S137
Brachytherapy - Physics
ESTRO 2026
various orientationsHalf-value layer (HVL)Linearity of measured dose with MU/timeIndividual applicator factorsBeam flatness and field sizeDepth-dose (DD) relationshipInterrupt testingMeasurements were performed using the PTW 23342 plane-parallel ionisation chamber and PTW UNIDOS Romeo electrometer, calibrated for air-kerma free in air in CCRI-50 x-rays. Beam flatness and field size were determined using Gafchromic™ EBT3 film. Percentage depth-dose was determined using RW3 slabs despite recognising them as not water-equivalent at low photon energies. Results: The system calibration factor was set to equate 3000 MU to 30 Gy. It was found that linearity (delivered MU against measured dose) was not proportional; at 2000 MU and 1000 MU, respectively, 1 % and 3,5 % less dose was measured, than expected.Output consistency, post calibration, was initially measured with the x-ray tube in “reference position” to avoid sources of error. Measurements were performed daily for three months prior to clinical introduction, and thereafter weekly, as part of regular quality assurance (QA). During the first two months, output dropped approximately 9 % before finally stabilising and has since then been within ± 2 % consistently. No clinically relevant differences in output with tube orientation relative to the reference position were observed, although an output of -1 % was obtained with the x- ray tube facing the floor.The machine handled beam delivery interruptions without remarks, although a single interruption caused an average signal loss of 1.7 % whereas two interruptions caused a 3 % loss.Remaining results are presented in Table 1.
Keywords: commissioning cxb dosimetry References:
[1] Gerard, JP, et al. Neoadjuvant chemoradiotherapy with radiation dose escalation with contact x-ray brachytherapy boost or external beam radiotherapy boost for organ preservation in early cT2-cT3 rectal adenocarcinoma (OPERA): a phase 3, randomised controlled trial. Lancet Gastroenterology & Hepatology 2023; 8(4):356-67 https://doi.org/10.1016/ S2468- 1253(22)00392-2.[2] Andreo, P, et al. Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water. International Atomic Energy Agency Technical Reports Series no 398, 2024. QA of brachytherapy source path in gynaecological applicators through tracking of the clinical source wire using x-ray imaging Sara Bornedal 1 , Maria Persson 1 , Åsa Carlsson- Tedgren 1,2 1 Department of Nuclear Medicine and Medical Physics, Karolinska University Hospital, Stockholm, Sweden. 2 Department of Oncology-Pathology, Karolinska Institute, Stockholm, Sweden Digital Poster 4144
Conclusion: Commissioning was performed during three months and resulted in a clinically accepted machine, consistent with our expectations. While treatment of rectal tumours generally will be done with 30 Gy (3000 MU), dose de-escalation requires reviewing, considering the non-proportional linearity of MU against measured signal. Most of the commissioning tests have been included in our periodic QA program.
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