ESTRO 2024 - Abstract Book

S351

Beachytherapy - Physics

ESTRO 2024

Material/Methods:

An in house designed and manufactured phantom was used for the dose measurements. It consists of a PMMA parallelepiped with dimensions of 21.5x12.1x12.1 cm 3 and with four slots to insert plastic needles connected to the afterloader. It is divided into two blocks between which a GAFChromicTM EBT-3 film is placed (Figure 1a). The irradiated plan consisted of 7 active dwell positions per catheter (28 in total), with the same dwell time for all of them, and a prescription dose of 3 Gy at the phantom center point. The irradiations were performed using a microSelectron V2 from Elekta for 192 Ir source and a MultiSource from BEBIG for 60 Co source. For film calibration, 7 pieces of film measuring 20x3.5 cm2 were employed. One piece of film was not irradiated and the other 6 pieces were irradiated with a 60 Co machine at the Radiation Physics Laboratory (RPL) of the Universidad Santiago de Compostela (USC). They were irradiated with doses ranging from 0.6 to 3.6 Gy. An Epson Expression 12000XL flatbed scanner was used for the scans. The software used for processing and analyzing film scans was Radiochromic.com. 3x3 mm 2 ROIs were performed in an area of ±2% dose homogeneity (Figure 1b) and the median pixel value was taken as the dose measurement. This value was compared with the same ROI in the TPS dose distribution (based on TG-43 formalism [1]). The uncertainty in the dose measurement was evaluated following the AAPM TG-235 recommendations [2] and the energy dependency reported by Buston et al. [3]. Four different pieces of film were irradiated with the same plan for 192 Ir and for 60 Co and the mean value obtained with this procedure was taken as the final result. Finally, a local gamma (2%,2mm) analysis [4] with a threshold of 10% of the maximum dose was performed comparing the TPS and the film absolute dose distributions.

Results:

Figure 1a shows the radiochromic film after the irradiation whereas Figure 1b shows the analysis performed with Radiochromic.com with the ROI selected for the dose measurement. The highlighted isodoses are 102% (purple) and 98% (green). The obtained dose values are shown in Table 1 along with the gamma analysis results. As observed, the measured doses are compatible with those obtained in the TPS.

(a)

(b)

Figure 1: (a) Radiochromic film placed in the phantom after the irradiation. Both parts of the phantom are placed together for the irradiation. (b) Radiochromic film dose distribution analyzed with the software Radiochromic.com. The highlighted isodoses are 102% (purple) and 98% (green). The ROI for the dose measurement is performed in this area of ±2 % dose homogeneity.