Is the dose-response relationship for local control of Hodgkin's disease obscured by lung inhomogeneity corrections?

Absence of a dose-response relationship has been reported for local control of Hodgkin's disease between 30 and 42 Gy delivered to the mantle field in definitive irradiation. These dose ranges were determined at the central ray using irregular field (IF) calculations without benefit of dosimetric correction for lung inhomogeneity. Detailed analysis was performed of dose delivered to hilar and mediastinal regions with mantle field irradiation incorporating lung inhomogeneity corrections for the indicated dose ranges using 60Co and 6 MV linear accelerator. Inclusion of lung inhomogeneity (LI) corrections revealed dose heterogeneity within the treatment volume, delivering higher total doses to hilar and lateral mediastinal regions. For a prescribed dose of 40 Gy to midline with 6 MV photons, the hila and mediastinum would receive 45 Gy with IF and LI corrections. Similar increases are observed with 60Co. Mantle field calculations should be performed with CT planning including lung inhomogeneity corrections to account for anatomic irregularity of the mediastinal contour and interposed lung. As lung inhomogeneity dosimetric data become available, conclusions drawn from clinical experience should be re-evaluated based upon location and extent of mediastinal disease involvement. Implications for treatment associated late toxicity using dose-volume histogram analysis should be considered.