Adaptive radiotherapy and the dosimetric impact of inter- and intrafractional motion on the planning target volume for prostate cancer patients

Felix Böckelmann,Sebastian Lettmaier,Karoline Kallis,Christoph Bert,Florian Putz,Rainer Fietkau

doi:10.1007/s00066-020-01596-x

Abstract

PurposeTo investigate the dosimetric influence of daily interfractional (inter) setup errors and intrafractional (intra) target motion on the planning target volume (PTV) and the possibility of an offline adaptive radiotherapy (ART) method to correct larger patient positioning uncertainties in image-guided radiotherapy for prostate cancer (PCa).Materials and methodsA CTV (clinical target volume)-to-PTV margin ranging from 15 mm in LR (left-right) and SI (superior-inferior) and 5–10 mm in AP (anterior-posterior) direction was applied to all patients. The dosimetric influence of this margin was retrospectively calculated by analysing systematic and random components of inter and intra errors of 31 consecutive intermediate- and high-risk localized PCa patients using daily cone beam computed tomography and kV/kV (kilo-Voltage) imaging. For each patient inter variation was assessed by observing the first 4 treatment days, which led to an offline ART-based treatment plan in case of larger variations.Results:Systematic inter uncertainties were larger (1.12 in LR, 2.28 in SI and 1.48 mm in AP) than intra systematic errors (0.44 in LR, 0.69 in SI and 0.80 mm in AP). Same findings for the random error in SI direction with 3.19 (inter) and 2.30 mm (intra), whereas in LR and AP results were alike with 1.89 (inter) and 1.91 mm (intra) and 2.10 (inter) and 2.27 mm (intra), respectively. The calculated margin revealed dimensions of 4–5 mm in LR, 8–9 mm in SI and 6–7 mm in AP direction. Treatment plans which had to be adapted showed smaller variations with 1.12 (LR) and 1.72 mm (SI) for Σ and 4.17 (LR) and 3.75 mm (SI) for σ compared to initial plans with 1.77 and 2.62 mm for Σ and 4.46 and 5.39 mm for σ in LR and SI, respectively.ConclusionThe currently clinically used margin of 15 mm in LR and SI and 5–10 mm in AP direction includes inter and intra uncertainties. The results show that offline ART is feasible which becomes a necessity with further reductions in PTV margins.

Highlights

Adaptive radiotherapy (ART) is an accepted method to treat patients with prostate cancer (PCa) since image-guided radiotherapy (IGRT) cannot compensate completely for patient-specific treatment variations [1, 2]
The systematic error was lower for intrafractional motion than for interfractional patient positioning (LR with 0.44 mm and 1.12 mm, SI with 0.69 mm and 2.28 mm and AP with 0.80 mm and 1.48 mm), whereas random errors were alike in LR and AP direction (LR with 1.91 mm and 1.89 mm and AP with 2.27 mm and 2.10 mm)
ART is the ideal intervention to account for specific interfractional discrepancy, as it has been reported that ART correction strategies reduce systematic and random errors [1, 2, 22, 32]

Summary

Introduction

Adaptive radiotherapy (ART) is an accepted method to treat patients with prostate cancer (PCa) since image-guided radiotherapy (IGRT) cannot compensate completely for patient-specific treatment variations [1, 2]. Offline and online ART strategies, such as offline planning target volume (PTV) modification, offline dose compensation and online plan adaption, have been introduced to diminish systematic and random errors [2, 3]. One of the first offline ART strategies was introduced by Yan et al [4] using daily CT and Martinez et al [5] by applying online portal imaging for reoptimizing treatment plans within the first week of the treatment scheme. They dealt with systematic errors generated K. Indication for radiotherapy, n (%) Primary treatment (%) Postoperative treatment Age (years) Mean (range) 71 (62–80)

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Discussion

Conclusion