Session Item

Assessment of cumulative doses to organs at risk (OARs) across multiple treatments is critical to safe re-irradiation, however there are multiple pathways to perform this assessment. We performed a multi-centre study to quantify the impact of clinical pathways on assessment of cumulative dose to OARs.

We provided DICOM planning CT, structures and dose for original and re-treatment courses for two patients with head & neck (HN) and lung cancer (Figure 1). Participants determined and reported cumulative physical and EQD2 doses (near-maximum, volumetric dose) to OARs using their clinical process and software. Standardised α/β values were provided for EQD2 calculation, and description of dose summation pathway was collected via a survey. Participants were asked to (optionally) submit physical dose distributions from the original course mapped onto the re-treatment course CT. A consistent workflow by one observer in a single software was used to sum the submitted registered dose distributions with dose from the second course dose, to assess isolated variation in image registration for dose mapping.

Figure 1: Summary of the two cases provided to participants

Cumulative dose assessment was performed by 21 participants using rigid (RIR) or deformable image registration (DIR)-based summation of 3D dose or isodose contours, or summation of DVH metrics extracted from each course. Registration was global or OAR-based. Three participants used tissue recovery factors (TRF) of 25-50% for the spinal cord only (n=1) or all organs (n=2). There was large variation in non-recovery corrected near-maximum cumulative dose (inter-observer ranges 14.5-24.1Gy for HN, 2.4-33.8Gy for lung OARs), which for lung OARs increased when converting to EQD2 (inter-observer ranges 9.4-42.9 Gy EQD2) (Figure 2, a-d). Cumulative mean doses were more consistent than near-maximum doses. Improved consistency in near-maximum dose was observed using a standardised workflow with submitted spatially mapped doses (inter-observer range 10.7-13.5Gy for HN, 0.4-9.8Gy for lung OARs) (Figure 2, e-f). 

Figure 2: Cumulative (a) physical and (b) EQD2 cumulative dose metrics for the head and neck (a & c) and lung cancer (b & d) cases reported by the participants (n=21) using their clinical pathway. (e) & (f) Cumulative physical dose metrics for doses summed and reported in a single treatment planning software (TPS) for the HNC and lung cancer cases respectively, based on mapped dose distributions.

Differences in methods for spatial mapping of dose between courses, conversion to biologically equivalent doses and use of tissue recovery factors resulted in substantial variations in cumulative dose assessment in two re-irradiation cases. The variations observed have implications for outcome analysis and our understanding of published doses concerning re-irradiation. Standardising the workflow using spatially registered doses may present a pathway for improved consistency in cumulative dose assessment.