ESTRO 2025 - Abstract Book

S8

Invited Speaker

ESTRO 2025

4664

Speaker Abstracts Impact of suboptimal process steps in film dosimetry Sabeena Beveridge, Andrew Alves, Sarah Judd, Katherine Collins Australian Clinical Dosimetry Service, Australian Radiation Protection And Nuclear Safety Agency, Melbourne, Australia Abstract: Film dosimetry is an excellent tool for radiotherapy treatment verification and quality assurance due to its high spatial resolution and two-dimensional (2D) dose distributions. There is a reluctance in the clinical environment to adopt film dosimetry programs which can be attributed to lack of resources and the time required for processing. Unless the user has a very good understanding of film dosimetry and the uncertainties associated with each component in the process, there is a high risk of incorrectly applying procedures and corrections which can lead to incorrect results and impact patient quality assessments. The Australian Clinical Dosimetry Service (ACDS) uses film as the primary dosimeter when assessing end-to-end type audits. Uncertainty contributions from the Linac, ion chamber, film, scanner, processing software, and positioning have been minimized to achieve a film dosimetry uncertainty of 2.3%. The largest reductions in uncertainty are from scanner corrections, film calibration corrections, and film-to-plan position accuracy. However, errors are still seen that allude our most experienced users. Quality control (QC) measures have been put in place throughout the ACDS film process to ensure the process remains consistent. A key methodology the ACDS uses to reduce film uncertainty is with the use of reference films. A calibration film set comprising of 12 doses is acquired using the on-site Linac (Electa Versa HD). Three reference films are irradiated at the auditing facility, which range from 10 Gy to 20 Gy (including a 0 Gy film), and are scanned with each audit film. Table 1 lists the estimated uncertainty for the components within this process. Some of these components can be reduced through improving procedures and applying corrections (Type A), however, some sources of error cannot be manipulated and must be accepted (Type B). The total uncertainty for the ACDS reference film procedure is ±0.83%, assuming that film uniformity and data entry are correct. QC within the process verifies that the film has not been rotated or positioned incorrectly on the scanner. With the release of EBT4 film (Ashland ISP Inc., Wayne, NJ, USA), it is expected that film uniformity is improved, however, the ACDS has evaluated reference film factors with both EBT3 and EBT4 film and have found no significant difference between the two types of film (p = 0.57, comparison of the means). Considerations in how film is positioned when comparing it to the planned dose is sometimes overlooked – where auto-positioning film to fit the planar dose is used to obtain best-fit results. Positioning errors due to IGRT misalignments can be ignored when the user relies on the software to process film results. Since SRS requires patient positioning to be within 1 mm, then film positioning should be within this tolerance to be valid. A recent study by Beveridge et al. 1 assessed the baseline film processing results from six Dosimetry Audit Networks (DANs) that had well-established film dosimetry programs. Only two out of the six DANs had consistent results within ±5% of the known dose. Most of the errors were due to software processing where users were not aware of how the software behaved during parts of the process. This has emphasized the importance of not relying on software to produce accurate results, but instead ensuring that appropriate quality measures are considered throughout the process.

1 Beveridge S, Alves A, Hussein M, Clark CH, Jornet N, Viegas CB, Reniers B, Alvarez PE, Azangwe G, Chelminski K, Dimitriadis A, Kazantsev P, Swamidas J. A framework for validating radiotherapy film dosimetry based on an international intercomparison. Medical Physics, Vol. 51(12), pp 9071 9087, 2024.