Accuracy of the recombination correction factor (ks) in FFF beams for three ion chamber types

Flattening filter free (FFF) beams pose particular considerations for absolute dosimetry. • These beams are capable to deliver high doses per pulse (DPP) which implies special consideration regarding recombination effects in ionization chambers. • We wanted to know which type of ionization chamber (Farmer, Semiflex or Pinpoint) would be more appropriate for commisioning our new FFF beam

Commissioning of small field size radiosurgery cones in a 6-MV flattening filter-free beam

This study aimed to describe the commissioning of small field size radiosurgery cones in a 6-MV flattening filter free (FFF) beam and report our measured values. Four radiosurgery cones of diameters 5, 10, 12.5, and 15¿mm supplied by Elekta Medical were commissioned in a 6-MV FFF beam from an Elekta Versa linear accelerator. The extraction of a reference signal for measuring small fields in scanning mode is challenging. A transmission chamber was attached to the lower part of the collimators and used for percentage depth dose (PDD) and profile measurements in scanning mode with a stereotactic diode. Tissue-maximum ratios (TMR) and output factors (OF) for all collimators were measured with a stereotactic diode (IBA). TMR and the OF for the largest collimator were also acquired on a polystyrene phantom with a microionization chamber of 0.016¿cm3 volume (PTW Freiburg PinPoint 3D). Measured TMR with diode and PinPoint microionization chamber agreed very well with differences smaller than 1% for depths below 20¿cm, except for the smaller collimator, for which differences were always smaller than 2%. Calculated TMR were significantly different (up to 7%) from measured TMR. OF measured with diode and chamber showed a difference of 3.5%. The use of a transmission chamber allowed the measurement of the small-field dosimetric properties with a simple setup. The commissioning of radiosurgery cones in FFF beams has been performed with essentially the same procedures and recommended ..

Development and clinical evaluation of a simple optical method to detect and measure patient external motion

A simple and independent system to detect and measure the position of a number of points in space was devised and implemented. Its application aimed to detect patient motion during radiotherapy treatments, alert of out-of-tolerances motion, and record the trajectories for subsequent studies. The system obtains the 3D position of points in space, through its projections in 2D images recorded by two cameras. It tracks black dots on a white sticker placed on the surface of the moving object. The system was tested with linear displacements of a phantom, circular trajectories of a rotating disk, oscillations of an in-house phantom, and oscillations of a 4D phantom. It was also used to track 461 trajectories of points on the surface of patients during their radiotherapy treatments. Trajectories of several points were reproduced with accuracy better than 0.3 mm in the three spatial directions. The system was able to follow periodic motion with amplitudes lower than 0.5 mm, to follow trajectories of rotating points at speeds up to 11.5 cm/s, and to track accurately the motion of a respiratory phantom. The technique has been used to track the motion of patients during radiotherapy and to analyze that motion. The method is flexible. Its installation and calibration are simple and quick. It is easy to use and can be implemented at a very affordable price. Data collection does not involve any discomfort to the patient and does not delay the treatment, so the system can be used routinely in all treatments. It has an accuracy similar to that of other, more sophisticated, commercially available systems. It is suitable to implement a gating system or any other application requiring motion detection, such as 4D CT, MRI or PET

Accuracy of the recombination correction factor (ks) in FFF beams for three ion chamber types

Flattening filter free (FFF) beams pose particular considerations for absolute dosimetry. • These beams are capable to deliver high doses per pulse (DPP) which implies special consideration regarding recombination effects in ionization chambers. • We wanted to know which type of ionization chamber (Farmer, Semiflex or Pinpoint) would be more appropriate for commisioning our new FFF beam

Commissioning of small field size radiosurgery cones in a 6-MV flattening filter-free beam

This study aimed to describe the commissioning of small field size radiosurgery cones in a 6-MV flattening filter free (FFF) beam and report our measured values. Four radiosurgery cones of diameters 5, 10, 12.5, and 15¿mm supplied by Elekta Medical were commissioned in a 6-MV FFF beam from an Elekta Versa linear accelerator. The extraction of a reference signal for measuring small fields in scanning mode is challenging. A transmission chamber was attached to the lower part of the collimators and used for percentage depth dose (PDD) and profile measurements in scanning mode with a stereotactic diode. Tissue-maximum ratios (TMR) and output factors (OF) for all collimators were measured with a stereotactic diode (IBA). TMR and the OF for the largest collimator were also acquired on a polystyrene phantom with a microionization chamber of 0.016¿cm3 volume (PTW Freiburg PinPoint 3D). Measured TMR with diode and PinPoint microionization chamber agreed very well with differences smaller than 1% for depths below 20¿cm, except for the smaller collimator, for which differences were always smaller than 2%. Calculated TMR were significantly different (up to 7%) from measured TMR. OF measured with diode and chamber showed a difference of 3.5%. The use of a transmission chamber allowed the measurement of the small-field dosimetric properties with a simple setup. The commissioning of radiosurgery cones in FFF beams has been performed with essentially the same procedures and recommended ..

A novel concept to include uncertainties in the evaluation of stereotactic body radiation therapy after 4D dose accumulation using deformable image registration

Purpose: To use four-dimensional (4D) dose accumulation based on deformable image registration (DIR) to assess dosimetric uncertainty in lung stereotactic body radiation therapy (SBRT) treatment planning. A novel concept, the Evaluation Target Volume (ETV), was introduced to achieve this goal. Methods: The internal target volume (ITV) approach was used for treatment planning for 11 patients receiving lung SBRT. Retrospectively, 4D dose calculation was done in Pinnacle v9.10. Total dose was accumulated in the reference phase using DIR with MIM. DIR was validated using landmarks introduced by an expert radiation oncologist. The 4D and three-dimensional (3D) dose distributions were compared within the gross tumor volume (GTV) and the planning target volume (PTV) using the D95 and Dmin (calculated as Dmin,0.035cc ) metrics. For lung involvement, the mean dose and V20 , V10 , and V5 were used in the 3D to 4D dose comparison, and Dmax (D0.1cc ) was used for all other organs at risk (OAR). The new evaluation target volume (ETV) was calculated by expanding the GTV in the reference phase in order to include geometrical uncertainties of the DIR, interobserver variability in the definition of the tumor, and uncertainties of imaging and delivery systems. D95 and Dmin,0.035cc metrics were then calculated on the basis of the ETV for 4D accumulated dose distributions, and these metrics were compared with those calculated from the PTV for 3D planned dose distributions. Results: The target registration error (TRE) per spatial component was below 0.5 ± 2.1mm for all our patients. For five patients, dose degradation above 2% (>4% in 2 patients) was found in the PTV after 4D accumulation and attributed to anatomical variations due to breathing. Comparison of D95 and Dmin,0.035cc metrics showed that the ETV (4D accumulated dose) estimated substantially lower coverage than the PTV (3D planning dose): in six out of the 11 cases, and for at least for one of the two metrics, coverage estimated by ETV was at least 5% lower than that estimated by PTV. Furthermore, the ETV approach revealed hot and cold spots within its boundaries. Conclusions: A workflow for 4D dose accumulation based on DIR has been devised. Dose degradation was attributed to respiratory motion. To overcome limitations in the PTV for the purposes of evaluating DIR-based 4D accumulated dose distributions, a new concept, the ETV, was proposed. This concept appears to facilitate more reliable dose evaluation and a better understanding of dosimetric uncertainties due to motion and deformation