Content from this work may be used under the terms of the CC BY 4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

19th Int. Conf. Accel. Large Exp. Phys. Control Syst.
ICALEPCS2023, Cape Town, South Africa
JACoW Publishing
ISBN: 978-3-95450-238-7
ISSN: 2226-0358
doi:10.18429/JACoW-ICALEPCS2023-MO3BCO07

FAST BEAM DELIVERY FOR FLASH IRRADIATIONSAT THE HZB CYCLOTRON
J. Bundesmann†, G. Kourkafas, A. Denker1, Helmholtz-Zentrum Berlin, Berlin, Germany
P. Mühldorfer, Berliner Hochschule für Technik, Berlin, Germany
J. Heufelder, A. Weber, Charité – Universitätsmedizin Berlin, Berlin, Germany
1
also at Berliner Hochschule für Technik, Berlin, Germany
Abstract
In the context of radiotherapy, Flash irradiations mean
the delivery of high dose rates of more than 40 Gy/s, in ashort time of less than one second. The expectation of theradio-oncologists are lesser side effects while maintaining
the tumour control when using Flash. Clinically acceptabledeviations of the applied dose to the described dose are lessthan 3%.
Our accelerator control system is well suited for the
standard treatment of ocular melanomas with irradiation
times of 30 s to 60 s. However, it is too slow for the shorttimes required in Flash. Thus, a dedicated beam delivery
control system has been developed, permitting irradiation
times down to 7 ms with a maximal dose variation of lessthan 3%.

effects. The medical physicists require that variations indose delivery should be less than 3%. Hence, in order tostart experiments with FLASH irradiations a new beam delivery control system had to be developed.

LAYOUT OF THE ACCELERATORS
Figure 1 shows the layout of HZB’s accelerator complex
for proton therapy. The k130 cyclotron is served by two
injectors: either a 6 MV Van-de-Graaff or a 2 MV Tandetron. Besides the treatment room is the experimental
room. The control system for the accelerator is based on
Vsystem [5].

INTRODUCTION
Proton Therapy of Ocular Melanoma
Proton therapy of ocular melanoma is a well-establishedand successful treatment. At the Helmholtz-Zentrum Berlin (HZB), patients are treated in cooperation with Charité– Universitätsmedizin Berlin since 1998. Overall, morethan 4500 patients have been treated. The local tumour
control is 96% after five years [1, 2, 3].
Protons permit the confinement of the dose to the tumour. At HZB, we use a proton beam with 68 MeV having
a range in water of 38 mm. With this energy, the dose drops
at the end of the Bragg peak from 90% to 10% within lessthan 1 mm. This permits the sparing of organs at risk likethe optical nerve or the macula.
For eye tumours the typically prescribed total dose is
60 Gy which is applied in four fractions over four days.
The irradiation time is 30 s to 60 s.

FLASH Irradiations
While the tumour control using protons for ocular melanoma is excellent, there is still a wish for further improvement: the reduction of side effects. A very vibrant research
field for reducing side effects while maintaining tumour
control is the so-called FLASH irradiation [4]: the dose rateis increased drastically, at least 40 Gy/s and the irradiation
time is less than 1 s. At the moment, neither the ideal dose
rate nor the ideal irradiation time is known. The normal
dose rate for ocular melanomas is less than 0.5 Gy/s. The
huge increase in dose rate provides challenges for the dosimetry like linearity of the dose monitors and saturation____________________________________________† bundesmann@helmholtz-berlin.de.

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178

Figure 1: Layout of the proton therapy accelerator complexof HZB. The red arrows mark the position of electrostatic
deflectors, the blue arrow the position of the fast beam
shutter.
SoftwareControl Frameworks for Accelerator & Experiment Control

There are 16 normal Faraday cups (FC) at various positions in the accelerator complex. They serve for intensity
measurements as well as beam dumps. Due to their 50 mm
stroke, their opening/closing time is 100 ms. For therapy
we use a so-called fast beam shutter in front of the treatment room. This opens within 20 ms and closes within
10 ms. These times match well the standard irradiation
times of at least 30 s. Furthermore, there are two electrostatic deflectors in front of the cyclotron: They can switch
off the beam within 50 ns [6, 7].

EXPERIMENTAL SET-UP

the beam line via an 80 µm Kapton foil, transverses thefirst ionisation chamber from PTW Freiburg, and the range
shifter. It passes the modulator wheel, which creates the
SOBP. Then comes a mirror for visualisation of the beam
spot on the samples. A second ionization chamber is used
for redundancy. Finally, the x- and y- dimensions of thebeam are shaped with an aperture. On the target position
either a camera to verify the lateral beam distribution, awater phantom for absolute dosimetry using a Markus
chamber with Unidos from PTW Freiburg, or the samples
to be irradiated are mounted.

TIMING ISSUES
In an ideal world, the opening or closing of the beam
shutters would happen instantaneously and without any delays. In reality, the following times have to be considered:
1. Delay from opening command until the device starts
to open.
2. Time of opening.
3. Irradiation time.
4. Delay from closing command until the device starts toclose.
5. Closing time

Figure 2: Experimental set-up for the FLASH irradiations,
here with water phantom and dosimetry chamber. The proton beam comes from the right.
Figure 2 depicts the set-up in the experimental room [8].
The experiments were performed like in the conventional
irradiation with a spread-out Bragg peak (SOBP). The
beam with a diameter of about 40 mm exits the vacuum of

Figure 3 illustrates this with an irradiation time of
500 ms using the fast beam shutter. The overall time forone experiment is one second. For the first 100 ms the Faraday cup just upstream of the fast beam shutter is still inplace, then it is opened. To make sure that it really has
moved out of the beam path, 200 ms pass before the command of opening the fast shutter is given. It then takes
about 40 ms until the device starts to open. The opening

Figure 3: Signals of the experiment over time for an irradiation time of 500 ms using the fast shutter. In the upper part,the signals from the two ionisation chambers (red and white) are displayed in arbitrary units. The lower part shows theswitching signals of the Faraday cup (green) and the fust shutter (blue). The vertical yellow line marks the end of theplanned irradiation time, in this case 500 ms after the shutter opening command.
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MO3BCO07
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19th Int. Conf. Accel. Large Exp. Phys. Control Syst.
ICALEPCS2023, Cape Town, South Africa
JACoW Publishing
ISBN: 978-3-95450-238-7
ISSN: 2226-0358
doi:10.18429/JACoW-ICALEPCS2023-MO3BCO07

Content from this work may be used under the terms of the CC BY 4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

19th Int. Conf. Accel. Large Exp. Phys. Control Syst.
ICALEPCS2023, Cape Town, South Africa
JACoW Publishing
ISBN: 978-3-95450-238-7
ISSN: 2226-0358
doi:10.18429/JACoW-ICALEPCS2023-MO3BCO07

time is about 20 ms, then the full signals in the ionisationchambers are visible. 500 ms after the shutter opening
command the signal to close both the Faraday cup and thefast shutter is given. However, it takes again about 40 ms
till the devices start closing and 10 ms until the beam isreally switched of.
The opening time of 20 ms of the fast beam shutter was
the reason to choose 200 ms for the first FLASH experiments [9]. Otherwise, the homogeneity of the lateral dose
profile, which has also to fulfil the 3% deviation limit,
would be compromised. In a later stage, an electrostatic
switch was available on both injector paths, enabling far
shorter irradiation times due to shorter switching times andsmaller delays.

HARDWARE AND CONTROL LOGIC
The system splits up into an embedded system and a PC
which are connected via TCP/IP. The embedded system isa sbRIO 9637 from National Instruments (NI). The ARM
Cortex A9 processor runs with NI Linux Real Time with areal time operation system [10]. The real accuracy of thereal time clock is 5 ppm. Via a DMA connection the processor has access to the FPGA. The programming platform
was LabVIEW. The FPGA samples with 10 kHz, giving atime resolution of 0.1 ms and streams the data every
100 ms to the PC with a two-dimensional array of1000 x 12 data points. The FPGA processes three analogue

signals from the two ionisation chambers and the FC using
I/U-converters DLPCA from FEMTO [11]. Thus, the unitof the ionisation chamber signal is mV per 100 µs, considered as counts. The FPGA operates three digital outputs forthe signals to open or close the FC, the fast shutter(BSATT), and the electrostatic deflector (pulser).
The PC receives and displays the streamed data. The amplification factors of the I/U converters for the ionisationchambers as well as for the FC are inserted manually. The
PC generates the start signal, saves the data into a file, calculates the calibration factors, which correlate the countsof the ionisation chamber to the dose. The PC displays also
the user interface (see Fig. 4). In the set-up fields the user
defines the following items:
 length of the pulse, the data acquisition time, the pre-trigger time, which is the time between startof data acquisition and opening command of either
fast shutter or electrostatic deflector,
 the time the last FC stays in the beam after beginning of the data acquisition for checks of the beam
intensity (FC_J2_out in Fig. 5). This time is also
used to determine the background of the ionisationchambers.

Figure 5: User interface of the FLASH control system displaying the results of the calibration using the electrostatic
deflector. A dose of 10 Gy was applied within 200 ms. The
calibration factors for the two ionization chambers as wellas the excess counts (Dose_1_NL and Dose_2_NL) aredisplayed. Also, the intensity on the Faraday cup before
and after the FLASH pulse is given (I_J2_start and
I_J2_end). For more details see text.

MO3BCO07

The user sees the streamed data of the ionisation chambers and the FC. The PC also displays if the beam intensity
is within the limits of the calibration (I_J2_ok). Otherwise,
it vetoes the beam (see below). The user chooses which
system to use for the FLASH, either the fast shutter or theelectrostatic deflector. Finally, the user defines the irradiation mode: calibration, verification or treatment.
Software180

Control Frameworks for Accelerator & Experiment Control

Figure 4: User interface of the FLASH control system forthe calibration using the fast shutter just after starting the
programme. For more details see text.

Figure 5 shows the user interface displaying the results
after the calibration. For this irradiation, the electrostatic
deflector was used. This yields far reaction times(see Fig. 5).

WORKFLOW
The following workflow looks tedious; however, it only
takes a few minutes, each irradiation taking only one second.

Calibration
Before starting the calibration mode, the beam intensity
is set to the anticipated dose rate. The water bath with theabsolute dosimetry is mounted on target position. The user
defines a desired, fixed time window between 10 ms and
500 ms. A FLASH is applied, and the measured absolutedose is entered into the user interface. From these data, thesystem correlates the counts from the ionisation chambers
to the dose. Also, the counts of the ionisation chambers after the switch-off signal are determined, the so-called excess counts.

Verification
The verification mode is only accessible after a successful calibration. Instead of a fixed time the user now chooses
an application dose with the absolute dosimetry still inplace. For safety reasons, this dose has to be slightly
smaller than the dose measured during the calibration
mode. From the calibration process the system gives theswitch-off signal when the counts in the ionisation chamber is equal to the counts corresponding to the desired dose
minus the excess counts. Again, the measured absolutedose is entered manually. This step is repeated several
times for statistical reasons. If the user is satisfied, he goesto the next step. Otherwise, the calibration procedure has
to be repeated.

To counteract any influences due to beam intensity fluctuations the beam intensity on the Faraday cup in front of
the treatment room is monitored for the first 100 ms of each
irradiation (see Fig. 5, upper plot, blue line). If it differs
more than 5% from the intensity during the calibrationmode, the FLASH is vetoed both in verification and intreatment mode.

RESULTS
Table 1 gives the results for various FLASH irradiation
times and doses for longer pulses. After the calibration, inthe verification mode 20 times a FLASH with the planned
dose was applied, and the measured dose and time compared with the planned values. For the times between
20 ms and 200 ms the variation in the applied dose was lessthan 1.5%. Thus, the requirements of the medical physicists are fulfilled.
Table 1: Comparison of Delivered Dose Versus Planned
Dose and Resulting Irradiation Time
Calibration
t
[ms]
200
150
100
50
40
30
20

D
[Gy]
116.8
88.0
60.8
28.0
23.2
16.8
11.8

Verification
D [Gy]
plan
105
79
55
25
21
15
11

D [Gy]
real
104.8
79.2
54.6
24.8
20.8
15.2
11.1

t [ms]

∆D

182.7
138.0
94.3
43.9
37.3
26.7
19.

0.19%
0.25%
0.73%
0.80%
0.95%
1.33%
0.91%

Treatment Mode
After a successful calibration and verification, the user
now enters the treatment mode. The water bath is replacedby the sample to be irradiated. Again, the applied dose mustbe smaller than the dose measured during the calibration
mode. The irradiation is performed if the beam intensity isstable and within the allowed limits (see below).

Safety Measures
The philosophy is that any possible errors lead only tominor changes in the applied dose. If any errors occur, theyshould preferentially result in a lower dose than intended:
whichever ionisation chamber reaches the switch-off level
first, this one triggers the stop of the irradiation. Thus, afailure of one chamber dose not result in an excess of dose.
In the unlikely event that both ionisation chambers fail simultaneously, the beam is switched off after the time usedduring the calibration. This is the reason why the dose to
be applied should be slightly smaller than the dose measured during the calibration mode with a fixed irradiation
time.
SoftwareControl Frameworks for Accelerator & Experiment Control

Figure 6: Results for irradiation times below 15 ms showing planned dose (black crosshairs), minimum delivered
dose (blue line) and maximum delivered dose (red line)
within these 20 shots.
The situation becomes more complicated with shorter irradiation times (see Fig. 6). The average applied dose is wellwithin the required limits. Down to 6 ms irradiation time
single shots deviate up to 3%, which is still acceptable.
However, the shorter the irradiation times, the larger theerrors. At 1 ms and 2 ms the error goes up to nearly 10%.
MO3BCO07
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19th Int. Conf. Accel. Large Exp. Phys. Control Syst.
ICALEPCS2023, Cape Town, South Africa
JACoW Publishing
ISBN: 978-3-95450-238-7
ISSN: 2226-0358
doi:10.18429/JACoW-ICALEPCS2023-MO3BCO07

Content from this work may be used under the terms of the CC BY 4.0 licence (© 2023). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

19th Int. Conf. Accel. Large Exp. Phys. Control Syst.
ICALEPCS2023, Cape Town, South Africa
JACoW Publishing
ISBN: 978-3-95450-238-7
ISSN: 2226-0358
doi:10.18429/JACoW-ICALEPCS2023-MO3BCO07

This is acceptable for physical experiments, but not forstudies on the effect of such irradiations.
The reason for these apparent large errors is the sampling
rate: For a shot of 1 ms with 10 kHz sampling, only 10 data
points are taken within the FLASH pulse itself which includes the rising flank as well as the falling flank. This isby far not enough.

CONCLUSION
A reliable fast beam delivery system for FLASH irradiations has been installed, implemented, and tested. The
system is not certified as a medical device. However, for
the implemented security it follows the same philosophy as
for medical devices. It has been used for experiments forradiation therapy on cells, organoids, and mice. For times
above 6 ms, the reproducibility of the dose applicationfrom one irradiation to the next is well within the limits as
required by the medical physicists and radiation oncologists.
Irradiations with shorter times are feasible, if the user accepts larger errors: about 5% down to 3 ms, and 10% forsmaller times. This can be overcome by increasing the sampling rate and is planned for the near future.

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dokumente/de­dlpca­200.pdf

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