TH-AB-BRB-12: A Novel 2D Expansion Technique Based On Beam Geometry for Generating Margins




Current practice of using 3D margins in radiotherapy with high-energy photon beams provides larger-than-required target coverage. Negligible dose changes for small CTV displacements in the beam direction allow for the design of 2D-based CTV-PTV margins. We investigate the impact of a novel 2D margin concept for prostate radiotherapy treatments.


Our implementation of 2D margins follows van Herk's (2000) margin recipe by considering only geometrical uncertainties perpendicular to the incident beam direction. The union of all beam-angle-dependent expansions defines our PTV. Five prostate radiotherapy plans were generated using IMRT inverse planning following clinical protocols with a prescription of 74Gy in 37 fractions. Systematic uncertainties of 1.1, 1.1, 1.5mm in the LR, SI, and AP directions, respectively, and 0.9, 1.1, 1.0mm for the random uncertainties, were used. A verification tool was used to simulate the effects of systematic and random errors, using a population of 10,000 identical geometries. The fraction of the population that satisfies or violates a certain dose or volume constraint was used for comparison.


The median observed PTV volume reduction between the 3D and 2D margins is 7.85% (range, 7.14–8.29%). Using the verification tool, all CTVs had close to 100% of the population satisfying D98% > 95% of the prescribed dose. When applying the verification tool to analyse the rectal doses delivered, a median relative reduction of 1.05% (range, 0.66–6.17%) of the population was observed for D2% above 70Gy; similarly for D2% above 74Gy, a median relative reduction of 31.7% (range, 13.3–52.6%) was observed.


We are able to ascertain sufficient dose coverage using our 2D margin concept for planning whilst lowering dose to nearby structures, though the amount of sparing depends largely on the patients’ anatomy. With the coverage observed using our novel technique, we believe additional room exists for sparing dose to target-adjacent OARs.

Research at The Institute of Cancer Research is supported by Cancer Research UK under Programme C33589/A19727. We acknowledge NHS funding to the NIHR Biomedical Research Centre at The Royal Marsden and The Institute of Cancer Research.