2.3. EQD2 sum plans and statistical analysis

Equivalent uniform dose in 2-Gy fraction (EQD2) sum plans were calculated for all courses of brain radiotherapy using MIM (MIM Software Inc. Version 6.7.9). The CT scans, dose distribution and the structure sets of all relevant courses were exported from ARIA to MIM. Then the EQD2 doses of each course were estimated in MIM. After inserting the number of fractions, the structures were matched with their different α/β-values and the equivalent uniform dose. To calculate the EQD2 the following formula was used:

where n is the number of fractions and d the fraction dose. Different EQD2 parameters were calculated based on different dose metrics (e.g. Dmean, D1cc etc.).

The α/β-values chosen were 10 Gy for GTV, 2 Gy for the myelon, brainstem and the optical nerve and 3 Gy for the body contour. Thereafter, EQD2 was calculated for the different structures in MIM and then transferred back to ARIA/Eclipse for final dose accumulation and further detailed dose-volume analysis.

The EQD2 plans were matched in Eclipse treatment planning system using rigid automatic bone match (translation and rotations) image registration. All organs at risk (OARs) were contoured on the most recent CT scan.

Dose parameters were then extracted for OARs (brain as defined as “whole brain minus GTV”, brainstem, chiasm, right and left optic nerves) and target volumes.