International Journal of Radiation Oncology*Biology*Physics
Physics ContributionDosimetric Impact of Interplay Effect on RapidArc Lung Stereotactic Treatment Delivery
Introduction
Stereotactic body radiotherapy (SBRT) for Stage I non–small-cell lung cancers has been shown in a meta-analysis to result in superior local control rates compared with conventionally fractionated radiotherapy (1). We have reported local control rates exceeding 90% using 10 to 12 static non-coplanar beams and risk-adapted fractionation schemes (2), and other groups have reported local control rates ranging from 88% to 92% 3, 4. As SBRT is increasingly being evaluated in fitter patients who have a lower likelihood of non–cancer-related death (5), the use of improved techniques that can reduce the risk of late toxicity after SBRT becomes more important.
RapidArc (Varian Medical Systems, Palo Alto, CA) is a volumetric modulated arc therapy technique that delivers the dose in one or more gantry rotations with variable multileaf collimator (MLC) positions, dose rates, and gantry speeds (6). RapidArc can achieve quality of treatment plans that is at least comparable to those generated with conventional intensity-modulated radiotherapy (IMRT) but with a substantial reduction in treatment time and a high precision of dose delivery 7, 8, 9, 10. Moreover, a study in patients with Stage I lung cancer showed that RapidArc SBRT could achieve better target dose conformity than when a conventional 10-field non-coplanar approach was used (11). However, a key concern with IMRT delivery in mobile tumors is the possibility of interplay between tumor motion and MLC motion. Such an interplay effect can cause hot/cold spots of dose within the target volume, and the resulting dose can vary up to 18% in a single fraction (12). Significant dose discrepancies of up to 29% have also been reported from the presence of longitudinal tumor motion during helical TomoTherapy (TomoTherapy Inc., Madison, WI) (13). It has been suggested that the interplay effect may have only a small dosimetric impact if treatments are delivered in 30 fractions, because the observed dose variations then decreased to 1% to 2% (14). However, this finding might not be applicable in RapidArc lung SBRT delivery, which is generally completed in 3 to 8 fractions.
RapidArc treatments commenced at our department in May 2008, and all plans are verified with film or ionization chamber array before the treatment. For SBRT via RapidArc, the quality-assurance (QA) procedures were performed by use of film measurements in different phantoms. The aim of this study was to evaluate the dosimetric accuracy between the delivered and calculated dose distributions for lung treatment, as well as the possible interplay between MLC motion and tumor motion. This interplay effect for RapidArc was investigated by measuring the RapidArc delivered dose distributions in a respiratory motion phantom capable of performing a programmable motion pattern.
Section snippets
Methods and Materials
All data from the first 20 patients treated with RapidArc for Stage I lung cancer were analyzed. All had undergone a four-dimensional (4D) computed tomography (CT) scan as reported previously (15). An internal target volume (ITV) that encompassed all motion was delineated, and a planning target volume (PTV) was created by adding a margin of 3 to 5 mm to the ITV. The amplitude of tumor motion for each patient was assessed by measuring the peak-to-peak tumor position from different phases of the
Results
Table 2 summarizes the results of the γ analysis both for static measurements vs. AAA calculations (n = 20 patients) and for dynamic measurements vs. convolutions based on static measurements (n = 15 patients). The data from the single arcs, CCW and CW, and the sum plans are shown.
For the single-arc analysis, the mean surface with γ greater than 1 was 2.9% (range, 0.0–12.7%). For the sum plan, this was reduced to 2.0% (range, 0.2–7.5%). The maximum dose deviation near the center of the PTV
Discussion
Accurate patient-specific QA for IMRT treatment of lung cancer is more complex than for most other treatment sites. Tissue is largely inhomogeneous, and the influence of organ/tumor motion has to be investigated. Dosimetric verification of RapidArc has been conducted at several institutes for various anatomic sites with good agreement 7, 17. Other studies to assess the accuracy of AAA for lung treatment showed that the deviations between measurement and AAA calculation in lung regions were well
Conclusions
Our findings suggest that the risk of underdosing/overdosing the tumor due to interplay effects is not a significant problem with RapidArc delivery. The interplay effect does not result in unexpected dose variation, and the dose gradient at the edge of the PTV, obtained from dynamic measurement, agreed within 1 mm compared with static-convolved measurement.
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Patient specific evaluation of breathing motion induced interplay effects
2023, Physica MedicaStatistical breathing curve sampling to quantify interplay effects of moving lung tumors in a 4D Monte Carlo dose calculation framework
2022, Physica MedicaCitation Excerpt :The approach is able to calculate the 4D dose for a certain breathing curve but not flexible enough to perform studies on interplay effects. Some studies report that the impact of interplay effects on target and OAR dose is relatively small and averages out over several fractions [13,25]. However, recent studies [26–28] observe dose deviations up to 3 % in case of fast, high-MU, flattening filter free (FFF) VMAT treatments (with up to 2300 MU/min).
A study of the interplay effect in radiation therapy using a Monte-Carlo model
2021, Physica MedicaCitation Excerpt :When averaging these profiles, results were similar to simulations without IE (from 99% to 102% of prescribed dose). This estimate of multi-fraction effect confirms literature findings based on other approaches [6,9–11,13,16,19–21,25]. This shows that the IE is smoothed if the treatment has multiple fractions.
Results of a multicentre dosimetry audit using a respiratory phantom within the EORTC LungTech trial
2019, Radiotherapy and OncologyCitation Excerpt :Another known cause of deviation between planned and delivered dose in dynamic irradiations is the (de)synchronization between MLC motion and target motion also known as interplay effect. Ong et al. [17] evaluated this effect in RapidArc flattened beam SBRT lung treatments and found this to be negligible; < to 3% gamma pass rate (3%/1 mm) for 1 fraction delivered using two arcs. They also found that this deviation diminished when increasing the number of fractions.
Conflict of interest: The VU University Medical Center is in research collaboration with Varian Medical Systems.