2951 - Respiratory Phase Variability of Organ-at-Risk Dose in Adrenal Stereotactic Body Radiotherapy

12:30pm - 01:45pm ET

Poster Hall - Exhibit Hall A

Screen: 20

POSTER

Presenter(s)

Arnaud Beddok, MD, PhD - Institute Jean Godinot, Reims 51056, France

H. Ben Amira¹, W. Mota¹, A. Licha², L. Ettalhaoui³, S. Guendouzen³, C. Invernizzi¹, P. Guilbert³, P. Torielli⁴, and A. Beddok⁵; ¹Institut Jean Godinot, reims, France, ²Institute Jean Godinot, Reims, France, ³Institut Jean Godinot, Reims, France, ⁴Institut Godinot, Reims, France, ⁵Department of Radiation Oncology, Institut Godinot, Reims, France

Purpose/Objective(s):

Respiratory motion can alter the position of abdominal organs at risk (OARs) during stereotactic body radiotherapy (SBRT), yet OAR dose is commonly evaluated on a single free-breathing planning CT. The objective of this study was to quantify respiratory-phase variability in kidney and stomach dose in patients treated with adrenal SBRT using 4DCT.

Materials/Methods:

Patients treated with adrenal SBRT (35–45 Gy in 3–5 fractions) with available 4DCT imaging were retrospectively analyzed at a single institution. The treatment plan was evaluated on all ten respiratory phases using the free-breathing planning CT as the reference dataset. For all patients, the ipsilateral kidney was contoured on each respiratory phase, and kidney mean dose was extracted. In left adrenal cases, the stomach was additionally contoured, and both stomach D1cc and stomach mean dose were recorded. For each patient, intra-patient variability was summarized using the amplitude of variation (?, maximum minus minimum) and the difference between the worst respiratory phase and the planning CT value. Kidney results were summarized using median and interquartile range (IQR), whereas stomach results were reported descriptively.

Results:

Seven patients were analyzed for ipsilateral kidney dose, including three patients with additional stomach analysis. Respiratory motion resulted in measurable variability in kidney dose across phases. The median intra-patient ?Dmean was 1.22 Gy (IQR 0.71–2.51 Gy; range 0.52–3.32 Gy). Worst-phase Dmean exceeded planning CT values in all patients, with a median increase of 1.91 Gy (range 0.39–5.12 Gy). In the three left-adrenal cases, respiratory motion led to substantial variability in stomach D1cc, with intra-patient ?D1cc ranging from 2.3 to 11.0 Gy. Worst-phase D1cc exceeded planning CT values by 2.9 to 16.2 Gy in all cases. Stomach mean dose showed more limited variability, with ?Dmean ranging from 1.3 to 2.4 Gy.

Conclusion:

Respiratory motion systematically increases abdominal OAR dose compared with planning CT estimates during adrenal SBRT. While kidney mean dose variability was moderate, high-dose exposure to small stomach volumes showed marked phase dependence. Phase-based dose assessment using 4DCT may improve the robustness of OAR evaluation and reduce the underestimation of clinically relevant dose in abdominal SBRT.