Dose summation strategies for external beam radiation therapy and brachytherapy in gynecologic malignancy: a review from the NRG Oncology and NCTN medical physics subcommittees.

Abstract

Definitive, non-surgical management of gynecologic malignancies involves external beam radiation therapy (EBRT) and/or brachytherapy (BT). Summation of the cumulative dose is critical to assess the total biologically effective dose (BED) to targets and organs-at-risk (OARs). Cumulative dose calculation from EBRT and BT can be performed with or without image registration (IR) and biological dose summation. Among these dose summation strategies, linear addition of dose-volume histogram (DVH) parameters without IR is the global standard for composite dose reporting. This approach stems from an era without image guidance and simple external beam and brachytherapy treatment approaches. With technological advances, EBRT and high dose rate (HDR) BT have evolved to allow for volume-based treatment planning and delivery. Modern conformal therapeutic radiation involves volumetric- or intensity-modulated EBRT, capable of simultaneously treating multiple targets at different specified dose levels. Therefore, given the complexity of modern radiation treatment, the linear addition of DVH parameters from EBRT and HDR BT is challenging to represent the combined dose distribution. Deformable image registration (DIR) between EBRT and image guided brachytherapy (IGBT) datasets may provide a more nuanced calculation of multi-modal dose accumulation. However, DIR is still nascent in this regard, and needs further development for accuracy and efficiency for clinical use. Biologic dose summation can combine physical dose maps from EBRT and each IGBT fraction, thereby generating a composite DVH from the BED. However, accurate radiobiological parameters are tissue-dependent and not well characterized. A combination of voxel-based DIR and biologically weighted dose maps may be the best approximation of dose accumulation but remains invalidated. The purpose of this report is to review dose summation strategies for EBRT and BT, including: (1) conventional EQD2 dose summation without image registration, (2) physical dose summation using 3D rigid IR and DIR, and (3) biological dose summation. We also provide general clinical workflows for IGBT with a focus on cervical cancer.