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ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
9319
Poster
Physics
00:00 - 00:00
A feasibility study of using TomoTherapy exit detector data for pretreatment verification
Marco Parisotto, Italy
PO-1371

Abstract

A feasibility study of using TomoTherapy exit detector data for pretreatment verification
Authors: Stefania Maggi.(Ospedali Riuniti, Medical Physics, Ancona, Italy), Valenti Marco.(Ospedali Riuniti, Medical Physics, Ancona, Italy), Marco PARISOTTO.(Ospedali Riuniti, Medical Physics, Ancona, Italy)
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Purpose or Objective

Consolidated recommendations [1-3] suggest pretreatment verification of IMRT plans be employed as part of a patient-specific quality assurance (QA) program. The TomoTherapy LINAC is able to deliver complex treatments using a compressed-air driven 64-leaf binary MLC, which modulates the radiation while the source is rotating around the patient. An arc-shaped array of 640 CT xenon detectors is located opposite the linear accelerator on a rotating slip-ring gantry. After treatment delivery, exit detector data can be extracted and processed for patient-specific QA. We presented a feasibility study for a TomoTherapy phantomless pretreatment verification using exit detector data.

Material and Methods

The DICOM-RT files of TomoTherapy treatments on two different anatomical sites were exported using the Accuray TPS. A homemade software (Matlab, The Mathworks Inc.) was developed to read the DICOM-RT file and generate a TomoTherapy procedure reproducing the plan, except the position of couch, which was forced to not move.  The couch was retracted to avoid any obstacle between source and detectors. The exit detector sinogram of the treatment projections was exported after delivery. A couple of ad-hoc procedures were created and executed in order to model the exit detectors array response to single and paired open leaves beams with couch retracted. The detectors array response sj,n due to a single open leaf j at projection n was extracted from the exit detector sinogram of the “single-leaf” procedure. The response rate Sj was modelled by averaging sj,n over 51 projections (i.e. a complete gantry rotation), normalized to the leaf open time. In order to account for the leaf fluence output factor (LFOF) a second “paired-leaves” procedure was cretated to extract the detector response pj,n due to paired open leaf j, j+1 at projection n. Similarly, the response rate Pj was defined by averaging sj,n over 51 projections, normalized to the leaves open time. Leaf open time and gantry rotation period were set to 0.314 and 20 s, respectively, for both procedures. Detector response rate contribution due to LFOF, was evaluated computing Lj = Pj – (Sj + Sj+1), see fig1. The expected exit sinogram of the plan was evaluated by superposition of Pj and Lj according to the programmed leaf open time after end of planning (EOF): indicating with Tj,n the open time of leaf j at projection n, the expected sinogram was given by:


Results

Gamma index pass rate (local DD=3%, DTA=1.25 mm, threshold 10%) of expected sinogram was 96.9% and 98.2% for a chest (50Gy/25fr) and H&N (66Gy/33fr) plans respectively.

Conclusion

The results obtained with this preliminar study were promising. Anyway, it needs further checks against measurements obtained with independent instruments. Furthermore, efforts should be addressed to investigate the sensibility against induced MLC errors and validate data for a significative amount of cases.

References:
[1] AAPM TG-218 (2018)
[2] AAPM TG-219 (2018)
[3] AAPM TG-120 (2011)