Verification method for Total Body Irradiation plans using TomoTherapy exit detectors
Diana Tovmasian,
Russian Federation
PO-1680
Abstract
Verification method for Total Body Irradiation plans using TomoTherapy exit detectors
Authors: Diana Tovmasian1,2, Anna Loginova1, Alexander Chernyaev2
1Dmitry Rogachev National Medical Research Center of Pediatric Hematology, Oncology and Immunology, Department of Radiotherapy, Moscow, Russian Federation; 2Lomonosov Moscow State University, Department of Physics, Moscow, Russian Federation
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Purpose or Objective
Total body irradiation (TBI) includes
irradiation of entire body except for organs at risk such as lungs, kidneys,
and lenses. In our department, TBI plans are created using TomoTherapy
(Accuray, USA) with Helical mode delivery.
The standard treatment plan verification includes the point absolute dose measurements
in the phantom. However, with the increasing complexity of TBI delivery, there
is a growing need for more accurate and informative plan verification. It seems
to be not enough to use a few point measurements for whole TBI plan.
In this study we present the
phantomless verification method for TBI plans using TomoTherapy exit detectors
data (ED) which ordinary used for visualization purpose.
Material and Methods
TBI treatment plans were
calculated for ten patients.
For each plan we performed the
Static Couch procedure (plan delivery in the absence of a phantom and couch
movement).
The signal from ED was extracted
using the TomoTherapy Quality Assurance tool. A hand-made software was designed
for processing raw data from ED as well as the sinogram data from treatment planning
station. The raw ED data were converted into synthetic sinogram by applying the
correction factors such as: background radiation, the shape of the detector,
the influence of neighboring collimators leaves and the leaf-channel dependence. Comparison
of planned and synthetic sinograms using Gamma-analysis (5% 1mm, global) was performed for each
plan with respect to different body regions.
Each plan was also verified by
standard method using Cheese Phantom and A1SL ion chambers. Planed and measured
absolute dose difference was found in at least four points corresponding to
different body regions.
Results
Result are reported in Table 1.
Table 1. Comparison of the
results for two plan verification methods: maximum dose difference in point
measurements and Gamma index between planned and synthetic sinograms for
different body regions.
Body region
| Maximum dose difference
in point measurements,
mean ± SD, %
| Gamma index between
planned and synthetic sinograms,
mean ± SD, % |
Head
| 95.5 ± 1.7 | 1.90 ± 0.11 |
Chest
| 87.6 ± 2.7 | 2.56 ± 0.35 |
Abdomen
| 87.8 ± 2.5 | 2.47 ± 0.09 |
Pelvis
| 97.2 ± 0.8
| 1.69 ± 0.14 |
Legs
| 96.0 ± 1.8 | 1.94 ± 0.41 |
All plans pass the standard dosimetry criteria
for phantom measurements: maximum dose difference did not exceed 3%. Regions including
high dose gradient such as chest and abdomen show highest values of dose
difference. This result correlated with the calculated gamma-index between
planed and synthetic sinograms. Passing Gamma criteria for developed method can
be determined with future data collection.
Conclusion
The data from ED store a large amount of information about the work of
the collimator and individual leaves. Using these data for dosimetric purposes
allows us to increase the quality and fullness of TBI plan’s verification.
Our
method can be used as additional plan verification tool for the cases with long
targets and complex dose delivery.