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

Highlights

• Preservation of renal function is essential in kidney SABR.

• A larger volume of ipsilateral kidney is spared when managing respiratory motion.

• Dose to OARs is lowered with motion management.

• Motion management is recommended when tumour motion amplitude is larger than 1 cm.

• Managing respiratory motion improves renal function by 4% per cm of motion.

Abstract

Purpose

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.

Methods

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.

Results

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).

Conclusions

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.

Introduction

Stereotactic ablative body radiotherapy (SABR) is a novel treatment for patients with medically inoperable renal cell cancer (RCC) [1], [2]. However, a large amount of radiation dose is commonly deposited to non-tumour ipsilateral kidney during SABR treatment [3], [4]. A relationship between radiation dose received to healthy kidney and renal function loss has recently been determined [3]. In particular, it has been shown that every 10 Gy of physical delivered dose results in a loss of 25–39% of SPECT activity depending on the prescribed dose. Any dose reduction to functional kidney volume would then limit the loss to renal function. Patients receiving SABR treatment typically have impaired kidney function and sometimes a solitary kidney. Therefore, dose reduction to the ipsilateral kidney is highly desirable to preserve renal function.

Kidneys move with respiration [5], [6], [7]. Respiratory motion is typically measured with respiratory binned four-dimensional CT (4DCT) images. Images are sorted into respiratory phases in order to reflect the breathing pattern. Respiratory motion in radiation therapy is typically managed through the use of an internal target volume (ITV), however advanced motion management such as respiratory gating [8], [9], [10], breath-hold [8], [9], [10], continuous positive airway pressure (CPAP) [11], multileaf collimator tracking [10], [12], ultrasound-based repositioning [13], or abdominal compression [10], [14] may be employed [15]. Eliminating the effect of respiratory motion through advanced motion management may lead to smaller treated volume, and thus to dose reduction to adjacent critical organs [16].

The purpose of this study is to quantify the gain in renal function if advanced motion management is employed, compared with free-breathing and an ITV. We investigate the scenario when the effects of respiratory motion are completely removed, as may be achievable with breath hold, respiratory gating, or real-time tracking. It is hypothesized that removing breathing motion will reduce dose received by the ipsilateral kidney and therefore improve renal function post SABR treatment. Motion management is requiring extra resources. By evaluating renal function gain obtain from dose reduction, we aim to help clinical decision-making in the use of advanced motion management and to optimise patient selection receiving SABR for primary RCC in the context of the additional resources required.