S2030
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Material/Methods: By separating and appropriately modifying/re-pairing the voxel mass and voxel dose values of a left lung structure coming from a left breast irradiation plan we modelled the special (extreme) cases of the spatial patterns of tissue density and dose distribution relative to each other and their common effects on volume and mass dosimetry. We distinguished the following scenarios:I. a homogeneous dose in a homogeneous density medium,II. an inhomogeneous dose in a homogeneous density medium,III. a homogeneous dose in an inhomogeneous density medium ,IV. an inhomogeneous dose varying with medium density,V. an inhomogeneous dose varying opposite to medium density.The method was tested on 102 left breast treatment plans using ‘classic’ two (medial and lateral) opposing fields and ’field-in-field’ (FiF) technique. Results: We defined new mass dosimetric qantities analogous to volumetric ones such as Dm-mean ‘mass-average dose’, BEDm-mean ’mass-average biological effective dose’, PTM ‘planning target mass’, D2g ’the minimum dose of the most exposed 2 g of the PTM’, CNm ’mass conformity number’. In cases I-III there is no difference between the results of the volume and mass dosimetric methodologies. Case IV means positive correlation between dose and density distribution, resulting DMH diverged upwards compared to DVH and consequently higher percentage and mean doses. In the case of negative correlation between dose and density distribution (case V) DMH runs below DVH, predicting actually smaller biological effect than that indicated by conventional dosimetry.
Figure 2. Conclusion:
Pediatric VMAT-TBI demonstrates excellent proximal stability with negligible rotation, while distal segments are more susceptible to longitudinal motion. Small, largely uniform margins appear adequate in cranial and thoracic regions, whereas slightly wider longitudinal margins should be considered for distal isocenters. Although region-specific margins are not strictly applicable in VMAT-TBI because the target is a continuous volume, the observed gradient underscores robust proximal setup accuracy and the need for heightened vigilance distally to avoid over- or underdosage at caudal junctions. Reinforced immobilization (e.g., shaped footrests, firmer vacuum cushions, pelvic belt) and prioritizing verification at inferior isocenters may further improve positional reproducibility and ensure consistent coverage along the body axis. Keywords: VMAT-TBI; Pediatric RT; Allogeneic HSCT Digital Poster 5121 The effect of tissue and dose inhomogeneity on mass dosimetry Gergely Antal 1,2 , Ferenc Lakosi 2 , Zsolt Cselik 1 , Georgina Fröhlich 3 1 Department of Radiotherapy, Veszprém County Csolnoky Ferenc Hospital, Veszprém, Hungary. 2 Department of Radiotherapy, Somogy County Kaposi Mór Teaching Hospital, Kaposvár, Hungary. 3 Centre of Radiotherapy, National Institute of Oncology, Budapest, Hungary Purpose/Objective: Previous studies have already established that the dose loads to the lung calculated on a volume (i.e. voxel) basis and calculated on a mass (i.e. voxel mass) basis may differ significantly in one direction (DVH<DMH) or another (DVH>DMH). We assumed that the direction and magnitude of this divergence is determined by the interrelationship between the anisotropic tissue density distribution and the also, but differently anisotropic dose distribution. In order to examine this problem area in more detail we needed to introduce new mass dosimetric concepts which are analogous to the 'traditional' volumetric ones.
For all left breast test plans volumetric dosimetry indicated higher DV-mean left lung volume-average dose comparing with Dm-mean mass-average dose values (25.2% (11.5%–43.0%)). The ‘traditional’ BEDV- mean volume-average biological doses were even higher than BEDm-mean values (28.7%, (13.0%– 47.4%)). Conclusion: Mass dosimetry is a promising method to enhance the evaluation of treatment plans for significantly inhomogeneous tissues like lungs, offering more accurate prediction of potential biological outcomes
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