WE-E-BRB-01: A Mathematical Model of the Optical Density Growth of EBT2 Film after Proton Irradiation

Gafchromic EBT2 is being used for radiation dosimetry including photon, electrons and recently in proton beams, based on its color changes from transparent to blue due to polymerization. It is critical to understand the temporal response of the polymerization after irradiation to proton beam for more accurate dosimetric results with EBT2.

Five small pieces cut from the same sheet (lot no. A08060905A) were irradiated with different doses (2 - 10 Gy) using a clinical proton beam with range 16 cm and spread-out-Bragg-peak 10 cm. An EPSON flatbed 10000XL was used for digitization in 48 bit color RGB images. The films were kept in the same position on the scanner for each scan. Within a 6-day period, measurements were taken every 3 minutes for the first hour, every 5 to 30 minutes for the first 1-12 hours and then randomly after 12 hours. Red, green and blue frequency read out of the scanner for each film was analyzed in ImageJ software.

A mathematical function (netOD =a + ß (1-exp (-t/Ta)) + γ (1-exp (-t/Tb)) was used to describe the temporal response. The polymerization is time dependent with two time constants, Ta (25 minutes) and Tb (1000 minutes), a, ß, and γ are dose dependent parameters. The percentage of OD change per 10 minutes remains less than 1% after the first hour. A 7%-ll% increase in the net OD was observed within the first 12 h when irradiated within 10 Gy, comparable to the 9% reported for EBT films.

The growth of OD post-irradiation may be explained by the rapid polymerization in EBT2 film. This characteristic can be described by a double exponential function which may be used to correct the temporal coloration effect to improve the accuracy for radiation dosimetry using EBT2 film in proton beam.