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S131

ESTRO 36 2017

_______________________________________________________________________________________________

and without a strong magnetic field. A magnetic field of

1.5T was used for PRESAGE® and FOX and 1.0T for BANG

TM

(1.5T was not feasible due to size constraints between the

pole pieces for BANG

TM

). PRESAGE® was irradiated with

doses to 5 Gy, FOX to 8 Gy, and BANG

TM

to 10 Gy.

Calibration curves fitting signal read-out and dose were

compared between 0T and 1.5/1.0T for each dosimeter

type.

Results

The optical signal was analyzed for PRESAGE® and FOX,

and the spin-spin relaxation rate R

2

(=1/T

2

) MR signal was

analyzed for BANG

TM

. For all three types of 3D dosimeters,

the calibration curves were linear. For PRESAGE®, the

percent difference between 0T and 1.5T was 1.5%

measured at the spectral peak of 632 nm; for FOX, there

was a 1.6% difference at 440 nm and 0.5% difference at

585 nm (R

2

= 1.00 for all optical calibration curves). The

greatest percent difference for a given point dose was

5.0% at 2 Gy for PRESAGE®, 2.3% at 6 Gy (440 nm) for FOX,

and 5.6% at 2 Gy (585 nm) for FOX. For BANG

TM

, the

percent difference between 0T and 1.0T was 0.7% (R

2

=

1.00). The greatest percent difference for a given point

dose was 0.3% at 10 Gy for BANG

TM

.

Conclusion

The same doses calculated for 0T were delivered for both

0T and 1.5/1.0T irradiations; the expected dose

difference with the strong magnetic field is up to about

0.5%. Considering this potential dose difference and other

uncertainties, the percent differences in response with

and without strong magnetic field were minimal for all

three 3D dosimeter systems, under 1.6% regarding the full

dose response curves and up to 5.6% for a single dose

point. All three dosimeter systems have already been used

for preliminary investigations on the MR-linac, including

electron return effect studies with an air cavity and

assessing changes in the field edges with and without the

strong magnetic field. This study encourages the

continued use of all three types of 3D dosimeters for MR-

IGRT applications without needing to apply a correction

factor for the signal acquired for any of the above 3D

dosimeter systems.

OC-0259 Online quantitative imaging on the MR-Linac

F. Koetsveld

1

, L.C. Ter Beek

2

, P.J. Van Houd t

1

, L.D. Van

Buuren

1

, U.A. Van der Heide

1

1

Netherlands Cancer Institute Antoni van Leeuwenhoek

Hospital, Radiotherapy department, Amsterdam, The

Netherlands

2

Netherlands Cancer Institute Antoni van Leeuwenhoek

Hospital, Radiology department, Amsterdam, The

Netherlands

Purpose or Objective

Quantitative MR imaging provides information on tumor

physiology and treatment response. For patients treated

in a MR-Linac, it is possible to perform daily quantitative

MR imaging. Online quantitative imaging provides valuable

input for radiomics studies, and can potentially be used

for adaptive dose painting.

Due to time constraints in an online setting, it is

challenging to obtain accurate images that can give

detailed information on tumor environment. Therefore we

assessed the accuracy of four quantitative imaging

sequences on the MR-Linac which are potentially relevant

for radiomics and response assessment: T2 mapping,

Dynamic Contrast Enhancement (DCE) including T1

mapping as input for pharmacokinetic modeling, and

Diffusion Weighted imaging (DWI).

Material and Methods

We compared phantom measurements on the MR-Linac

with two diagnostic scanners: a 3T Philips Achieva dStream

and a 1.5T Philips Achieva.

We tested T2 mapping using a slow but accurate Carr-

Purcell-Meiboom-Gill sequence on the Eurospin T05

phantom (Eurospin TO5, Diagnostic Sonar, Livingston,

Scotland). We compared the results of this sequence with

an accelerated sequence optimized to clinically feasible

acquisition times by reducing the amount of spin echoes.

We tested T1 mapping with an Inversion Recovery series,

which is more accurate but too slow to be used in clinical

practice. We compared this with the fast and clinically

applicable Variable Flip Angle (VFA) T1 mapping

technique.

For DCE we further tested stability of the signal during a

7 min sequence on a phantom containing different

contrast agent concentrations.

For DWI, we tested the accuracy of Apparent Diffusion

Coefficient (ADC) measurements of water at 0

o

C with an

Echo Planar Imaging (EPI) sequence and with a Turbo Spin

Echo (TSE) DWI sequence, which is less susceptible to

geometric distortions.

Finally, to demonstrate the image quality and clinical

applicability on the MR-Linac, we made T1 and T2 maps of

the pelvis on a healthy volunteer.

Results

The phantom results for all four sequences are shown in

figure 1. The accelerated T2 mapping is accurate to within

2% standard deviation on all systems. As expected the VFA

sequence shows a bias of 10-15%. This sequence has similar

precision (within 10% standard deviation) on all three

systems. The DCE sequence on the MR-Linac is shown to

be stable over a 7min long dynamic series like the regular

MR scanners, with a coefficient of variation of < 1% for all

contrast agent concentrations. For diffusion, it shows that

the literature value ADC of water of 1.13*10-3 mm

2

/s (Holz

et al, 2000) can be accurately measured using both EPI and

TSE on the MR-Linac.

Figure 2 shows a T1 and a T2 map of the prostate of a

healthy volunteer, using a scan with 2x2x2 mm voxels.