Online

ESTRO 2020

Session Item

Physics track: Dose measurement and dose calculation
9319
Poster
Physics
00:00 - 00:00
Dosimetric measurements with Gafchromic EBT-3 films for the Papillon+ intraoperative therapy system
Claudine Colnard-Hofverberg, France
PO-1391

Abstract

Dosimetric measurements with Gafchromic EBT-3 films for the Papillon+ intraoperative therapy system
Authors: Claudine COLNARD-HOFVERBERG.(CENTRE ANTOINE LACASSAGNE, MEDICAL PHYSICS, NICE, France), Mathieu Gauthier.(CENTRE ANTOINE LACASSAGNE, MEDICAL PHYSICS, NICE, France), Joël Herault.(CENTRE ANTOINE LACASSAGNE, MEDICAL PHYSICS, NICE, France)
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Purpose or Objective

The Papillon+ system is a mobile kilovoltage X-ray unit for Contact Radiotherapy with a high dose-rate delivery. The first prototype was acquired by Centre Antoine Lacassagne, in Nice France. It is operated at 50kV for partial breast intraoperative irradiation using spherical applicators of diameters between 30 and 50mm. The dose-rate at the applicators surface ranges from 10 to 46Gy/min.
Methods have been developed for commissioning and quality assurance prior to integration in the clinical practice. A feasibility study is presented here for using Gafchromic EBT-3 films as a dosimetry tool and to assess their utility for in-vivo dose verification during treatment with the Papillon+ unit. The procedure which has been developed for handling and analyzing the films is discussed.

Material and Methods

A Gafchromic EBT-3 films lot has been calibrated against an independent 50kV X-ray device to determine the dose calibration curve between 0 and of 30 Gy as the standard prescription dose for partial breast intraoperative therapy is 20 Gy.
Absolute dosimetry was carried out with the calibrated film lot and compared with measurements using a PTW 0.02cm³ soft X-ray chamber type 23342 for validation.
Homogeneity of the irradiation was estimated by using strips of films placed around the spherical applicators at the equator and two orthogonal meridians.
Depth dose profiles for all applicator sizes have been acquired in a water tank with pieces of films parallel to the beam central axis. The same measurements were also made with a dedicated phantom in water and in air for comparison and to determine the effects of photon backscattering.
Handling of the films was done following recommendations from the AAPM TG-55 report. Films have been digitized with an Epson 10000XL flatbed scanner and analyzed with the FilmQA-Pro software using a “One-Scan” protocol that employs two reference films to re-scale the calibration curve. The green color channel was chosen to measure the high doses considered.

Results

Absolute dose measurements with Gafchromic EBT-3 films are in accordance with ionization chamber values. The irradiation at the breast applicators surface is homogeneous with a relative standard deviation less than 3%. Percentage depth dose measurements made in water improve with increasing applicator size. The dose at 10mm is 39.9%, 37.46%, 33.4%, and 26.25% of the dose at the applicator surface for diameters of 50, 45, 40 and 35mm, respectively. Depth dose measurements with the phantom in air showed the importance of the backscatter factor at the used beam quality.

Conclusion

Gafchromic EBT-3 films are reliable for dosimetry at 50kV with the Papillon+ system. A reproducible and accurate method has been developed that use Gafchromic EBT-3 films for dose measurements at commissioning and during quality assurance tests. This procedure is now used in the clinic for in-vivo dosimetry during partial breast intraoperative irradiation with the Papillon+ unit.