Radiation Dose Response of Renal Parenchyma Assessed By CT Contrast Enhancement Change Following Kidney Stereotactic Body Radiotherapy
Abstract
Purpose
To quantify renal parenchyma functional changes in response to radiation dose for patients treated with kidney stereotactic body radiotherapy (SBRT) by analyzing the paired pre- and post-treatment contrast-enhanced CT scans. We hypothesize that radiation induced injury manifests as a measurable, dose-dependent reduction in local parenchymal contrast enhancement, potentially suggesting new dosimetric objectives for kidney functional sparing.
Methods
Twelve patients treated with kidney SBRT were retrospectively analyzed in two prescription cohorts: 36Gy in 3 fractions (n=6) and 40Gy in 5 fractions (n=6). Pre- and post-treatment portal venous phase CT scans were deformably registered. To reduce inter-scan contrast variability, the post-treatment CT was normalized on a slice-by-slice basis to align mean abdominal aorta Hounsfield Unit (HU) with that of the pre-treatment CT. Renal parenchyma was extracted on the pre-treatment CT using a window of [20HU, 200HU] to exclude cysts and collecting systems. Voxel-wise change in parenchymal enhancement (ΔHU = Post – Pre) was computed across all patients in each cohort using 3Gy intervals to generate population-level dose response curves.
Results
A non-linear dose response was observed in both cohorts. In the 36 Gy/3fx cohort, mean ΔHU decreased progressively from -7 HU in low-dose regions (<10Gy) to -16HU at 22.5Gy, with diminishing incremental change beyond this dose level. The 40 Gy/5fx cohort exhibited a steeper decline: ΔHU remained stable at -7 HU in low-dose regions (<15Gy) before dropping sharply to -20HU at 26.5Gy, where it reaches a plateau.
Conclusion
Voxel-wise analysis of paired pre- and post-treatment contrast-enhanced CTs demonstrates a dose-dependent reduction in renal parenchyma contrast enhancement following kidney SBRT. The observed non-linear dose–response suggests that limiting the volume of kidney receiving doses within the intermediate dose range associated with the sharp change in renal parenchyma contrast enhancement may be considered for kidney-sparing radiotherapy planning.