S1628
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Results:
Phys2023, 50, 8034–8043. DOI: 10.1002/mp.166744. Kaiyum, R.; Hoehr, C.; Tattenberg, S.; Shum, R. L.; Mermut, O.; Rink, A. Commun Mater2025, [In Press].5. Hsing, C.-H.; Oanh, L. D. H.; Chao, T.-C.; Lee, C.-C.; Hong, J.-H.; Cheng, C.-C.; Tseng, C.-K.; Tung, C.-J. Physica Medica2021, 81, 185–190. DOI: 10.1016/j.ejmp.2020.12.007 Keywords: radiochromic materials, dosimetry, diacetylenes Digital Poster Highlight 716 Effects of non-coplanar radiation beams on potential foetal dose during cranial radiotherapy treatments for pregnant patients Tanya Kairn, Daniel Bryant, Catherine Bettington, Scott B Crowe Cancer Care Services, Royal Brisbane and Women's Hospital, Brisbane, Australia Purpose/Objective: With reductions in the number of MU required to deliver VMAT treatments compared to IMRT or wedged treatments, and the increasing availability of flattening-filter-free (FFF) beams which avoid flattening filter scatter, it is becoming common to deliver cranial radiotherapy treatments to pregnant patients using a VMAT technique [1,2].Noncoplanar beams through the patient in the superior-inferior direction from ~90o couch angles (aka: table or floor angles), while common in cranial VMAT, are clearly unacceptable in these cases. However, evaluation of potential effects of smaller couch rotations, including beams though the patient in the inferior-superior direction remains crucial. Material/Methods: Head, neck and thorax sections of an Alderson RANDO phantom (1974 model) were set up with slabs of water-equivalent plastic positioned to allow calibrated ionisation chamber measurements 50 cm inferior of isocentre. A 6 MV FFF radiation beam with geometry derived from cranial VMAT beam characteristics [3] (3x3 cm2 MLC aperture inside 6x6 cm2 jaw field) was directed to a hypothetical off-axis cranial target from various gantry and couch angles.
Figure 1: Measurement results at 10 cm depth, indicating how noncoplanar beams may affect foetal dose estimates. (Blue text, arrows and data relate to positive couch rotations. Green text, arrows and data relate to negative couch rotations.)
Figure 2: Measurement results on surface, indicating how noncoplanar beams may affect in vivo dosimetry. (Same colour coding as figure 1.)Figures 1-2 summarise the results of this study, with asymmetries (different results on the positive/negative sides of gantry zero) exemplifying the effects of an off-axis target position.Generally, beams that were directed into the phantom from the superior towards the inferior due to the rotated couch showed a trend of increasing dose to the foetal region with increasing couch angle, a clear effect of internal patient scatter [4,5,6].Beams directed in the opposite direction also showed increasing dose to the foetal region with increasing couch angle, with leakage dose increasing dramatically as the linac head moved closer to the
phantom [7]. Conclusion:
These results provide a warning against using non- coplanar beams in the rare cases where the delivery of cranial radiotherapy to pregnant patients is unavoidable:1. Beams directed in the superior-inferior direction result in increased internal scattered
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