Author(s):
1. Goran Kolarević, Republic of Srpska, Bosnia and Herzegovina

Abstract:
Background/Aim. Accurate pre-treatment image verification of patient positioning in radiotherapy (RT), known as Image-guided radiotherapy (IGRT), has consistently remained crucial for ensuring the precise administration of RT treatment to the patient. Electronic Portal Imaging Devices (EPID) have largely taken over the role of film-based verification, resulting in enhanced image quality and improved depiction of critical information regarding the tumor volume's position. The imaging dose delivered to the patient can vary based on the chosen verification method (such as two orthogonal mega-voltage scans, two orthogonal kilo-voltage scans, or Cone beam CT) and the energy level of the radiation photons being used. This study aimed to measure the pre-treatment verification doses administered to patients using MV photon radiation. Methods. In our RT center, we use the medical linear accelerator True Beam (Varian Medical Systems, Palo Alto CA, USA), which has a-Si 1200 EPID. All measurements were conducted using the lowest photon energies available, specifically 2.5 MV flattening filter free (FFF) and 6 MV with a flattening filter (FF). For the purposes of the study, a heterogeneous phantom CIRS Thorax 002LFC (Computerized Imaging Reference Systems Inc, Norfolk VA, USA) was used. This phantom is designed to anatomically and dimensionally simulate an average human thorax, measuring 30 cm in length, 30 cm in width, and 20 cm in height. It is made of plastic water, lungs and bone. Within the phantom, there are 10 cylindrical inserts designed to accommodate ionization chambers for the precise measurement of absolute dose. To ascertain the absolute dose at ten specified measurement locations within the Thorax 002LFC phantom, an Exradin A-19 farmer-type ionization chamber (0.62 cm3 in volume) paired with a SuperMax electrometer (Standard Imaging Inc, Middleton WI, USA), was used. All dose measurements were carried out utilizing the isocentric technique for every measurement point. This involved acquiring measurements from two projections: Anterior-Posterior (AP) and Lateral (LAT)-with a single monitoring unit (MU) for each projection. The field dimensions were set at 16x16 cm². Absolute doses were computed following the guidelines outlined in Technical Reports Series No. 398, which provides the protocol for absorbed dose determination in external beam radiotherapy. Results. The depth dose percentages at 10 cm and 20 cm (10x10 cm2, SSD 100 cm), for photon beams 2.5 MV FF and 6 MV FF are: 52.7 % and 22.7 % vs 66.3 % and 38.1 %. Quality index (D20/D10) was 0.485 vs 0.669 (2.5 MV FFF vs 6 MV FF). The measured absorbed doses (2.5 MV FFF) in the soft tissue, for two orthogonal projections (1 MU each), were 14.1-16.7 mGy. Using identical measurement conditions in the lungs, absorbed doses ranging from 16.8 to 18 mGy were measured. In the bone region, 11.4 mGy was measured. For 6 MV FF, under the same conditions, all measured doses were found to be higher by 29 % in comparison to the doses measured for 2.5 MV FFF. For an average number of about 20 RT treatments (fractions), the patient receives an additional imaging dose of 300 mGy (2.5 MV), i.e. about 400 mGy (6 MV). Conclusion. In the field of external photon beams radiotherapy (RT), Image-guided radiotherapy (IGRT) stands as the recognized gold standard. Delivering treatment without image verification is deemed unacceptable due to the crucial role it plays in ensuring the precise administration of the dose to the tumor volume. If using MV photon beams for pre-treatment image , it is advisable to opt for lower MV energy whenever feasible. Imaging modalities based on kV photon beams (kV-kV and kV CBCT), This approach facilitates a concurrent reduction in the margin between the clinical target volume (CTV) and the planning target volume (PTV), aligning with the ALARA principle (As Low As Reasonably Achievable) for minimizing radiation exposure. In clinical practice, the majority of radiotherapy (RT) centers commonly rely exclusively on kV imaging, except when dealing with treatment fields.

Key words:
radiotherapy, MV imaging, dose, MV photon beam.

Date of abstract submission:
15.08.2023.

Conference:
Contemporary Materials 2023 - Savremeni materijali

Applied paper from author