Evaluation of interplay and organ motion effects by means of 4D dose reconstruction and accumulation

Abstract

Pencil beam scanned proton therapy (PBS-PT) treatment quality might be compromised by interplay and motion effects. Via fraction-wise reconstruction of 4D dose distributions and dose accumulation, we assess the clinical relevance of motion related target dose degradation in thoracic cancer patients. Methods and materials: For the ten thoracic patients (Hodgkin lymphoma and non-small cell lung cancer) treated at our proton therapy facility, daily breathing pattern records, treatment delivery log-files and weekly repeated 4DCTs were collected. Patients exhibited point-max target motion of up to 20 mm. They received robustly optimized treatment plans, delivered with five-times rescanning in fractionated regimen. Treatment delivery records were used to reconstruct 4D dose distributions and the accumulated treatment course dose per patient. Fraction-wise target dose degradations were analyzed and the accumulated treatment course dose, representing an estimation of the delivered dose, was compared with the prescribed dose. Results: No clinically relevant loss of target dose homogeneity was found in the fraction-wise reconstructed 4D dose distributions. Overall, in 97% of all reconstructed fraction doses, D98 remained within 5% from the prescription dose. The V95 of accumulated treatment course doses was higher than 99.7% for all ten patients. Conclusions: 4D dose reconstruction and accumulation enables the clinical estimation of actual exhibited interplay and motion effects. In the patients considered here, the loss of homogeneity caused by interplay and organ motion did not show systematic pattern and smeared out throughout the course of fractionated PBS-PT treatment. Dose degradation due to anatomical changes showed to be more severe and triggered treatment adaptations for five patients.

Towards the clinical implementation of intensity-modulated proton therapy for thoracic indications with moderate motion: Robust optimised plan evaluation by means of patient and machine specific information

“…Besides anatomical information provided by weekly 4DCTs, patient intra-fractional motion variability is also considered by recording patient breathing patterns and acquiring machine log files for each delivered treatment fraction. This tool has been used for a fraction-wise treatment evaluation of thoracic indications, providing longitudinal treatment course quality control and aiding in the clinical decisions for plan adaptation [56]. As future research, by reducing the applied margins and the magnitude of the IMPT robust optimisation parameters, we will proceed to investigate less conservative, and consequently more conformal planning strategies for thoracic IMPT.…”

Dosimetric influence of deformable image registration uncertainties on propagated structures for online daily adaptive proton therapy of lung cancer patients

“…Besides intra-fractional motion, the dose calculation in low-density lung tissue has large uncertainties and large inter-fractional changes are expected to occur [24,[51][52][53]. Each of these challenges are discussed separately in the following paragraphs.…”

“…Intensity modulated proton therapy treatment plans with 60 Gy-RBE (RBE=1.1) in 2 Gy-RBE per fraction were optimised on a planning target volume (PTV), which was generated by a homogeneous extension of the CTV by 5 mm [33]. Plans consisting of three individually selected fields [24,[33][34][35] were optimised using an in-house developed planning system and analytical dose calculation algorithm [27,36].…”

“…Motion mitigation techniques such as rescanning [17], gating [13], tracking [18], 4Doptimisation [19,20] or deep-inspiration breath-hold (DIBH) [21] can handle the latter, while for the former fast online daily plan adaption can restore the target coverage [22,23]. Previous studies showed that the dose degradation caused by changing anatomy is more severe than that caused by intrafractional changes (patients treated in free breathing with rescanning) and makes treatment plan adaption necessary [24].…”

A study to investigate the influence of cardiac motion on the robustness of pencil beam scanning proton plans in oesophageal cancer

“…We did not investigate the interplay effects between pencil beam scanning delivery and the cardiac motion phase. The interaction between pencil beam scanning and breathing motion was studied previously in patients with oesophageal and lung cancer, resulting in a limited impact on the prescription dose [22] , [23] , [24] . Given the small displacement of the heart compared to breathing motion, the influence of interplay effects for cardiac motion is probably even more limited.…”