Four‐dimensional dose distributions of step‐and‐shoot IMRT delivered with real‐time tumor tracking for patients with irregular breathing: Constant dose rate vs dose rate regulation

Abstract

Purpose: Dose-rate-regulated tracking (DRRT) is a tumor tracking strategy that programs the MLC to track the tumor under regular breathing and adapts to breathing irregularities during delivery using dose rate regulation. Constant-dose-rate tracking (CDRT) is a strategy that dynamically repositions the beam to account for intrafractional 3D target motion according to real-time information of target location obtained from an independent position monitoring system. The purpose of this study is to illustrate the differences in the effectiveness and delivery accuracy between these two tracking methods in the presence of breathing irregularities. Methods:Step-and-shoot IMRT plans optimized at a reference phase were extended to remaining phases to generate 10-phased 4D-IMRT plans using segment aperture morphing (SAM) algorithm, where both tumor displacement and deformation were considered. A SAM-based 4D plan has been demonstrated to provide better plan quality than plans not considering target deformation. However, delivering such a plan requires preprogramming of the MLC aperture sequence. Deliveries of the 4D plans using DRRT and CDRT tracking approaches were simulated assuming the breathing period is either shorter or longer than the planning day, for 4 IMRT cases: two lung and two pancreatic cases with maximum GTV centroid motion greater than 1 cm were selected. In DRRT, dose rate was regulated to speed up or slow down delivery as needed such that each planned segment is delivered at the planned breathing phase. In CDRT, MLC is separately controlled to follow the tumor motion, but dose rate was kept constant. In addition to breathing period change, effect of breathing amplitude variation on target and critical tissue dose distribution is also evaluated. Results: Delivery of preprogrammed 4D plans by the CDRT method resulted in an average of 5% increase in target dose and noticeable increase in organs at risk (OAR) dose when patient breathing is either 10% faster or slower than the planning day. In contrast, DRRT method showed less than 1% reduction in target dose and no noticeable change in OAR dose under the same breathing period irregularities. When ±20% variation of target motion amplitude was present as breathing irregularity, the two delivery methods show compatible plan quality if the dose distribution of CDRT delivery is renormalized. Conclusions: Delivery of 4D-IMRT treatment plans, stemmed from 3D step-and-shoot IMRT and preprogrammed using SAM algorithm, is simulated for two dynamic MLC-based real-time tumor tracking strategies: with and without dose-rate regulation. Comparison of cumulative dose distribution indicates that the preprogrammed 4D plan is more accurately and efficiently conformed using the DRRT strategy, as it compensates the interplay between patient breathing irregularity and tracking delivery without compromising the segment-weight modulation.

Planning 4D intensity-modulated arc therapy for tumor tracking with a multileaf collimator

“…Although we only tested on a limited number of cases, we expect that this conclusion will hold true for most cases where the target is subject to respiratory-induce motion. Our study, as a continuation of the previous works (Yang et al 2012, Gui et al 2010) describing 4D planning for fixed-field IMRT, demonstrates the efficacy of the DAD method in 4D planning, and verifies the feasibility of this method in 4D-IMAT planning.…”

“…A 4D-IMAT plan is generated by transforming the segments of a 3D plan optimized on a reference-phase image set of 4D CT. A direct aperture deformation (DAD) method is used for segment transformation. The DAD method, in which both rigid and deformable organ motions are considered, was originally created to compensate for interfractional variations for online replanning (Feng et al 2006, Ahunbay et al 2008), and was later successfully used for 4D planning of fixed-field IMRT (Gui et al 2010, Yang et al 2012). …”

An experimentally validated couch and MLC tracking simulator used to investigate hybrid couch‐MLC tracking

“…Yi et al . and Yang et al . proposed to use a 4D treatment plan and dynamically adjust the dose rate during treatment delivery to synchronize the planned target motion with the real‐time observed.…”

Evaluation of Dose Uncertainty to the Target Associated With Real-Time Tracking Intensity-Modulated Radiation Therapy Using the CyberKnife Synchrony System

“…Therefore, 4D planning or other additional analysis may be needed. 25 The maximum and minimum dose values are particularly sensitive to single voxel hot/cold spots. Further investigation including a dose index about organs at risk using a 4D planning software are warranted in the future.…”

“…It is necessary to ensure respiratory stability, including the baseline shift and phase control, to guarantee the correlation between them during irradiation. [24][25][26] Malinowski et al 27 reported that both tumor motion and the relationship between tumor and respiratory surrogate displacements change in most treatment fractions with patient in-room time of 30 minutes. In our facility, we have used an audio guidance system and continuous positive airway pressure to stabilize the correlation mentioned previously as well as doing IMRT planning using 1 type of collimator so that the irradiation time is within 30 minutes.…”