to the original doses.
calculation algorithm, with a dose matrix equal to 1 mm ,
3
without heterogeneity correction. 3. Results
To compare the fluence maps and to evaluate the Table 1 shows the percentage of points approved in the
change in the structures’ DVHs due to displacements, 3D gamma analysis (3%/2 mm/10%) for each simulated
another 30 plans were made using the same configurations planning with errors inserted in relation to the original plan.
described above but including the displacements. In each
new plan, known displacement errors in one direction Of the 30 simulated plans, 10 were approved in the
were inserted. These displacements could be in the lateral, proposed analysis. Of these, the lowest percentage value
vertical, or longitudinal axes of the table, in increments of approved points was 97.12%, reaching 100% approval.
of 1 mm, varying from −5 to +5 mm from the origin. Such plans refer to displacements of up to ±2 mm in
Each new plan was created with errors in only one of the both the lateral and vertical directions and ±1 mm in the
directions. longitudinal direction, which presented more significant
restrictions in relation to the approval of 3DGI.
The files were exported from the TPS in the electronic
medical image storage standard (DICOM) of the treatment Out of the 20 failed plans, four were met with failure
plans (DICOM RT plan), the calculated dose matrices with a rate of up to approximately 9% in relation to the total
(DICOM RT dose) in the volumetric format with 0.1 cm number of points evaluated; in six plans, the differences
3
resolution, as well as the sets of structures (DICOM RT were between 10% and 20%; in eight plans, the differences
structure set) for each generated plan. were 20 – 30%; and in two plans, the difference presented
was more than 30%.
The dose percentages obtained from the structures’
DVH were also exported in relation to the volumes of 98% Figures 2-4 present the differences in behaviors of the
(D ), 95% (D ), 10% (D ), and 2% (D ) in addition to evaluated structures’ DVH, which were identified from the
98
95
2
10
the mean dose (D mean ) of the PTV, and for the rectum and comparisons between the original clinically approved plan
bladder, in relation to the volumes of 30% (D ), 10% (D ), and the simulated plans with errors inserted.
30
10
and 2% (D ) and the respective average doses. To obtain 3.1. PTV
2
the reading of the values in a practical and precise way, a
script was written in the TPS itself to present the DVH data In the comparison between the plans with errors inserted
in table format. in relation to the original plan, the values of the smallest
Table 1. Percentage of points approved in three-dimensional gamma analysis
Axis Plan with displacement (mm)
−5 −4 −3 −2 −1 +1 +2 +3 +4 +5
Lateral 69.18 77.22 86.92 97.15 99.99 100.00 97.12 87.03 77.59 69.39
Vertical 71.70 80.77 90.77 98.25 100.00 100.00 98.30 90.90 80.85 71.64
Longitudinal 70.85 76.36 83.23 91.15 99.82 99.82 91.19 83.27 76.41 70.79

Volume 2 Issue 4 (2024) 3 doi: 10.36922/arnm.4005