ESTRO 36 Abstract Book

S113

ESTRO 36 2017

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OC-0228 DVH criteria for prostate in vivo EPID

dosimetry

R.F.M. Van Oers

, E. Van der Bijl

, I. Olaciregui-Ruiz

, A.

Mans

Netherlands Cancer Institute, Radiation Oncology,

Amsterdam, The Netherlands

Purpose or Objective

In our department

in vivo

EPID dosimetry is used for dose

verification of all treatment plans. The algorithm uses

EPID images acquired behind the patient to reconstruct

the in vivo 3D dose distribution. This is then automatically

compared to the planned dose distribution and alerts are

generated when deviations are detected.

These alerts are based on γ-analysis. Gamma values

combine dose-difference and distance-to-agreement in a

single metric, but this metric contains no information on

the clinical relevance of deviations. Furthermore, γ-

analysis can be insensitive to systematic under- or

overdoses in plans with inhomogeneous dose distributions.

Dose-volume histograms (DVHs) are widely used for

evaluation of treatment plans, and readily understood by

clinicians. Moreover, differences in DVH parameters can

be linked more straightforwardly to clinical relevance.

This makes DVH-based criteria an attractive alternative to

γ-criteria for in vivo EPID dosimetry alerts.

In this study we investigated the correlation between γ-

and DVH-parameters of the PTV for prostate treatments,

and compared the alert rates for different criteria in order

to propose a suitable set of DVH-criteria for clinical

implementation.

Material and Methods

In vivo 3D dose distributions were reconstructed for the

first three fractions of 95 prostate VMAT treatments, and

then averaged for the evaluation of each treatment. The

γ-analysis was done with global 3%/3mm settings. DVHs

were obtained for the PTV (prostate + seminal vesicles).

Calculated γ-parameters were mean γ, the near-maximum

γ (γ1%), and the γ-passrate (γ%<1); our current criteria

also include the isocenter dose difference (ΔDisoc).

Calculated DVH-parameters were the difference in near-

maximum dose (ΔD2), median dose (ΔD50), and near-

minimum dose (ΔD98). We obtained alert rates for

different sets of criteria on these DVH-parameters. These

were compared to alert rates for the current γ-based

criteria. There are two alert levels, higher-priority “error”

and lower-priority “warning”.

Results

The strongest correlation was found between γ-mean and

|ΔD50|, Pearson’s r=0.95. All other γ- and DVH

parameters were also strongly correlated, with r values

around 0.85.

Figure 1:

Relation between γ-mean and ΔD50 for the

analyzed prostate VMAT plans. The indicated alerts are

generated by the “error level” sets of γ- and DVH-criteria.

Table 1 shows alert rates for different sets of γ- and DVH-

criteria. The two highlighted sets of γ-criteria are the ones

currently used in our clinic to generate alerts for prostate

treatments, juxtaposed with sets of DVH-criteria of similar

alert rate.

Table 1:

Alert rates (% of treatments) for different sets

of γ- and DVH-criteria. The top set of γ-criteria

corresponds to “warning level” alerts, the bottom set

corresponds to “error level” alerts.

Conclusion

A strong correlation was found between γ- and DVH-

parameters of the PTV; a set of DVH-criteria that performs

comparably to the current γ-criteria can easily be chosen.

OC-0229 EPID dose response in the MR-Linac with and

without presence of a magnetic field

I. Torres Xirau

, I. Olaciregui-Ruiz

, B. J. Mijnheer

, U. A.

van der Heide

, A. Mans

Netherlands Cancer Institute Antoni van Leeuwenhoek

Hospital, Department of Radiation Oncology,

Amsterdam, The Netherlands

Purpose or Objective

Image-guided radiotherapy systems are being investigated

and clinically implemented aiming for online and real-time

adaptation of the treatment plan. The use of Electronic

Portal Imaging Devices (EPIDs) for independent in vivo

dose verification in the Elekta MR-Linac is being

developed. One of the challenges for MR-Linac portal

dosimetry is the presence of a small magnetic field at the

EPID level. In the presence of a magnetic field, the

secondary electrons that actually deposit the dose in the

scintillator of the EPID will be affected by the Lorentz

force possibly leading to a B-field induced dose

redistribution. The aim of this study was to analyze and

quantify the effects of the B-field on the EPID images

acquired on the Elekta MR-Linac.

Material and Methods

The Elekta/Philips MR-Linac combines a 1.5T magnetic

resonance imaging scanner with a linear accelerator and

is equipped with an on-board EPID. A magnetometer

(MetroLab THM1176) was used to measure the strength of

the magnetic B-field at the surface of the EPID. To assess

the reproducibility of the panel readouts, a 10x10 cm

field was irradiated 10 times in two consecutive days and

the value of the on-axis region (averaged 5x5 pixels) of

EPID images was recorded. During the installation of the

MR-Linac in our institute, EPID images were acquired

before the B-field was ramped up and repeated with B-

field one month later. To study the on-axis response of the

EPID as function of field size with and without the

magnetic B-field, square fields were irradiated with field

sizes varying from 2 to 20 cm. Furthermore, EPID images

acquired with and without B-field were compared by

means of a 2-D γ-analysis (local 2%,1mm, 20% isodose) and

X-Y EPID lateral profiles were compared by visual

inspection.

Results

The magnetic field measured on top of the panel did not

exceed 2.5 mT, yielding an electron trajectory radius of

approximately 1.20 m. The reproducibility of EPID central

axis values for ten irradiated 10x10 cm

fields was 0.3% (1

SD).

The normalized on-axis EPID response as function of field

size with and without the presence of the magnetic field

is shown in

Figure 1

together with their ratio.