![]() We performed the simulations using our 4D interplay simulation program with 11 patients sampled from 112 patients with stage III nonsmall cell lung cancer previously treated at MDACC. In the present study, we assessed dynamic dose distributions in the presence of interplay effect, taking into account dose fractionation, respiration irregularity, and isolayer repainting with clinically adopted treatment planning procedures and beam-delivery system details at PTC-H. 12 Applying what this theoretical derivation suggests to practical clinical settings requires further investigations. It was theoretically shown using a generalized model that the dynamic dose converges to the 4D composite dose in multiple deliveries through fractionation and rescanning despite interplay. According to previous interplay studies, however, it is necessary that this method be reassessed for evaluating dynamic dose distributions under the influence of interplay effect. 9–11 For example, dose calculation may be repeated in each respiratory phase in a 4DCT set, and the doses in different phases are transformed to and averaged on the reference phase to obtain a 4D composite dose. Use of composite doses or other methods based on four-dimensional (4D) CT images and image registration has been proposed for evaluating the actual delivered dose in the presence of respiratory motion. 6,7 For the spot scanning treatment at the University of Texas MD Anderson Cancer Center (MDACC) Proton Therapy Center, Houston, Texas (PTC-H), we adopted an isolayer repainting style (ILR) in which all planned positions are first visited once by a deliverable spot, then repeated as needed until all planned MUs are delivered before the next energy layer can be initiated. ![]() 8 Also, studies focused on proton scanning beams demonstrated that highly heterogeneous dose distributions are likely, but may be significantly reduced by fractionation and repainting. In reality, however, interplay effect may be largely diminished by fractionation and repainting. They showed that under extreme situations, parts of tumors could be completely missed by irradiation in the presence of respiratory motion. 8 statistically studied interplay effect with dynamic photon beams. This interplay between periodic anatomical changes and beam scanning adds complications in predicting the effects of organ motion on the delivered dose distributions. Consequently, the actual delivered dose may seriously deviate from the nominal dose distribution calculated on the same free-breathing or average CT images with which the treatment plan is designed. ![]() Therefore, a scanning spot could be delivered to a location not as planned because the spot “sees” an anatomy that may be quite different from that in the planning computed tomography (CT) images. In patients with lung cancer, intrafractional motion of tumors and organs is highly correlated with respiration, and the time scales of this motion are at the same level as the spot-scanning processes. 3–5 A dominant concern that hinders the application of IMPT to lung cancers is potential local regions of under- and overdoses resulting from the effect of interplay between proton spot scanning and intrafractional respiratory motion in dose delivery.Ī scanning proton beam covers a three-dimensional target volume laterally by sequentially delivering a series of narrow, nearly monoenergetic beams called scanning spots, and longitudinally layer by layer via altering proton energy. 1,2 In treatment of lung cancer, IMPT could further increase the dose delivered to the tumor while minimizing that delivered to surrounding healthy organs, such as the lung, esophagus, and spinal cord. ![]() Intensity modulated proton therapy (IMPT), typically based on scanning proton beams, provides greater control over dose distributions than the techniques based on collimated broad beams (e.g., the double scattering proton therapy).
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