Session Item

Specification of the PTW INVIVO rectum probe type 9112 tells us that temperature dependence of calibration factors of diodes is <1%/K. In worst scenario, for example, if this factor equal 0.9%/K, it leads to result that correction factor for increasing temperature from calibration 20° C to 37° C is equal to essential value 15.2%. The purposes of this work are to clarify the temperature dependence of diode outputs in our probe, and answer the question whether we need introduce temperature correction or not.

In figure you can see the idea of measurement setup based on water flow-through concept. Water goes from Reservoir 5 for out coming water and quickly change to required temperature the Perfusor® Stringe 50ml with inserted into it rectum probe 7 and temperature sensor 4. In quite shot time (5-10 min) after water flow beginning, the temperature stabilizes and measurements can be done. The distance between rectum probe and radiation source is approximately 4 cm, which is correspond to clinical situation.  Geometrical shape of the basis plate for fixation Perfusor designed this way that, if we suggest that even if there is a temperature change of Perfusor dimensions, it will result the movement of the Perfusor to the direction perpendicular to the radiation from the source and does not yield to changing of distance between source and diodes.

Figure.   Scheme of setup for measurement of temperature dependence calibration factor of PTW INVIVO rectum probe. 1- Reservoir for incoming water; 2, 6 – tubes for water ; 3 – Original-Perfusor® Syringe 50 ml shell; 4- temperature sensor; 5 – Reservoir for out coming water ; 7 – rectum probe with R1-R5 diodes; 8 – Flexitron afterloader:; 9 - catheter for radiation source; 10- radiation source.

On the table, you can see sequence of all temperature stop-points during experiment. Reading of R2 diode is higher, because its distance to radiation source is shorter.  Readings of the diodes demonstrate some “not stable” manners, since dwell-position of the source precisely not the same during many circles of “send-out- send–back” for source procedures of the investigation, and in spite of that the general trend of that the temperature dependence calibration factor of order 0.1%/K is clearly seen.

Table.   Measured values of rectum probe (diodes R1, R2, R3, R4, R5) readings for all temperature stop-points sequence during experiment.

1. Measured values for temperature dependence of calibration factors of diodes are ~0.1%/K. As results, we need to correct minus 1-2% of INVIVO diodes readings in treatment procedure, if we want to be precise. However, up-to-now in our clinic we do not do this on our practice, because there are many others factors of uncertainties.

2. Found temperature dependence of calibration factors ~0.1%/K is in disagreement with results (~ 0.5%/K) of authors [1, page 160].  Good point for dispute.

[1] - Fröhlich et al., „In vivo dosimetry of the rectum...“, Reports Prac. Oncol&Radiother., Vol.24, p. 158, 2019