Histomorphometric evidence of growth plate recovery potential after fractionated radiotherapy: an in vivo model.

This study evaluated the hypothesis that early growth plate radiorecovery is evident by growth rate, histomorphometric and immunohistochemical end points after exposure to clinically relevant fractionated radiation in vivo. Twenty-four weanling 5-week-old male Sprague-Dawley rats were randomized into eight groups. In each animal, the right distal femur and proximal tibia were exposed to five daily fractions of 3.5 Gy (17.5 Gy) with the left leg serving as a control. Rats were killed humanely at 7, 8, 9, 10, 11, 14, 15 and 16 days after the first day of radiation exposure. Quantitative end points calculated included individual zonal and overall growth plate heights, area matrix fraction, OTC-labeled growth rate, chondrocyte clone volume and numeric density, and BrdU immunohistochemical labeling for proliferative index. Transient postirradiation reductions occurred early and improved during observation for growth rate, proliferative indices, transitional/hypertrophic zone matrix area fraction, proliferative height, and clonal volume. Reserve and hypertrophic zone height remained increased during the period of observation. The current model, using a more clinically relevant fractionation scheme than used previously, shows early evidence of growth plate recovery and provides a model that can be used to correlate temporal changes in RNA and protein expression during the early period of growth plate recovery.