Reducing the impact on renal function of kidney SABR through management of respiratory motion

Stereotactic ablative body radiotherapy (SABR) is as a viable treatment option to treat kidney cancer. This study quantifies dose reduction to non-tumour ipsilateral kidney and estimated renal function gain from elimination of respiratory motion.

We reviewed 62 previously treated kidney SABR patients. The gross tumour volume (GTV) was segmented in each phase of a four-dimensional CT (4DCT). Tumour motion amplitude (TMA) was obtained from the GTV centroid on each phase. Low modulation, motion managed (MM) plans were generated on the exhale phase image. Internal target volume (ITV)-based plans were generated on the 4DCT average intensity projection. To estimate delivered kidney dose, the ITV-based plan was copied ten times to the exhale phase image, with isocentre located at the GTV centroid position in each phase. The dose was calculated and averaged to result in non-motion managed plans. Difference in ipsilateral kidney volume receiving 50% of the prescription dose (V50%) and estimated glomerular filtration rate (GFR) change were compared between ITV and MM plans.

The mean ± st.dev. TMA was 0.79 ± 0.49 cm. Removing respiratory motion reduced ipsilateral kidney V50% (slope of the difference = 12 cc/cm of TMA, Pearson-r = 0.69, p-value <10^-9), and estimated GFR was improved (slope = 4.4 %/cm of TMA, Pearson-r = 0.85, p-value < 10^-10).

We have quantified the improvement in healthy kidney dose when removing respiratory motion from kidney SABR plans, and demonstrated an expected gain in GFR of 4.4 %/cm of motion removed.