Session Item

Pencil beam scanning (PBS) proton therapy (PT) in patients with intra-fraction, breathing-induced tumour motion might result in unrecognized deviations from the planned dose distribution. Our work pursued the clinical roll-out of a 4D log file-based proton dose reconstruction (4DlogReco). By that, we monitor the interplay effect and study its relevancy in an ongoing clinical study at the University Proton Therapy Dresden (UPTD).

We had developed and experimentally validated a 4DlogReco (in RayStation v.8) based on amplitude-sorted 4DCTs, PBS machine log files and synchronized motion log files. The workflow and data handling was verified in the clinical treatment planning system by a retrospective analysis of four complete PBS-PT treatment series (incl. lung, oesophageal and pancreatic carcinoma; mean motion ≤5mm; 20-33 fractions) of patients who received weekly in-room 4DCTs for monitoring interfraction changes. For the final 4DlogReco translation into clinical practice, we initiated the MOBIL study (Monitoring Of Breathing for Interplay study with Logfiles).

Available patient data were analysed fraction-wise (Fig1). We considered individual critical organs at risk (OAR; lung, heart, spinal cord, kidneys, oesophagus), the clinical target volume (CTV) coverage, mean dose, near-maximum dose and homogeneity index [D₉₈, D_mean, D₁, HI=(D₁-D₉₈)/D_prescribed] and the deviations from the plan in the fraction-wise worst case (Δ_wc) and in the accumulated dose (Δ_acc).

The 4DlogReco was successfully translated into clinical application. So far, two patients (oesophageal and pancreatic carcinoma; mean motion ≤5mm; 19 and 30 fractions) had been treated within the MOBIL study. Daily 4DlogReco took about 15min incl. data processing and dose calculation, and should speed up by an automatic log file retrieval and GPU-based dose calculation after TPS upgrade.

For the six investigated patients (incl. the four workflow test patients), intra-fraction motion led to CTV parameter differences in the worst-case fractions of Δ_wc(D₉₈)=(‑15.5 – ‑3.4)pp, Δ_wc(D₁)=(‑0.1 – 3.5)pp and Δ_wc(HI)=0.04 – 0.17, while there were smaller changes in the accumulated dose of Δ_acc(D₉₈)=(‑2.6 – -0.6)pp, Δ_acc(D₁)=(-1.3 – 0.4)pp and Δ_acc(HI)=0.00 – 0.03 (Fig2). The individually relevant OAR dose parameters remained uncritical in line with the so far investigated minor motion amplitudes.