ESTRO 36 Abstract Book

S406

ESTRO 36 2017

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rates. Irradiations were performed with a

Co PICKER unit

in a secondary standard calibration laboratory. The

samples were divided into groups of two and each group

was placed at a different distance (56.65 - 427 cm) from

the

Co source at a 5cm depth within a water phantom.

Irradiation times varied in order to deliver the same dose

of 1 Gy at the center of all cuvettes with dose rates in the

range of 0.018 – 1.0 Gy/min. For the high dose rate study,

a similar methodology was employed. Four couples of

PRESAGE cuvettes were placed within a slab in a solid

water phantom and irradiated at different dose-rates by

varying the dose delivery rate of an ELEKTA Versa HD FFF

linac from 2.5 up to 19 Gy/min. Dose delivery of 1 Gy for

all dose rates was verified by ion chamber measurements.

Irradiation induced optical density (OD) change was

measured from pre- and post-irradiation scans with a

digital spectrophotometer operated at 633 nm. Mean OD

change for each group was normalized to the value for the

highest dose rate in each study.

Results

Results presented in figure 1 show a trend of increasing

PRESAGE dose sensitivity with decreasing dose rate with

the over-response reaching up to 16% at 0.018 Gy/min.

Although in a first approach such low dose rates could be

considered extremely low in external radiotherapy, recent

studies have shown that in advanced radiotherapy

techniques (e.g. VMAT) dose rate varies drastically across

dose distributions delivered and a considerable

contribution of the delivered dose could come from very

low dose rates (0.01 - 0.1Gy/min). Regarding the high dose

rate study, all responses agree within experimental

uncertainties, indicating that PRESAGE sensitivity is not

significantly affected.

Figure 1: Dose rate dependence of PRESAGE response for

both studies included in this work. Error bars correspond

to 1 standard deviation of all experimental uncertainties

involved.

Conclusion

Results of this study indicate a significant over-response

of this PRESAGE formulation in very low dose rates that

should be considered when they are used in applications

involving wide range of dose delivery rates.

Acknowledgement: This work was financially supported by

the State Scholarships Foundation of Greece through the

program ‘Research Projects for Excellence IKY/SIEMENS’.

PO-0775 Contributions to detector response in

arbitrary photon fields

S. Wegener

, O.A. Sauer

University Hospital, Radiation Oncology, Würzburg,

Germany

Purpose or Objective

Due to their small active volumes, diodes are often the

detectors of choice for many commissioning tasks

including the measurement of output factors, especially in

small fields. However, high-atomic number material in the

chip, detector shielding or other components and a finite

active volume size have been found to alter the signal

compared to the dose ratios measured in water in the

absence of such a detector. As a consequence, correction

factors need to be applied to correct the obtained signals.

Using three experimental setups (fig. 1), the different

contributions to the detector signals were separated and

analyzed: the response to scatter, the primary beam and

the combination of both.

Material and Methods

Signal ratios were obtained for three different

experimental setups (fig. 1): First, the standard open field

geometry. Secondly, fields in which the central part of the

beam was blocked out by a 4 mm aluminum pole and the

detector was positioned in the dose minimum

below. Finally, the detector in air instead of water with

a PMMA cap fitted on top. A range of typically used

detectors were analyzed, namely a microDiamond, a

PinPoint ionization chamber, an EDGE diode, as well as

three shielded and three unshielded diode detectors. EBT3

Gafchromic film served as reference. Measurements were

carried out on a PRIMUS linac at a photon beam quality of

6 MV with field sizes between 0.8 and 10 cm.

Responses in the blocked field and the PMMA setup were

combined to calculate the response in the open square

fields. The results were interpolated to a general matrix

from which responses in any field could be calculated.

Examples of such fields were measured for comparison.

Results

A higher detector overresponse and increasing detector to

detector differences were observed when the primary

beam was blocked out, whereas almost identical response

was seen for all detectors in the primary beam. A

combination of the responses in those two setups in a

detector-dependent ratio reproduced the values obtained

in the open field geometry with less than 1% deviation for

all detectors studied and all quadratic field sizes. For

rectangular and offset fields the agreement is still almost

within 1 %. Only when the detector was close to the field

edge larger deviations occurred (fig. 2).

Conclusion

Detector responses in open fields could be calculated from

the response to scatter and in the primary beam with 1%

agreement in all studied square fields and for all studied

detectors. The calculation was extended to rectangular

and non-symmetric fields yielding results in agreement

with the measurements for a wide range of fields. This

method suggests a way to calculate correction factors for

arbitrary fields.