Performance parameters of a liquid filled ionization chamber array

PurposeThe aim of this study was to evaluate experimentally the accuracy of the dose calculation algorithm AcurosXB in small field highly modulated Volumetric Modulated Arc Therapy (VMAT).MethodThe 1000SRS detector array inserted in the rotational Octavius 4D phantom (PTW) was used for 3D dose verification of VMAT treatments characterized by small to very small targets. Clinical treatment plans (n = 28) were recalculated on the phantom CT data set in the Eclipse TPS. All measurements were done on a Varian TrueBeamSTx, which can provide the jaw tracking technique (JTT). The effect of disabling the JTT, thereby fixing the jaws at static field size of 3 × 3 cm2 and applying the MLC to shape the smallest apertures, was investigated for static fields between 0.5 × 0.5−3 × 3 cm2 and for seven VMAT patients with small brain metastases. The dose calculation accuracy has been evaluated by comparing the measured and calculated dose outputs and dose distributions. The dosimetric agreement has been presented by a local gamma evaluation criterion of 2%/2 mm.ResultsRegarding the clinical plans, the mean ± SD of the volumetric gamma evaluation scores considering the dose levels for evaluation of 10%, 50%, 80% and 95% are (96.0 ± 6.9)%, (95.2 ± 6.8)%, (86.7 ± 14.8)% and (56.3 ± 42.3)% respectively. For the smallest field VMAT treatments, discrepancies between calculated and measured doses up to 16% are obtained. The difference between the 1000SRS central chamber measurements compared to the calculated dose outputs for static fields 3 × 3, 2 × 2, 1 × 1 and 0.5 × 0.5 cm2 collimated with MLC whereby jaws are fixed at 3 × 3 cm2 and for static fields shaped with the collimator jaws only (MLC retracted), is on average respectively, 0.2%, 0.8%, 6.8%, 5.7% (6 MV) and 0.1%, 1.3%, 11.7%, 21.6% (10 MV). For the seven brain mets patients was found that the smaller the target volumes, the higher the improvement in agreement between measured and calculated doses after disabling the JTT.ConclusionFixing the jaws at 3 × 3 cm2 and using the MLC with high positional accuracy to shape the smallest apertures in contrast to the JTT is currently found to be the most accurate treatment technique.

Section: Performance Parameters Of the Octavius 4d Systemsupporting

confidence: 92%

Section: Methodsmentioning

confidence: 90%

Section: Methodsmentioning

confidence: 99%

See 1 more Smart Citation

Influence of the jaw tracking technique on the dose calculation accuracy of small field VMAT plans

Swinnen

Öllers

Roijen

et al. 2017

“…The White Paper states: “Due to the highly irregular nature and limited resolution of the energy fluence matrix, deviations between calculated and measured dose in the Delta 4 detector positions are more frequent in the IMRT case, … Deviations are primarily localized to regions of the rapid dose changes, indicating distances to agreement less than the pixel size, i. e.

< 6 mm

.” It is clear that when the penumbra of individual segments is not represented with sufficient resolution, the superposition of many segments in a modulated beam would lead to rather inaccurate calculations. This is effectively equivalent to calculating modulated beams with a

6 \times 6 {mm}^{2}

pixel size, while it is well established in the literature that segmented beam calculations require 2.5 mm voxel grid resolution to faithfully reproduce encountered gradients 28 , 29 , 30 . This explains poor dosimetric agreement for VMAT plans found in Table 2 and 4, 5, 6.…”

Section: Discussionmentioning

confidence: 99%

Evaluation of semiempirical VMAT dose reconstruction on a patient dataset based on biplanar diode array measurements

Stambaugh

Opp

Wasserman

et al. 2014

We report the results of a preclinical evaluation of recently introduced commercial tools for 3D patient IMRT/VMAT dose reconstruction, the Delta4 Anatomy calculation algorithm. Based on the same initial measurement, volumetric dose can be reconstructed in two ways. Three‐dimensional dose on the Delta4 phantom can be obtained by renormalizing the planned dose distribution by the measurement values (D4 Interpolation). Alternatively, incident fluence can be approximated from the phantom measurement and used for volumetric dose calculation on an arbitrary (patient) dataset with a pencil beam algorithm (Delta4 PB). The primary basis for comparison was 3D dose obtained by previously validated measurement‐guided planned dose perturbation method (ACPDP), based on the ArcCHECK dosimeter with 3DVH software. For five clinical VMAT plans, D4 Interpolation agreed well with ACPDP on a homogeneous cylindrical phantom according to gamma analysis with local dose‐error normalization. The average agreement rates were 98.2%±1.3% (1 SD), (range 97.0%‐100%) and 92.8%±3.9% (89.5%‐99.2%), for the 3%/3 mm and 2%/2 mm criteria, respectively. On a similar geometric phantom, D4 PB demonstrated substantially lower agreement rates with ACPDP: 88.6%±6.8% (81.2%‐96.1%) and 72.4%±8.4% (62.1%‐81.1%), for 3%/3 mm and 2%/2 mm, respectively. The average agreement rates on the heterogeneous patients' CT datasets are lower yet: 81.2%±8.6% (70.4%‐90.4%) and 64.6%±8.4% (56.5%‐74.7%), respectively, for the same two criteria sets. For both threshold combinations, matched analysis of variance (ANOVA) multiple comparisons showed statistically significant differences in mean agreement rates (p<0.05) for D4 Interpolation versus ACPDP on one hand, and D4 PB versus ACPDP on either cylindrical or patient dataset on the other hand. Based on the favorable D4 Interpolation results for VMAT plans, the resolution of the reconstruction method rather than hardware design is likely to be responsible for D4 PB limitations.PACS number: 87.55Qr

“…The approach should possess high spatial resolution as the lesions could be of the order of 1 cm or less in size. At the same time, the targets could be fairly wide spread, which negates the advantages of dedicated “stereotactic” detector arrays with small detector pitch, that typically have a relatively small active area under the assumption that the lesion would be located at isocenter 15. Moreover, the targets randomly placed in three dimensions naturally call for a 3D verification approach.…”

Section: Introductionmentioning

confidence: 99%

A hybrid volumetric dose verification method for single‐isocenter multiple‐target cranial SRS

Ahmed

Kapatoes

Zhang

et al. 2018

A commercial semi‐empirical volumetric dose verification system (PerFraction [PF], Sun Nuclear Corp.) extracts multi‐leaf collimator positions from the electronic portal imaging device movies collected during a pre‐treatment run, while the rest of the delivered control point information is harvested from the accelerator log files. This combination is used to reconstruct dose on a patient CT dataset with a fast superposition/convolution algorithm. The method was validated for single‐isocenter multi‐target SRS VMAT treatments against absolute radiochromic film measurements in a cylindrical phantom. The targets ranged in size from 0.8 to 3.6 cm and in number from 3 to 10 per plan. A total of 17 films rotated at different angles around the cylinder axis were analyzed. Each of 27 total targets was intercepted by at least one film, and 2–4 different films were analyzed per plan. Film dose was always scaled to the ion chamber measurement in a high‐dose, low‐gradient area deliberately created at the isocenter. The planar dose agreement between PF and film using 3%(Global dose‐difference normalization)/1 mm gamma analysis was on average 99.2 ± 1.1%. The point dose difference in the low‐gradient area in the middle of every target was below 3%, while PF‐reconstructed and film dose centroids for individual targets showed submillimeter agreement when measured on a well aligned accelerator. Volumetrically, all voxels in all plans agreed between PF and the primary treatment planning system at the 3%/1 mm level. With proper understanding of its advantages and shortcomings, the tool can be applied to patient‐specific QA in routine radiosurgical clinical practice.