ESTRO 2025 - Abstract Book

S2874

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

scans were rigidly registered to assess dose distribution in the overlapping regions and validate whole-body coverage. The isocenter coordinates were fixed in the anterior-posterior (A-P) and left-right (L-R) axes, while in the craniocaudal (C-C) direction, the distance between the isocenters was set at 25 cm for the HFS position and 25 or 30 cm for the FFS position, depending on the patient's height. The prescribed doses were 2 Gy x 1 fraction for three patients, and 8 Gy x 4 fractions for two patients. Raystation treatment planning system (TPS) generated the progressive dose gradient (PDG) in the junction regions, which is influenced by the isocenter distance. Offset errors of 5, 7, 8, and 10 mm were simulated along all axes to assess their impact on whole-body coverage. The acceptable deviation from the initial dosimetry was set at 2% (V95% and V110%). The distances between the isocenters tested were 27, 33, 35 and 40 cm, and the PDG's length and shape were assessed.

Results:

Results showed that offset errors of ≤ ±5 mm had minimal impact on the V95% (deviation < 0.2 Gy) in all directions. For offsets between 5 and 7 mm, coverage remained stable in the L-R direction, but there was up to a 2.5% reduction in the C-C direction.Offsets greater than ±7 mm caused significant changes in coverage, with a 10 mm offset resulting in a 7% decrease in V95%.The PDG in the overlap regions attenuated the overdosage, with V110% increasing by no more than 1.5% for a 10 mm offset. Conclusion: The study concluded that the PDG from the TPS helps mitigate the risk of over- or underdosage due to patient positioning errors. We accept maximum 5 mm offset without repositioning. We apply an offset of 5-7 mm in the L-R