Impact of plan parameters on the dosimetric accuracy of volumetric modulated arc therapy

Currently dynamic conformal arcs (DCA) and volumetric modulated arc therapy (VMAT) are two popular planning techniques to treat lung stereotactic body radiation therapy (SBRT) patients. Of the two, DCA has advantages in terms of multi‐leaf collimator (MLC) motion, positioning error, and delivery efficiency. However, VMAT is often the choice when critical organ sparing becomes important. We developed a hybrid strategy to incorporate DCA component into VMAT planning, results were compared with DCA and VMAT plans. Four planning techniques were retrospectively simulated for 10 lung SBRT patients: DCA, Hybrid‐DCA (2/3 of the doses from DCA beams), Hybrid‐VMAT (2/3 of the doses from VMAT beams) and VMAT. Plan complexity was accessed by modulation complexity score (MCS). Conformity index (CI) for the planning target volume (PTV), V 20 and V 5 for the lung, V 30 for the chestwall, and maximum dose to all other critical organs were calculated. Plans were compared with regard to these metrics and measured agreement between the planned and delivered doses. DCA technique did not result in acceptable plan quality due to target location for five patients. Hybrid‐DCA produced one unacceptable plan, and Hybrid‐VMAT and VMAT produced no unacceptable plans. The CI improved with increasing VMAT usage, as did the dose sparing to critical structures. Compared to the VMAT technique, a total MU reduction of 14%, 25% and 37% were found for Hybrid‐VMAT, Hybrid‐DCA and DCA techniques for 54 Gy patient group, and 9%, 23% and 34% for 50 Gy patient group, suggesting improvement in delivery efficiency with increasing DCA usage. No significant variations of plan complexity were observed between Hybrid‐DCA and Hybrid‐VMAT (P = 0.46 from Mann–Whitney U‐test), but significant differences were found among DCA, Hybrid and VMAT (P < 0.05). Better agreements between the planned and delivered doses were found with more DCA contributions. By adding DCA components to VMAT planning, hybrid technique offers comparable dosimetry to full VMAT, while increasing delivery efficiency and minimizing MLC complexity.

Section: Methodsmentioning

confidence: 99%

“…The MSC for VMAT plans was originally defined by McNiven for step‐and‐shoot IMRT plans,14 and then modified by Masi in order to apply for VMAT plans 15. The LSV is defined based on the difference in position for adjacent MLC leaves in the same bank for each control point (CP).…”

Section: Definition Of Modulation Metrics (Mcs)mentioning

confidence: 99%

A hybrid planning strategy for stereotactic body radiation therapy of early stage non‐small‐cell lung cancer

Liu

Sintay

Pearman

et al. 2018

“…The modulation complexity, described by the Modulation Complexity Score (MCS), 23 , 24 for IMRT and VMAT:

M C S_{b e a m} = \sum_{i = 1}^{Z - 1} (\frac{L S V_{C P_{i} +} L S V_{C P_{i + 1}}}{2}, \frac{A A V_{C P_{i} +} A A V_{C P_{i + 1}}}{2}, \frac{M U_{i, i + 1}}{M U_{b e a m}})

with the Leaf Sequence Variability (LSV) per control point

C P_{i}

, for a total number Z of control points:

L S V_{C P} = {[\frac{\sum_{n}^{Z - 1} (p o s_{max} - | p o s_{n} - p o s_{n + 1} |)}{(N - 1) \times p o s_{max}}]}_{l e f t} \cdot {[\frac{\sum_{n}^{Z - 1} (p o s_{max} - | p o s_{n} - p o s_{n + 1} |)}{(N - 1) \times p o s_{max}}]}_{r i g h t}

where

p o s_{n}

is the position of the left/right leaf n and

p o s_{m a x}

is the maximal position variability of the left/right leaf bank at control point

C P_{i}

, for a total number N of open leaves.The Aperture Area Variability (AAV) per control point is given by:

A A V_{C P} = true \sum_{n = 1}

…”

Section: Methodsmentioning

confidence: 99%

EPID based in vivo dosimetry system: clinical experience and results

Celi

Costa

Wessels

et al. 2016

Mandatory in several countries, in vivo dosimetry has been recognized as one of the next milestones in radiation oncology. Our department has implemented clinically an EPID based in vivo dosimetry system, EPIgray, by DOSISOFT S.A., since 2006. An analysis of the measurements per linac and energy over a two‐year period was performed, which included a more detailed examination per technique and treatment site over a six‐month period. A comparison of the treatment planning system doses and the doses estimated by EPIgray shows a mean of the differences of 1.9% (±5.2%) for the two‐year period. The 3D conformal treatment plans had a mean dose difference of 2.0% (±4.9%), while for intensity‐modulated radiotherapy and volumetric‐modulated arc therapy treatments the mean dose difference was −3.0 (±5.3%) and −2.5 (±5.2%), respectively. In addition, root cause analyses were conducted on the in vivo dosimetry measurements of two breast cancer treatment techniques, as well as prostate treatments with intensity‐modulated radiotherapy and volumetric‐modulated arc therapy. During the breast study, the dose differences of breast treatments in supine position were correlated to patient setup and EPID positioning errors. Based on these observations, an automatic image shift correction algorithm is developed by DOSIsoft S.A. The prostate study revealed that beams and arcs with out‐of‐tolerance in vivo dosimetry results tend to have more complex modulation and a lower exposure of the points of interest. The statistical studies indicate that in vivo dosimetry with EPIgray has been successfully implemented for classical and complex techniques in clinical routine at our institution. The additional breast and prostate studies exhibit the prospects of EPIgray as an easy supplementary quality assurance tool. The validation, the automatization, and the reduction of false‐positive results represent an important step toward adaptive radiotherapy with EPIgray.PACS number(s): 87.53.Bn, 87.55.Qr, 87.56.Fc, 87.57.uq

“…PPS depends on the modulation complexity score (MCS),39 leaf travel index (LTI),39 and the maximal MLC leaf speed of a 4D DCAT plan with collimator angle at

θ_{c o l l}

and couch angle at

θ_{c h}

(eq. (6)).…”

Section: Methodsmentioning

confidence: 99%

“…MCS ranges from 0 to 1, and it approaches 0 for increasing degree of treatment plan complexity 39, 40, 41. LTI ranges from 0 to 1, and it approaches 0 for increasing total MLC leaf travel distance 39.…”

Section: Methodsmentioning

confidence: 99%

Effects of collimator angle, couch angle, and starting phase on motion‐tracking dynamic conformal arc therapy (4D DCAT)

Yao

Podgorsak

et al. 2017

PurposeThe aim of this study was to find an optimized configuration of collimator angle, couch angle, and starting tracking phase to improve the delivery performance in terms of MLC position errors, maximal MLC leaf speed, and total beam‐on time of DCAT plans with motion tracking (4D DCAT).Method and materialsNontracking conformal arc plans were first created based on a single phase (maximal exhalation phase) of a respiratory motion phantom with a spherical target. An ideal model was used to simulate the target motion in superior‐inferior (SI), anterior‐posterior (AP), and left‐right (LR) dimensions. The motion was decomposed to the MLC leaf position coordinates for motion compensation and generating 4D DCAT plans. The plans were studied with collimator angle ranged from 0° to 90°; couch angle ranged from 350°(−10°) to 10°; and starting tracking phases at maximal inhalation (θ=π/2) and exhalation (θ=0) phases. Plan performance score (PPS) evaluates the plan complexity including the variability in MLC leaf positions, degree of irregularity in field shape and area. PPS ranges from 0 to 1, where low PPS indicates a plan with high complexity. The 4D DCAT plans with the maximal and the minimal PPS were selected and delivered on a Varian TrueBeam linear accelerator. Gafchromic‐EBT3 dosimetry films were used to measure the dose delivered to the target in the phantom. Gamma analysis for film measurements with 90% passing rate threshold using 3%/3 mm criteria and trajectory log files were analyzed for plan delivery accuracy evaluation.ResultsThe maximal PPS of all the plans was 0.554, achieved with collimator angle at 87°, couch angle at 350°, and starting phase at maximal inhalation (θ=π/2). The maximal MLC leaf speed, MLC leaf errors, total leaf travel distance, and beam‐on time were 20 mm/s, 0.39 ± 0.16 mm, 1385 cm, and 157 s, respectively. The starting phase, whether at maximal inhalation or exhalation had a relatively small contribution to PPS (0.01 ± 0.05).ConclusionsBy selecting collimator angle, couch angle, and starting tracking phase, 4D DCAT plans with the maximal PPS demonstrated less MLC leaf position errors, lower maximal MLC leaf speed, and shorter beam‐on time which improved the performance of 4D motion‐tracking DCAT delivery.