Room: AAPM ePoster Library
Purpose: Deformable registration and dose accumulation are key tools for estimating composite dose to targets and organs at risk (OARs). In particular, treatment regimens combining external beam radiotherapy (EBRT) and brachytherapy, such as for locally-advanced cervix cancer (LACC), rely on estimating the composite dose-volume (DVH) metrics, such as D2cc to OARs. A fundamental understanding of how these metrics behave under deformable mappings is critical for the clinical implementation of dose accumulation and adaptive radiotherapy.
Methods: Guided by the treatment of LACC, several fundamental dose deformation scenarios were investigated in a commercial treatment planning system (RayStation v. 6.1, RaySearch). Scenarios included solid and hollow spherical and cylindrical test objects (mimicking the bladder and rectum). Deformation vector fields (DVFs) were calculated that perfectly mapped outer-to-outer and inner-to-inner surfaces. The geometries were specified such that local volume preservation could be enforced (iso-volumetric walls) or not (iso-thick walls). A brachytherapy source distribution was placed at pre-defined source-to-surface distances (SSDs) and D2cc was computed on the undeformed and deformed ROIs.
Results: If local volume preservation of the DVF is violated, differences in D2cc between the target and reference structures can be large (up to 50%) in very high dose gradients (SSD=5 mm) and still considerable (20%) for SSD=3 cm. If local volume is preserved, D2cc differences are within expected interpolation uncertainties (7% and 1%, respectively). These deviations arise even if the objects are well registered and can be related to the local deformation gradient tensor (Jacobian) of the DVF.
Conclusion: DVH metrics such as D2cc rely on local volume preservation and are inadequate for reporting dose values if this condition is violated. This is particularly prominent in high dose gradients and large deformations. DVFs must undergo careful quality control within accumulated ROIs to ensure consistent behavior with the expected biomechanics of the tissue in question.