Retrospective Planning Study of Patients with Superior Sulcus Tumours Comparing Pencil Beam Scanning Protons to Volumetric-Modulated Arc Therapy

Abstract

Aims Twenty per cent of patients with non-small cell lung cancer present with stage III locally advanced disease. Precision radiotherapy with pencil beam scanning (PBS) protons may improve outcomes. However, stage III is a heterogeneous group and accounting for complex tumour motion is challenging. As yet, it remains unclear as to whom will benefit. In our retrospective planning study, we explored if patients with superior sulcus tumours (SSTs) are a select cohort who might benefit from this treatment. Materials and methods Patients with SSTs treated with radical radiotherapy using four-dimensional planning computed tomography between 2010 and 2015 were identified. Tumour motion was assessed and excluded if greater than 5 mm. Photon volumetric-modulated arc therapy (VMAT) and PBS proton single-field optimisation plans, with and without inhomogeneity corrections, were generated retrospectively. Robustness analysis was assessed for VMAT and PBS plans involving: (i) 5 mm geometric uncertainty, with an additional 3.5% range uncertainty for proton plans; (ii) verification plans at maximal inhalation and exhalation. Comparative dosimetric and robustness analyses were carried out. Results Ten patients were suitable. The mean clinical target volume D95 was 98.1% ± 0.4 (97.5–98.8) and 98.4% ± 0.2 (98.1–98.9) for PBS and VMAT plans, respectively. All normal tissue tolerances were achieved. The same four PBS and VMAT plans failed robustness assessment. Inhomogeneity corrections minimally impacted proton plan robustness and made it worse in one case. The most important factor affecting target coverage and robustness was the clinical target volume entering the spinal canal. Proton plans significantly reduced the mean lung dose (by 21.9%), lung V5, V10, V20 (by 47.9%, 36.4%, 12.1%, respectively), mean heart dose (by 21.4%) and thoracic vertebra dose (by 29.2%) ( P < 0.05). Conclusions In this planning study, robust PBS plans were achievable in carefully selected patients. Considerable dose reductions to the lung, heart and thoracic vertebra were possible without compromising target coverage. Sparing these lymphopenia-related organs may be particularly important in this era of immunotherapy.

Imaging Strategies in Proton Therapy for Thoracic Tumors: A Mini Review

“…Patients lay on the table with a gap between the head and pelvic vacuum bags to minimize material in the beam path ( 13 , 14 ). The type of cushions is critical because they could introduce uncertainties in the alignment of different tissues along the beam path ( 15 ).…”

“…The free-breathing patients underwent CT simulation with 4D-CT to account for respiratory motion with deformation ( 12 , 15 , 17 – 24 ). Regarding the motion assessment, a tumor motion range less than 5 mm is considered acceptable for free-breathing delivery ( 15 ).…”

“…The free-breathing patients underwent CT simulation with 4D-CT to account for respiratory motion with deformation ( 12 , 15 , 17 – 24 ). Regarding the motion assessment, a tumor motion range less than 5 mm is considered acceptable for free-breathing delivery ( 15 ). Patient respiratory waveforms were monitored throughout the procedures and recorded with a respiratory gating system ( 14 , 17 , 19 , 20 , 25 – 27 ).…”

“…In selecting imaging CT for planning, the uncertainty related to the proton-stopping-power conversion of the Hounsfield units plays an essential role. Currently, algorithms using single energy CT photon to proton stopping power calculations implement a 3%–3.5% uncertainty for each centimeter (cm) of beam path length ( 15 ). However, for planning purposes, one technical advance that undoubtedly may improve proton treatment for NSCLC is the employment of dual-energy CT (DECT) or other techniques that reduce range uncertainty for treatment delivery ( 35 , 36 ).…”