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Purpose or Objective:

Modern radiation therapy modalities regularly produce SRT/SRS/SBRT plans with highly irregular and steep dose gradient distributions consisting of many small beam apertures. Accurate verification of such complex treatment fields is still challenging and Gafchromic EBT3 and EBT-XD film have taken their role as dosimetry media with highest spatial resolution and SRT optimized dose range. Purpose of this work is to evaluate whether well-known temperature dependence of former Gafchromic film media are present with EBT3 and EBT-XD film. The observed systematic patterns of temperature dependence are characterized in their effect for accurate dose readout and relevance in pre-treatment verification.

Material and Methods:
An Epson V750 pro flatbed scanner was used to perform scan studies with 125 consecutive scans to purposely warm up the scanner bed with scanning to evaluate temperature dependent readout variation. A two channel thermometer was used to measure temperature in the upper and lower region of the scanner bed. Square film pieces with irradiation dose form 0 Gy to 64 Gy of 1 cm size were placed in the central axis of the scanner bed. Evaluation was performed with the software “Image J” with a 20 by 20 pixel ROI on the film pieces. The film pieces were evaluated in all three colour channels in 8100 measurements in total.
Results:
Temperature dependent readout differences known from former type Gafchromic film media are still present with EBT3 and EBT-XD film type. Temperature dependence is strongly dependent on dose. More temperature results in darker film readout in most cases. Interestingly EBT3 red colour channel reverses temperature dependence direction from approximately 16 Gy irradiation dose on, meaning that more temperature results in less dose readout. Figure 1 and Figure 2 illustrate the temperature dependent transmission error in dependence of irradiation dose. Raw transmission differences are depicted, therefore positive values represent less dose readout with increasing temperature.

Conclusion:
The results show that all doses in a clinical pre-treatment verification will be influenced differently by temperature. The common practice in commercial dosimetry software of recalibrating a calibration curve to reference regions with help of 1-3 reference film pieces with known dose with the same temperature as the film to evaluate is therefore not sufficient to remove temperature dependent readout error. In SRT/SRS/SBRT severe side effects are likely when OAR dose is underestimated. Therefore highest possible precision in dosimetry is not only required in high dose region, but also in medium and low dose areas (OAR relevant) at the same time. For highest precision we therefore suggest to always use temperature as a dosimetry parameter to achieve highest possible precision in Gafchromic film dosimetry.