S1666
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
TPS within stated uncertainties. In Phase 2, beads and CC04 remained in close agreement and revealed a systematic TPS underestimation that grows with distance from the field edge. From our data, for cumulative dose, IPEM’s CIED low-risk levels [i] are typically reached ≥ 4 cm for VMAT (6 MV) and ≥ 3 cm for SABR (10 MV FFF) from edge of high dose area of PTV. If a CIED lies closer than these distances, a heightened-care pathway is recommended: independent OOF measurement (beads/IC), cardiology liaison/interrogation, and plan or shielding modifications. Because TPS alone may misclassify risk near the field edge, measured dose should guide stratification, especially as device dose approaches 5– 10 Gy.Beads allowed multiple measurement points in a single treatment delivery, whereas data for IC needed multiple deliveries. References:
Results: Phase 1 (reference validation). Micro-silica bead TLDs’ measured doses agreed within 3% of both the ion chamber (IC) and TPS values, with a combined measurement uncertainty of ±3.0% (k = 2).Phase 2 (2 cm depth, CIED-relevant extended measurement to OOF). For both plans, there was close agreement between beads, CCO4 and TPS across the length of the target volume, but then TPS dose started to deviate from the measured values beyond the edge of the target, with deviation increasing with increasing distance, with an average 48% underestimation for VMAT and an order of magnitude for SABR (Fig. 2).
[i] Radiotherapy Board (The Royal College of Radiologists, IPEM & Society and College of
Radiographers) 2025, Guidance on the management of patients with cardiac implantable electronic devices receiving radiotherapy, January 2025, London. Available at: Radiotherapy Board Publications (accessed 27 Oct 2025). Keywords: Out-of-field dosimetry; Silica Bead TLDs; CIED
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In-vivo electron dose measurements with micro silica bead TLD’s embedded in surface patches. John A Mills 1 , Mehdi SABERI 2 , Fatemeh NAZARI 3 , Shakhardokht JAFARI 3 1 PEAS, James Watt House, HINCKLEY, United Kingdom. 2 Physics, GenesisCare, MILTON KEYNES, United Kingdom. 3 True-invivo, Halpern House, PORTSMOUTH, United Kingdom Purpose/Objective: Micro silica bead dosimeters of 1.6mm outer diameter can be embedded into 2mm thick 3D printed patches which fully conform to the surface of a patient. The small size of detectors enables high resolution measurements in the penumbra region of the treatment field.This paper describes the generation of a patch from the surface scan of a patient and measurements in a phantom and on a patient. Material/Methods: Micro silica bead dosimeters can be used for absolute dose measurements in electron beams (1). There is minimal perturbation of the electron beam (2). Surface Scanning of patients (3) provides an accurate surface file, used to produce moulds for a plaster cast or a low melting point alloy mask (4). By embedding location cavities for micro-beads within a thin patch,
Conclusion: Phase 1 confirmed micro-silica bead TLDs are quantitatively reliable under reference conditions, agreeing with the calibrated CC04 ion chamber and
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