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.

Autoplanning in the setting of stereotactic body radiation therapy for lung cancer

PURPOSE

Automatic planning (AP) has been compared to manual planning (MP) in lung stereotactic body radiation therapy (SBRT) to validate the former and to implement it in clinical practice.

METHODS

A new developing Guided Planning System (GPS) engine was used to reoptimize 20 lung SBRT plans with the RayStation™ treatment planning system (TPS). The original manual plans were optimized to deliver 60 Gy in 5 or 8 fractions to the target with constraints on organs at risk (OARs) based on an internal protocol. AP plans were compared to MP based on (i) planning target volume (PTV) and OARs dosimetric evaluation, (ii) clinician's blind plan comparison, (iii) Plan QA results, and (iv) plan quality metrics. Differences between continuous variables were explored through the Mann-Whitney test (p < 0.05).

RESULTS

Target and OARs dosimetry showed no significant difference, with the only exception of the spinal cord maximum dose that was significantly lower for AP in the 5 fractions scheme (MP: 8.93 Gy ± 3.94 Gy vs AP: 6.45 Gy ± 2.72 Gy, p = 0.034). In the blind comparison, AP was preferred in 45 % of cases while MP in 35 % of cases (no preference was expressed in 20 % of cases). A trend towards lower monitor units (MUs) was found for AP in the 5 fractions scheme (MP: 3383 ± 943 vs AP: 2662 ± 588, p = 0.059). No significant difference was found in any of the plan quality metrics.

CONCLUSIONS

AP plans were not inferior to MP plans; therefore, GPS is ready for clinical use in a pulmonary SBRT setting, reducing the planning workload and harmonizing the planning procedure.