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.