Prescription dose and optimisation strategies in MR-guided online adaptive radiotherapy for kidney tumours: a two-step planning analysis

Abstract

Background: Stereotactic radiotherapy (SRT) for kidney cancer, particularly when tumours are situated near critical organs-at-risk (OARs), presents significant challenges in achieving optimal dose delivery. MR-guided online adaptive radiotherapy (MRgoART) offers a promising solution by allowing real-time anatomical modification and plan reoptimisation. However, the ideal strategy for prescription dose selection and reoptimisation remains unclear.

Methods: This single-centre planning study evaluated kidney tumours located within 1 cm of gastrointestinal OARs. In Step 1, prescription doses for MRgoART were compared: the target dose (26 Gy) versus the planned dose (adjusted during pre-treatment planning to satisfy OAR constraints). In Step 2, two optimisation strategies were assessed: (1) covering 99% of the planning target volume (PTV) with the prescription dose (99%_xGy_Plan) and (2) delivering the full target dose with acceptable partial PTV coverage accepting dose heterogeneity (26Gy_x%_Plan), both respecting OAR constraints. Dose-volume parameters and blinded expert preferences were evaluated.

Results: Of 22 patients assessed, 14 patients with 18 tumours met the inclusion criteria. Among these, 36 MRgoART plans for 12 tumours were analysed in Step 1. Reoptimisation using the target dose resulted in significantly higher mean tumour doses, improved dose gradients, and PTV coverage metrics compared to reoptimisation based on the planned dose. In Step 2, 54 plans were assessed. Although the 26Gy_x%_Plan demonstrated superior mean tumour and PTV dose, it exhibited lower conformity. Radiation oncologists preferred the 26Gy_x%_Plan in 48% of cases, following 26% deemed almost equal, indicating its clinical advantage.

Conclusion: For kidney tumours adjacent to gastrointestinal OARs, MRgoART planning should favour reoptimisation based on the target dose. A high-dose strategy with partial PTV coverage (26Gy_x%_Plan) was generally preferred by radiation oncologists, balancing therapeutic effectiveness with OAR protection.

Keywords: Adaptive radiotherapy; Kidney tumour; MR-Linac; MR-guided online adaptive radiotherapy; Planning study; Stereotactic radiosurgery.

Fig. 1

Flowchart of the two-step analysis. Step 1 compares reoptimisation using the target dose vs. planned dose. Step 2 compares the 99%_xGy_Plan vs. the 26Gy_x%_Plan. Abbreviations: MRgoART, MR-guided online adaptive radiotherapy; PTV, planning target volume; OAR, organ at risk

Fig. 2

Examples of dose distributions: (A) pre-treatment planning, (B) planned dose reoptimisation, (C) target dose reoptimisation, which is identical to the 99%_xGy_Plan used in Step 2, (D) partial PTV coverage strategy (26Gy_x%_Plan). Colour isodose lines: brown (30 Gy), red (26 Gy), yellow (22 Gy), light blue (18 Gy), purple (14 Gy), blue (10 Gy). Contours: pink (PTV), white (duodenum), yellow-green (small bowel), green (colon). Doses covering 99% of the PTV for (A), (B), (C), and (D) were 18.1 Gy, 19.8 Gy, 21.3 Gy, and 22.3 Gy, respectively. Doses covering 95% and 50% of the PTV for (A), (B), (C), and (D) were 22.1 Gy and 26.0 Gy, 20.8 Gy and 23.8 Gy, 23.4 Gy and 27.1 Gy, and 26.4 Gy and 31.1 Gy, respectively