Raffaele Novario

Experimental method to obtain scattering contribution in portal dose images

A method for evaluating scattered dose contribution in portal images acquired under clinical conditions (phantom-device distance of 30 cm) is presented. This method is based on radiographic film and ionisation chamber measurements and is valid for homogenecus polystyrene phantoms and square fields of different size. The portal imaging device consisted of a radiographic film placed between slabs of polystyrene under full build-up conditions (1.5 cm for 6 MV beam and 3 cm for 18 MV and 1 cm of polystyrene backscatter material. First the primary dose image in the portal plane P(i,j) is obtained using a projection algorithm, then the scattered dose image S(i,j) is found by subtracting the primary dose image in the portal plane P(i,j) from the total dose image acqu red in the portal plane T(i,j). The ratio S(i,j)/T between the scattered dose distribution and the dose value measured on the bearn axis in the portal plane was found to be uniform within the radiation field for all the geometrical configuration of phantoms and fields studied. Under these conditions the mean value of the scatter fraction S/T evaluated within a ROI centred on the bearn axis accurately describes the scatter fraction distribution S(i,j)/T within the whole radiation field. S/T ranges from 7.4% to 31.1% in the 6 MV beam and from 8.9% to 30.8% in the 18 MV beam. Finally an analytical method to evaluate the ratio S/T has been developed from the experimental results. It comprises phantom, accelerator head and portal imaging device contributions and depends on field size and phantom thickness.

The use of photostimulable phosphor systems for periodic quality assurance in radiotherapy

The fusion of radiological and optical images can be achieved through charging a photostimulable phosphor plate (PSP) with an exposure to a field of X- or gamma-rays, followed by exposure to an optical image which discharges the plate in relation to the amount of incident light. According to this PSP characteristic, we developed a simple method for periodic quality assurance (QA) of light/radiation field coincidence, distance indicator, field size indicators, crosshair centering, coincidence of radiation and mechanical isocenter for linear accelerators. The geometrical accuracy of radiological units can be subjected to the same QA method. Further, the source position accuracy for an HDR remote afterloader can be checked by taking an autoradiography of the radioactive source and simultaneously an optical image of a reference geometrical system.

A new sonographic method for the evaluation of skin permeability changes after irradiation with low-frequency ultrasound

The irradiation of the skin with low-frequency ultrasound (42 MHz) increases the skin permeability, allowing the US stimulated drug delivery (sonophoresis). The changes in the skin permeability is generally demonstrated with the measurement of corneometry and transepidermal water loss (TEWL). A novel ultrasound scanner with a 50 MHz probe was used for acquiring images of the skin (penetration depth few millimeters). The images show the different epidermal and dermis layers. A specially designed plexiglas basin containing an US transducer was used. Water was used as matching medium. The transducer was set to generate a 42 MHz continuous wave US beam with an intensity of 150 mW/cm2 for a chosen preset time. Ninety healthy volunteers were submitted to exposure of the back of the hand. The back of the hand of each person was scanned at 50 MHz before the irradiation and after 1, 15 and 60 minutes. A significant variation of the stratum corneum and the derma on the sonographic image was found. A particular software code was developed in order to quantify the amount of the variation in the image, using different parameters (entropy, energy, skewness, kurtosis, etc.) related to the pixel value in different regions of interest and to the cumulated profile along lines perpendicular to the skin surface. The variations in the parameters so defined were demonstrated to be statistically significant and with a sensibility much higher than corneometry and TEWL. This new approach allow to better understand the mechanism and quantify the changes in the skin permeability.

Fast treatment planning with IVUS imaging in intravascular brachytherapy

The planned target volume in intracoronary brachytherapy is the vessel wall. The success of the treatment is based on the need of delivering doses possibly not lower than 8 and not higher than 30 Gy.nAn automatic procedure in order to acquire intravascular ultrasound images of the whole volume to be irradiated is pointed out; a motor driven pullback device, with velocity of the catheter of 0.5 and 1 mm/s allows to acquire the entire target volume of the vessel with a number of slices normally ranging from 400 to 1600.nA semiautomatic segmentation and classification of the different structures in each slice of the vessel is proposed. The segmentation and the classification of the structures allows the calculation of their volume; this is very useful in particular for plaque volume assessment in the follow-up of the patients. A 3D analyser tool was developed in order to visualize the walls and the lumen of the vessel. The knowledge, for each axial slice, of the position of the source (in the centre of the catheter) and the position of the target (vessel walls) allows the calculation of a set of source-target distances. Given a time of irradiation, and a type of source a dose volume histogram (DVH) describing the distribution of the doses in the whole target can be obtained. The whole procedure takes few minutes and then is compatible with a safe treatment of the patient, giving an important indication about the quality of the radiation treatment selected.