On the interplay effects with proton scanning beams in stage III lung cancer

Abstract

Purpose:To assess the dosimetric impact of interplay between spot-scanning proton beam and respiratory motion in intensity-modulated proton therapy (IMPT) for stage III lung cancer. Methods: Eleven patients were sampled from 112 patients with stage III nonsmall cell lung cancer to well represent the distribution of 112 patients in terms of target size and motion. Clinical target volumes (CTVs) and planning target volumes (PTVs) were defined according to the authors' clinical protocol. Uniform and realistic breathing patterns were considered along with regular-and hypofractionation scenarios. The dose contributed by a spot was fully calculated on the computed tomography (CT) images corresponding to the respiratory phase that the spot is delivered, and then accumulated to the reference phase of the 4DCT to generate the dynamic dose that provides an estimation of what might be delivered under the influence of interplay effect. The dynamic dose distributions at different numbers of fractions were compared with the corresponding 4D composite dose which is the equally weighted average of the doses, respectively, computed on respiratory phases of a 4DCT image set. Results: Under regular fractionation, the average and maximum differences in CTV coverage between the 4D composite and dynamic doses after delivery of all 35 fractions were no more than 0.2% and 0.9%, respectively. The maximum differences between the two dose distributions for the maximum dose to the spinal cord, heart V40, esophagus V55, and lung V20 were 1.2 Gy, 0.1%, 0.8%, and 0.4%, respectively. Although relatively large differences in single fraction, correlated with small CTVs relative to motions, were observed, the authors' biological response calculations suggested that this interfractional dose variation may have limited biological impact. Assuming a hypofractionation scenario, the differences between the 4D composite and dynamic doses were well confined even for single fraction. Conclusions: Despite the presence of interplay effect, the delivered dose may be reliably estimated using the 4D composite dose. In general the interplay effect may not be a primary concern with IMPT for lung cancers for the authors' institution. The described interplay analysis tool may be used to provide additional confidence in treatment delivery.

Beam‐specific planning target volumes incorporating 4D CT for pencil beam scanning proton therapy of thoracic tumors

“…As multiple beam paintings of four are clinically achievable, we believe that the dosimetric advantage of PBS treatment in this patient will be able to be maintained despite the interplay phenomenon. These observations are consistent with that reported by many researchers 4 , 18 , 19 , 20 , 21 . Because the breathing period and beam duration can vary from patient to patient, detailed assessment of the interplay effect for multiple patients and multiple motion mitigation strategies is the subject of current investigation within our group.…”

“… 18 , 19 , 20 , 21 In this study we demonstrate such an interplay phenomenon for a representative thoracic patient.…”

“…The method we used to evaluate beam interplay is similar to Zou et al (33) for conventional lung SBRT applications and other researchers for PBS applications 11 , 12 , 18 , 19 , 21 . A representative patient is shown in Fig.…”

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

“…These uncertainties are highlighted in lung cancers (101)(102)(103)(104). In particular, several studies have demonstrated the heightened sensitivity of IMPT to changes in heterogeneity and motion interplay effects as compared with PS-PT (105)(106)(107)(108)(109).…”

Future of medical physics: Real‐time MRI‐guided proton therapy

“…With pencil beam scanning treatments comes the potential for interplay effects. 31,32,33 These effects, i.e., possible significant hot or cold spots within the treatment volume accumulated over the course of a single fraction, are shown to be mitigated by fractionation, 32 multiple rescanning, 33 and retracking. 34 In the case of MRPT, the real-time MR images at the rate of 4-8 frames per second may in fact be available to assist in the management of interplay effects.…”