S125
Brachytherapy - Physics
ESTRO 2026
10.1002/mp.15674[10] Hartmann GH et al. Med Phys. 2021 Nov;48(11):7461-7475.
results which cannot not be used in setups differing from those at determination. Few publications report kQBT,Q0 at different source distances[3-9] or in different detector orientations[4,5]. Material/Methods: Recent assessments of the beam quality correction factor are reviewed. A new formalism based on the cema approach to cavity theory[10] allows determination of kQBT,Q0 for calibrated ionization chambers using a decomposition of Newly defined perturbation factors pint and pext correct for volume averaging in the sensitive volume and for perturbations of the radiation fluence by detector structures[3]. The perturbation factors are analyzed for multiple chamber positions and orientations in relation to the source[3,4]. Comparison of pint for cavities composed of water or detector medium under different measuring conditions allows separate assessment of volume averaging caused by geometry and by further fluence perturbations caused by the detector material. Results: The behavior of the perturbation factors in the radiation field of a BT source suggests a standardized pathway of calculating kQBT,Q0 with Monte Carlo simulations. While older determinations of kQBT,Q0 require intermediate theoretical calculation steps (e.g. definition and application of intermediate dose conversion factors[2]) to fulfil approximate conservation of Bragg-Gray conditions between calibration and measurement, the fluence analysis, differential in energy, shows that direct MC simulation yields correct results without such assumptions and can be used to calculate kQBT,Q0 under the varying measuring conditions in brachytherapy. Conclusion: Analysis of the perturbation factors can help selecting appropriate calibrated detectors for the determination of a reference dose rate from a brachytherapy source in water and detectors for measurement of its relative dose distributions in a phantom. Keywords: Brachytherapy, dosimetry, beam quality correction References: [1] IAEA Technical Report Series No. 398 Rev. 1. International Atomic Energy Agency, Vienna 2024[2] Sarfehnia A et al. Med. Phys. 37,4 (2010)1924-1932.[3] Failing T et al. Phys. Med. Biol. 69 (2024) 165002[4] Failing T et al. Phys. Med. Biol. 70 (2025) 115020[5] Rossi G et al. Z. Med. Phys. 33 (2023) 463– 78 https://doi.org/10.1016/j.zemedi.2022.07.004[6] Thrapsanioti Z et al. J. Appl. Clin. Med. Phys. (2024) e14575.[7] Chofor N et al.. Z. Med. Phys. 26,3 (2016) 238-250.[8] Kaveckyte V et al. Med. Phys. 45,1 (2018) 429-37. DOI: 10.1002/mp.15674 [9 ) Kaveckyte V et al. Med Phys. 49 (2022) 4715–30. DOI:
Digital Poster 1876
Comparison of dosimetric quality and planning efficiency between surface HDR brachytherapy and VMAT for post-operative shoulder keloid: A case study Zulaikha Jamalludin 1,2 , Nur Liyana Amir 2 , Saiful Azli Bistamam 2 , Zulhaziq Hilmi Zulkifli 2 , Nur Syahirah Mohammad Sanusi 2 , Muhammad Adib Abd Manaf 2 , Adlinda Alip 1,2 1 Department of Clinical Oncology, Universiti Malaya, Kuala Lumpur, Malaysia. 2 Department of Clinical Oncology, Universiti Malaya Medical Centre, Kuala Lumpur, Malaysia Purpose/Objective: Aggressive keloids can cause pain, functional limitations and significant psychosocial impact, often requiring repeated surgical excisions. Radiotherapy has been shown to reduce regrowth risk by targeting residual keloid cells post-surgery. Surface brachytherapy and photon volumetric modulated arc therapy (VMAT) are among the available options. For extensive or anatomically complex lesions, such as shoulder keloid exceeding 25 cm, treatment planning becomes challenging. Moreover, the first radiation fraction must be delivered within 24 hours after surgery, making planning time a critical factor. This study compares the planning efficiency and dosimetric quality between surface brachytherapy and photon VMAT in an aggressive shoulder keloid case at Universiti Malaya Medical Center. Material/Methods:
A retrospective VMAT plan was generated for the same patient previously treated with surface HDR brachytherapy. Time from contouring to clinician
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