Dosimetric Impact of Interplay Effect on RapidArc Lung Stereotactic Treatment Delivery

doi:10.1016/j.ijrobp.2010.02.059

International Journal of Radiation OncologyBiologyPhysics

Volume 79, Issue 1, 1 January 2011, Pages 305-311

https://doi.org/10.1016/j.ijrobp.2010.02.059 Get rights and content

Purpose

Volumetric modulated arc therapy (RapidArc; Varian Medical Systems, Palo Alto, CA) allows fast delivery of stereotactic radiotherapy for Stage I lung tumors. We investigated discrepancies between the calculated and delivered dose distributions, as well as the dosimetric impact of leaf interplay with breathing-induced tumor motion.

Methods and Materials

In 20 consecutive patients with Stage I lung cancer who completed RapidArc delivery, 15 had tumor motion exceeding 5 mm on four-dimensional computed tomography scan. Static and dynamic measurements were performed with Gafchromic EBT film (International Specialty Products Inc., Wayne, NJ) in a Quasar motion phantom (Modus Medical Devices, London, Ontario, Canada). Static measurements were compared with calculated dose distributions, and dynamic measurements were compared with the convolution of static measurements with sinusoidal motion patterns. Besides clinical treatment plans, additional cases were optimized to create excessive multileaf collimator modulation and delivered on the phantom with peak-to-peak motions of up to 25 mm. γ Analysis with a 3% dose difference and 2- or 1-mm distance to agreement was used to evaluate the accuracy of delivery and the dosimetric impact of the interplay effect.

Results

In static mode film dosimetry of the two-arc delivery in the phantom showed that, on average, fewer than 3% of measurements had γ greater than 1. Dynamic measurements of clinical plans showed a high degree of agreement with the convolutions: for double-arc plans, 99.5% met the γ criterion. The degree of agreement was 98.5% for the plans with excessive multileaf collimator modulations and 25 mm of motion.

Conclusions

Film dosimetry shows that RapidArc accurately delivers the calculated dose distribution and that interplay between leaves and tumor motion is not significant for single-fraction treatments when RapidArc is delivered with two different arcs.

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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Cited by (94)

Patient specific evaluation of breathing motion induced interplay effects
2023, Physica Medica
In lung SABR, interplay between target motion and dynamically changing beam parameters can affect the target coverage. To identify the potential need for motion-management techniques, a comprehensive methodology for pre-treatment estimation of interplay effects has been implemented.
In conjunction with an alpha-version of VeriSoft and OCTAVIUS 4D (PTW-Freiburg, Germany), a method is presented to calculate a virtual, motion-simulated 3D dose distribution based on measurement data acquired in a stationary phantom and a subsequent correction with time-dependent target-motion patterns. In-house software has been developed to create user-defined motion patterns based on either simplistic or real patient-breathing patterns including the definition of the exact beam starting phase. The approach was validated by programmed couch and phantom motion during beam delivery.
Five different breathing traces with extremely altered beam-on phases (0 % and 50 % respiratory phase) and a superior-inferior motion altitude of 25 mm were used to probe the influence of interplay effects for 14 lung SABR plans. Gamma analysis (2 %/2mm) was used for quantification.
Validation measurements resulted in >98 % pass rates. Regarding the interplay effect evaluation, gamma pass rates of <92 % were observed for sinusoidal breathing patterns with <25 number of breaths per delivery time (NBs) and realistic patterns with <18 NBs.
The potential influence of interplay effects on the target coverage is highly dependent on the patient’s breathing behaviour. The presented moving-platform-free approach can be used for verification of ITV-based treatment plans to identify whether the clinical goals are achievable without explicit use of a respiratory management technique.
Dosimetric robustness of lung tumor photon radiotherapy evaluated from multiple event CT imaging
2022, Physica Medica
Intrafractional respiratory motion is a concern for lung tumor radiotherapy but full evaluation of its impact is hampered by the lack of images representing the true motion. This study presents a novel evaluation using free-breathing images acquired over realistic treatment times to study the dosimetric impact of respiratory motion in photon radiotherapy.
Cine-CT images of 14 patients with lung cancer acquired during eight minutes of free-breathing at three occasions were used to simulate dose tracking of four different planning methods. These methods aimed to deliver 54 Gy in three fractions to $D_{50 %}$ of the target and were denoted as robust 4D (RB4), homogeneous fluence to the ITV (FLU) and an isodose prescription to the ITV with a high central dose (ISD), concurrently renormalized (IRN). Differences in dose coverage probability and homogeneity between the methods were quantified. Correlations between underdosage and attributes regarding the tumor and its motion were investigated.
Despite tumor motion amplitudes being larger than in the 4DCT all but FLU achieved the intended CTV $D_{50 %}$ for the cohort average. For all methods but IRN at least 93% of the patients would have received 95% of the intended dose. No differences in $D_{50 %}$ were found between RB4 and ISD nor IRN. However, RB4 led to better homogeneity.
Tumor motion in free-breathing not covered by the 4DCT had a small impact on dose. The RB4 is recommended for planning of free-breathing treatments. No factor was found that consistently correlated dose degradation with patient or motion attributes.
Statistical breathing curve sampling to quantify interplay effects of moving lung tumors in a 4D Monte Carlo dose calculation framework
2022, Physica Medica
Citation 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).
The interplay between respiratory tumor motion and dose application by intensity modulated radiotherapy (IMRT) techniques can potentially lead to undesirable and non-intuitive deviations from the planned dose distribution. We developed a 4D Monte Carlo (MC) dose recalculation framework featuring statistical breathing curve sampling, to precisely simulate the dose distribution for moving target volumes aiming at a comprehensive assessment of interplay effects.
We implemented a dose accumulation tool that enables dose recalculations of arbitrary breathing curves including the actual breathing curve of the patient. This MC dose recalculation framework is based on linac log-files, facilitating a high temporal resolution up to 0.1 s. By statistical analysis of 128 different breathing curves, interplay susceptibility of different treatment parameters was evaluated for an exemplary patient case. To facilitate prospective clinical application in the treatment planning stage, in which patient breathing curves or linac log-files are not available, we derived a log-file free version with breathing curves generated by a random walk approach. Interplay was quantified by standard deviations $σ$ in D_5%, D_50% and D_95%.
Interplay induced dose deviations for single fractions were observed and evaluated for IMRT and volumetric arc therapy ( $σ_{D 95 %}$ up to 1.3 %) showing a decrease with higher fraction doses and an increase with higher MU rates. Interplay effects for conformal treatment techniques were negligible ( $σ < 0.1 %)$ . The log-file free version and the random walk generated breathing curves yielded similar results (deviations in $σ <$ 0.1 %) and can be used as substitutes for interplay assessment.
It is feasible to combine statistically sampled breathing curves with MC dose calculations. The universality of the presented framework allows comprehensive assessment of interplay effects in retrospective and prospective clinically relevant scenarios.
Estimation of delivered dose to lung tumours considering setup uncertainties and breathing motion in a cohort of patients treated with stereotactic body radiation therapy
2021, Physica Medica
Dose-response relationships for local control of lung tumours treated with stereotactic body radiotherapy (SBRT) have proved ambiguous, however, these have been based on the prescribed or planned dose. Delivered dose to the target may be a better predictor for local control. In this study, the probability of the delivered minimum dose to the clinical target volume (CTV) in relation to the prescribed dose was estimated for a cohort of patients, considering geometrical uncertainties.
Delivered doses were retrospectively simulated for 50 patients treated with SBRT for lung tumours, comparing two image-guidance techniques: pre-treatment verification computed tomography (IG1) and online cone-beam computed tomography (IG2). The prescribed dose was typically to the 67% isodose line of the treatment plan. Simulations used in-house software that shifted the static planned dose according to a breathing motion and sampled setup/matching errors. Each treatment was repeatedly simulated, generating a multiplicity of dose-volume histograms (DVH). From these, tumour-specific and population-averaged statistics were derived.
For IG1, the probability that the minimum CTV dose (D_98%) exceeded 100% of the prescribed dose was 90%. With IG2, this probability increased to 99%.
Doses below the prescribed dose were delivered to a considerably larger part of the population prior to the introduction of online soft-tissue image-guidance. However, there is no clear evidence that this impacts local control, when compared to previous published data.
A study of the interplay effect in radiation therapy using a Monte-Carlo model
2021, Physica Medica
Citation 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.
In modulated radiotherapy, breathing motion can lead to Interplay (IE) and Blurring (BE) effects that can modify the delivered dose. The aim of this work is to present the implementation, the validation and the use of an open-source Monte-Carlo (MC) model that computes the delivered dose including these motion effects.
The MC model of the Varian TrueBeam was implemented using GATE. The dose delivered by different modulated plans is computed for several breathing patterns. A validation of these MC predictions is achieved by a comparison with measurements performed using a dedicated programmable motion platform, carrying a quality assurance phantom. A specific methodology was used to separate the IE and the BE. The influence of different motion parameters (period, amplitude, shape) and plan parameters (volume margin, dose per fraction) was also analyzed.
The MC model was validated against measurement performed with motion with a mean 3D global gamma index pass rate of 97.5% (3%/3 mm). A significant correlation is found between the IE and the period and the antero-posterior amplitude of the motion but not between the IE and the CTV margin or the shape of motion. The results showed that the IE increases D2% and decreases the D98% of CTV with mean values of +6.9% and −3.3% respectively.
We validated the feasibility to assess the IE using a MC model. We found that the most important parameter is the number of breathing cycles that must be greater than 20 for one arc to limit the IE.
Results of a multicentre dosimetry audit using a respiratory phantom within the EORTC LungTech trial
2019, Radiotherapy and Oncology
Citation 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.
The EORTC 22113-08113 LungTech trial assesses the safety and efficacy of SBRT for centrally located NSCLC. To insure protocol compliance an extensive RTQA procedure was implemented.
Twelve centres were audited using a CIRS008A phantom. The phantom was scanned using target inserts of 7.5 mm and 12.5 mm radius in static condition. For the 7.5 mm insert a 4DCT was acquired while moving according to a cos⁶ function. Treatment plans were measured using film and an ionization chamber. Wilcoxon’s signed-rank tests were performed to compare the three plans across institutions. A Spearman correlation was calculated to evaluate the influence of factors such as PTV, slice thickness and total number of monitor units on the dosimetric results.
The reference output dose median [min, max] variation was 0.5% [−1.1, +1.5]. The median deviations between chamber doses and point-planned doses were 1.8% [−0.1; 6.7] for the 7.5 mm and 1.1% [−2.8; 5.0] for the 12.5 mm sphere in static situation and 3.2% [−3.2; 15.7] for the dynamic situation. Film gamma median pass rates were 92.0% [68.0, 99.0] for 7.5 mm static, 96.2% [73.0, 99.0] for 12.5 mm static and 71.0% [40.0, 99.0] for 7.5 mm dynamic. Wilcoxon’s signed-rank tests showed that the dynamic irradiations resulted in significantly lower gamma pass rates compared to the 12.5 mm static plan (p = 0.001). The total number of MUs per plan was correlated to both film and IC results.
An end-to-end audit was successfully performed, revealing important variations between institutions especially in dynamic irradiations. This shows the importance of dosimetry audits and the potentials for further technique and methodology improvements.

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: Conflict of interest: The VU University Medical Center is in research collaboration with Varian Medical Systems.

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Physics ContributionDosimetric Impact of Interplay Effect on RapidArc Lung Stereotactic Treatment Delivery

Purpose

Methods and Materials

Results

Conclusions

Introduction

Section snippets

Methods and Materials

Results

Discussion

Conclusions

Radiother Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Radiother Oncol

Radiother Oncol

Semin Radiat Oncol

Radiother Oncol

Int J Radiat Oncol Biol Phys

Physics Contribution
Dosimetric Impact of Interplay Effect on RapidArc Lung Stereotactic Treatment Delivery