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Plastic Scintillation Detector Calibration Procedure for Brachytherapy in Vivo Dosimetry

A Herreros1, J Perez-Calatayud2*, F Ballester3, (1) Hospital Clinic Barcelona, Barcelona, ,ES, (2) Hospital La Fe, Valencia, ,ES, (3) University of Valencia, Burjassot, ,ES

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To establish plastic scintillation detector (PSD) calibration procedure for brachytherapy in vivo dosimetry with a Farmer chamber in a new designed PMMA phantom.

Methods: A PMMA phantom was constructed to calibrate in a robust and reproducible manner in vivo PSD probes from NU-RISE (Aveiro, Portugal). The phantom incorporates 4 holes located in the phantom periphery for Ir-192 source insertion through needles of 1.22 mm internal diameter and one central accessory to alternate PSD and PTW (Freiburg, Germany) PMMA Farmer ionization chamber for cross-calibration. Dw and Dair were computed with PENELOPE/penEasy using an Ir-192 source model to obtain a kQQ0 quality factor. Through IAEA TRS-398 formalism, ionization measured in the PMMA phantom with the Farmer chamber calibrated in a Co-60 beam allows absorbed dose to water determination in full scatter conditions. Dry air composition and liquid water used were those recommended in Supplement 2 of the AAPM TG43. Measured ionization ratios were checked against their corresponding Monte Carlo obtained Dair ratios in both geometries. For this purpose ELEKTA (Stockholm, Sweden) microSelectron v3 afterloader with a Mallinckrodt microSelectronv2 (Petten, The Netherlands) source was used. A pseudo AAPM TG43 geometry was recreated with a water scanning phantom. Positioning accessories were constructed for imitation of the PMMA phantom geometry in the water phantom.

Results: The discrepancies between measured ionization ratio and Monte Carlo obtained Dair ratios in both geometries are well within the combined experimental and Monte Carlo uncertainties (±2.3% with k=1). Monte Carlo uncertainties include: material composition, mass energy-absorption coefficients, tally statistics and volume averaging. Farmer-measurements uncertainties considered are: phantom holes drilling accuracy, source positioning, measurement reproducibility and ND,w(Co-60) calibration by the standard laboratory.

Conclusion: Instead of using an indirect calibration through SK to Dw transference via AAPM TG43, a faster, robust and direct Dw measurement procedure was developed for PSD calibration.

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Keywords

In Vivo Dosimetry, Brachytherapy

Taxonomy

TH- Brachytherapy: Calibration & Quality Assurance

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