Characterization of a cable-free system based on p-type MOSFET detectors for "in vivo" entrance skin dose measurements in interventional radiology

PURPOSE

During radiological interventional procedures (RIP) the skin of a patient under examination may undergo a prolonged x-ray exposure, receiving a dose as high as 5 Gy in a single session. This paper describes the use of the OneDose(TM) cable-free system based on p-type MOSFET detectors to determine the entrance skin dose (ESD) at selected points during RIP.

METHODS

At first, some dosimetric characteristics of the detector, such as reproducibility, linearity, and fading, have been investigated using a C-arc as a source of radiation. The reference setting (RS) was: 80 kV energy, 40 cm × 40 cm field of view (FOV), current-time product of 50 mAs and source to skin distance (SSD) of 50 cm. A calibrated PMX III solid state detector was used as the reference detector and Gafchromic(®) films have been used as an independent dosimetric system to test the entire procedure. A calibration factor for the RS and correction factors as functions of tube voltage and FOV size have been determined.

RESULTS

Reproducibility ranged from 4% at low doses (around 10 cGy as measured by the reference detector) to about 1% for high doses (around 2 Gy). The system response was found to be linear with respect to both dose measured with the PMX III and tube voltage. The fading test has shown that the maximum deviation from the optimal reading conditions (3 min after a single irradiation) was 9.1% corresponding to four irradiations in one hour read 3 min after the last exposure. The calibration factor in the RS has shown that the system response at the kV energy range is about four times larger than in the MV energy range. A fifth order and fourth order polynomial functions were found to provide correction factors for tube voltage and FOV size, respectively, in measurement settings different than the RS. ESDs measured with the system after applying the proper correction factors agreed within one standard deviation (SD) with the corresponding ESDs measured with the reference detector. The ESDs measured with Gafchromic(®) films were in agreement within one SD compared to the ESDs measured using the OneDose(TM) system, as well. The global uncertainty associated to the OneDose(TM) system established in our experiments, ranged from 7% to 10%, depending on the duration of the RIP due to fading. These values are much lower than the uncertainty commonly accepted for general diagnostic practices (20%) and of about the same size of the uncertainty recommended for practices with high risk of deterministic side effects (7%).

CONCLUSIONS

The OneDose(TM) system has shown a high sensitivity in the kV energy range and has been found capable of measuring the entrance skin dose in RIP.

Is the in vivo dosimetry with the OneDosePlusTM system able to detect intra-fraction motion? A retrospective analysis of in vivo data from breast and prostate patients

Background

The OneDosePlus^TM system, based on MOSFET solid-state radiation detectors and a handheld dosimetry reader, has been used to evaluate intra-fraction movements of patients with breast and prostate cancer.

Methods

An Action Threshold (AT), defined as the maximum acceptable discrepancy between measured dose and dose calculated with the Treatment Planning System (TPS) (for each field) has been determined from phantom data. To investigate the sensitivity of the system to direction of the patient movements, fixed displacements have been simulated in phantom. The AT has been used as an indicator to establish if patients move during a treatment session, after having verified the set-up with 2D and/or 3D images. Phantom tests have been performed matching different linear accelerators and two TPSs (TPS1 and TPS2).

Results

The ATs have been found to be very similar (5.0% for TPS1 and 4.5% for TPS2). From statistical data analysis, the system has been found not sensitive enough to reveal displacements smaller than 1 cm (within two standard deviations). The ATs applied to in vivo treatments showed that among the twenty five patients treated for breast cancer, only four of them moved during each measurement session. Splitting data into medial and lateral field, two patients have been found to move during all these sessions; the others, instead, moved only in the second part of the treatment. Patients with prostate cancer have behaved better than patients with breast cancer. Only two out of twenty five moved in each measurement session.

Conclusions

The method described in the paper, easily implemented in the clinical practice, combines all the advantages of in vivo procedures using the OneDosePlus^TM system with the possibility of detecting intra-fraction patient movements.