Research Article: Comparison of detector performance in small 6 MV and 6 MV FFF beams using a Versa HD accelerator

Date Published: March 11, 2019

Publisher: Public Library of Science

Author(s): Paula Monasor Denia, María del Carmen Castellet García, Carla Manjón García, Juan David Quirós Higueras, Noelia de Marco Blancas, Jorge Bonaque Alandí, Xavier Jordi Juan Senabre, Agustín Santos Serra, Juan López-Tarjuelo, Qinghui Zhang.


Investigate the applicability of a series of detectors in small field dosimetry and the possible differences between their responses to FF and FFF beams. This work extends upon the series of detectors used by other authors to also include metal-oxide-semiconductor field-effect transistors (MOSFETs) detectors and radiochromic film. We also included a later correction of output factors (OFs) recommended by the recently published IAEA´s code of practice TRS 483 on dosimetry of small static fields used in external beam radiotherapy.

The OFs, profiles, and PDDs of 6 MV and 6 MV FFF beams were measured with 11 different detectors using field sizes between 0.6 × 0.6 cm2 and 10 × 10 cm2.

The OFs of the FFF beams were lower than those of the FF beams for field sizes larger than 3 × 3 cm2 but higher for field sizes smaller than 3 × 3 cm2. After applying the IAEA´s TRS 483 corrections, the final OFs were compatible with our initial results when considering uncertainties involved. Small-volume detectors are preferable for measuring the penumbra of these small fields where this attribute is higher in the crossline direction than in the inline direction. The R100 of equivalent-quality FFF beams was higher compared to the corresponding flattened beams.

We observed no difference for the dose responses between 6 MV and 6 MV FFF beams for any of the detectors. OF results, profiles and PDDs were clearly consistent with the previously published literature regarding the Versa HD linac. Correcting our first OFs, taken as ratio of detector charges, with the IAEA´s TRS 483 corrections to obtain the final OFs, did not make the former significantly different.

Partial Text

With technological advances there has been an increase in the use of techniques such as static and dynamic intensity-modulated radiotherapy, volumetric modulated arc therapy, and stereotactic cranial and extra-cranial radiotherapy, as well as the use of flattening filter-free (FFF) beams [1–3]. These techniques have the common characteristic of using fields and small segments to maximally optimise patient treatments by varying the fluence without requiring homogeneous flat beams.

We used a Versa HD linear accelerator (Elekta, Stockholm, Sweden) equipped with 6 MV and 6 MV FFF energy beams and an Agility head (Elekta, Stockholm, Sweden) with a multileaf collimator (MLC) with 160 leaves of 5-mm thick interdigitation-capable tungsten projected into the isocenter. The sheets move at a maximum speed of 3.5 cm/s and the MLC does not have a backup jaw. Its dose rate at 6 MV can reach 600 UM/min and this reaches up to 1400 UM/min with the 6 MV FFF beam. The remaining geometric and dosimetric properties of the Versa HD accelerator are described elsewhere [32–35]. The beam quality for the 6 MV beam is a tissue phantom ratio (TPR)20/10 of 0.684 and the TPR20/10 for the 6 MV FFF beam is 0.674. The accelerator was calibrated to administer 1 cGy/MU at a 10-cm depth in water, for a 10 × 10 cm2 field, and at a source-to-surface distance of 90 cm.

Firstly, with respect to OFs determination, by the time we were commissioning our 6 MV/6 MV FFF Versa HD, the piece of literature related with this topic was still somewhat heterogeneous. To our knowledge, only Lechner et al. work [27] was sufficiently systematic by covering a wide set of detectors and by reporting a complete series of corrections. This publication was really useful for us to notice detector behaviour and led some of our decisions when providing input for our therapy planning system. However, we decided to use our raw estimates for OFs until an institutional response like an IAEA’s code of practice was available. As mentioned before, it has come while preparing this manuscript, so we decided to keep our first determinations, which are representative of what users traditionally did in the absence of calculations of correction factors, and also present the real OFs in the way TRS 483 establishes [18] along with their uncertainties as a basis for a novel comparison.

There were no substantial differences in the dose responses for FF and FFF beams that could have any clinically relevant consequences for any of the detectors investigated. Both the results of the OF and for the profiles and PDDs are clearly consistent with previously published data relating to the Versa HD, and thus these findings will help other professionals who are commissioning new Versa HD linacs. These data provide valuable insight into accurate beam modelling, which in turn, determines treatment outcomes and patient safety.




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