Evaluation and mitigation of the interplay effects of intensity modulated proton therapy for lung cancer in a clinical setting

Abstract

Purpose The primary aim of this study was to evaluate the impact of interplay effects for intensity-modulated proton therapy (IMPT) plans for lung cancer in the clinical setting. The secondary aim was to explore the technique of iso-layered re-scanning for mitigating these interplay effects. Methods and Materials Single-fraction 4D dynamic dose without considering re-scanning (1FX dynamic dose) was used as a metric to determine the magnitude of dosimetric degradation caused by 4D interplay effects. The 1FX dynamic dose was calculated by simulating the machine delivery processes of proton spot scanning on moving patient described by 4D computed tomography (4DCT) during the IMPT delivery. The dose contributed from an individual spot was fully calculated on the respiratory phase corresponding to the life span of that spot, and the final dose was accumulated to a reference CT phase by using deformable image registration. The 1FX dynamic dose was compared with the 4D composite dose. Seven patients with various tumor volumes and motions were selected. Results The CTV prescription coverage for the 7 patients were 95.04%, 95.38%, 95.39%, 95.24%, 95.65%, 95.90%, and 95.53%, calculated with use of the 4D composite dose, and were 89.30%, 94.70%, 85.47%, 94.09%, 79.69%, 91.20%, and 94.19% with use of the 1FX dynamic dose. For the 7 patients, the CTV coverage, calculated by using single-fraction dynamic dose, were 95.52%, 95.32%, 96.36%, 95.28%, 94.32%, 95.53%, and 95.78%, using maximum MU limit value of 0.005. In other words, by increasing the number of delivered spots in each fraction, the degradation of CTV coverage improved up to 14.6%. Conclusions Single-fraction 4D dynamic dose without re-scanning was validated as a surrogate to evaluate the interplay effects for IMPT for lung cancer in the clinical setting. The interplay effects can be potentially mitigated by increasing the number of iso-layered re-scanning in each fraction delivery.

Reducing Dose Uncertainty for Spot-Scanning Proton Beam Therapy of Moving Tumors by Optimizing the Spot Delivery Sequence

“…All measurements were compared with the 4D composite dose (4D dose) (11, 21, 22), which is the averaged sum of the doses calculated on all N individual phases of the breathing cycle using the planned fluence, with the corresponding motion pattern. Assuming 10 phases in each respiratory cycle, by definition the 4D dose can be determined by averaging 10 stationary measurements at each of the 10 phases for every motion condition, or, alternatively, by making 1 stationary measurement and then shifting and averaging according to the breathing trace.…”

“…The sum of all 30 fractional doses was calculated as accumulated dose. The absolute dose error between the 4D dose and each individual delivery and the absolute dose error between the 4D dose and the accumulated dose were calculated and compared with layer-by-layer rescanning results as previously reported (21). …”

Advances in radiotherapy techniques and delivery for non-small cell lung cancer: benefits of intensity-modulated radiation therapy, proton therapy, and stereotactic body radiation therapy

“…Several methods are available to mitigate these uncertainties: robust beam angle selection utilizing waterequivalent thickness optimization, 4DCT-based robust optimization, layer or volumetric "repainting" delivery, spotsequence delivery optimization, increased fractionation, spot-size modulation, mini-ridge filter utilization, and respiratory gating or breath-hold-based treatment, to name a few (105,(110)(111)(112)(113)(114). Unfortunately, most of these methods require additional treatment planning software and devices or increase time and logistical burden on planning, quality assurance, and treatment delivery.…”

Novel Hybrid Scattering- and Scanning-Beam Proton Therapy Approach

“…We also observed a reduced dose in the lung case. For both lung cancer and esophageal cancer, the interplay effect is the major motion uncertainty of the scanning beam technique [26,27]. By combining the scanning beam with the scattering beam, this uncertainty will be reduced, as was observed with the photon therapy hybrid technique.…”