Copenhagen, Denmark
Onsite/Online

ESTRO 2022

Session Item

Implementation of new technology and techniques
7002
Poster (digital)
Physics
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 ± 1.71.90 ± 0.11
Chest
87.6 ± 2.72.56 ± 0.35
Abdomen
87.8 ± 2.52.47 ± 0.09
Pelvis
97.2 ± 0.8
1.69 ± 0.14
Legs
96.0 ± 1.81.94 ± 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