Firstly,
the results of the validation are discussed.
In the animated gif in Fig. 3 ,
the 3D representation of the PSD map,
calculated by the dose management system DOSE for the plexiglas (adult) phantom (previously seen in Fig. 1 ),
is displayed.
The map was compared against the irradiated fields on the film and found to correspond well with the experimental measurements.
The maximum geometrical difference was 2.2cm for the checked field dimensions and distances.
The overestimation of the dimensions that was noticed in cases of rectangular fields is caused by the fact that currently all fields are approached by the PSD tool as squares with the same area,
due to the lack of shape information in the RDSR,
at least in the case of this interventional device.
Fig. 3: Representation on a 3D phantom of the PSD map calculated in DOSE dose management system. Each irradiated field and the distance between them were measured using both excel and imageJ software and compared against the same parts of the film that was used during the experiments.
References: Qaelum NV - Leuven/BE
Regarding the peak skin dose value,
the PSD tool of the dose management system showed good accuracy with all absolute dose deviations within 10% when compared against a set of five manual calculations that we performed starting from the Air Kerma at Reference Point and taking into account the source to skin distance,
backscatter factor,
table and pad attenuation and f-factor for each event.
Additionally,
the dose deviations between the PSD tool and the Piranha dosimeter indications were also smaller than 5% for the three irradiations checked.
In the case of the 10yo CIRS phantom,
the 3D representation of the PSD map in DOSE is shown in the animated gif of Fig. 4 .
All the irradiations of this map are displayed with a purple color which corresponds to dose values below 100mGy.
In Fig. 5 ,
the corresponding irradiated film of this case is shown together with the evaluation measurements of the geometry.
It is obvious that the map is a good representation of the film.
Maximum geometric deviation for all fields of this pediatric case was 1.1cm.
Fig. 4: Representation of the PSD map on a 10yo-sized 3D phantom as calculated and shown in DOSE. The map represents the PSD calculation of DOSE for the experimental irradiation of the 10yo CIRS phantom.
References: Qaelum NV - Leuven/BE
Fig. 5: The film that was irradiated with the 10yo CIRS phantom was used to validate the PSD map for geometry. The dimensions and distances of the irradiated areas on the film were measured and compared with manual measurements of the PSD map in ImageJ software.
References: Qaelum NV - Leuven/BE
The second part will discuss the evaluation of a size-corrected phantom instead of a standard adult phantom for the PSD estimation.
In Fig. 6 ,
the PSD map of the 12yo patient as calculated and presented in DOSE is presented.
For the 4 clinical cases checked,
the PSD was relatively low (lower than 100 mGy).
Regarding the testing of using an adult phantom instead,
only small deviations were observed between the PSD with adult versus pediatric models.
In particular,
a deviation of 4.8% was found in the case of the 12yo patient,
with an overestimation of skin dose when using the adult phantom.
This is due to the fact that the adult phantom is bigger in size which leads to higher backscatter factor and smaller source to skin distance,
especially when it comes to lateral views.
Fig. 6: The PSD map as calculated and presented in DOSE for the 12 yo patient. The skin dose in all cases is below 100mGy and this is why all the irradiated areas are shown in purple.
References: Qaelum NV - Leuven/BE
Third part: Evaluation of the influence of the correct phantom size on PSD calculation for different techniques with “what if” scenarios.
In the animated images of figures 7-9 ( Fig. 7 ,
Fig. 8 , Fig. 9),
the two cases: "no overlap" and "overlap" are shown for the newborn,
10yo and adult respectively.
In case of no overlapping fields,
the dose is significantly higher for the same x-ray beam when estimated for an adult phantom,
due to smaller source to skin distance and higher backscatter radiation (100% higher than the PSD estimation on a baby phantom).
However,
when evaluating a different technique with lateral views that cause overlapping fields for the pediatrics (due to longer source to skin distance that increases the fields),
the dose is doubling for the baby phantom and is more or less in the same range of the one for the adult.
This can happen even with a small difference of 1 cm in the lateral movement of the table.
This is indicated in Fig. 7 with the white edges that appear in the overlapping case.
For the adult ( Fig. 9 ),
the fields are not overlapping in any of the two cases,
thus the dose is the same.
For the 10yo,
there is also a slight overlapping which also increased the dose ( Fig. 8 ).
Fig. 7: Animated gif that shows how the PSD estimation on a baby phantom changes with the technique when a) fields are not overlapping, b) fields are overlapping. In the second case, the white columns that appear show that the PSD is now much higher.
References: Qaelum NV - Leuven/BE
Fig. 8: Animated gif that shows how the PSD estimation on a 10yo phantom changes with the technique when a) fields are not overlapping, b) fields are slightly overlapping.
References: Qaelum NV - Leuven/BE
Fig. 9: Animated gif that shows how the PSD estimation on an adult phantom changes with the technique. In the second case, the fields are closer to each other but still not overlapping, thus, PSD remains the same. On the contrary, this has an effect on the real PSD of pediatrics, but cannot be evaluated if the correct size-phantom is not used.
References: Qaelum NV - Leuven/BE