[Stereotactic radiotherapy with linear accelerators. Dosage considerations on collimators and photon beam energy]. / La radioterapia stereotassica con acceleratori lineari. Considerazioni dosimetriche sui collimatori e sull'energia di fasci di fotoni.

The small X-ray beam profiles produced by linear accelerators and used in stereotactic radiosurgery were studied. Circular focusing and not-focusing beams outlined by additional collimators (diameter ranging 10 to 30 mm) and square field outlined by the jaws (field size 10 to 30 mm) were measured. 6 and 18-MV X-ray beams from a dual energy accelerator and 6 and 15-MV X-ray beams produced by 2 single-energy accelerators were used. Measurements were carried out with Kodak X Omat V films in a perspex phantom in the same conditions for all the fields. To study the profile characteristics we introduced the V80/V20 = (R80/R20)3 ratio where R80 and R20 are respectively the mean distance of the 80% and 20% isodose lines from the beams axis. Measurement results show that 6-MV X-ray beams have a higher V80/V20 ratio than the ones obtained with the other energies. There is no significant difference between the beams produced with focused collimators and those produced with not-focused collimators. Square fields outlined by the jaws have a V80/V20 ratio greatly dependent on the accelerator used. This ratio is generally worse than the ones obtained with beams outlined by additional collimators.

[Advantages of a system of positioning and localization made of thermo-conformable materials for irradiation of cervicofacial and encephalic lesions treated with stereotactic fractionated radiation]. / Vantaggi offerti da un sistema di posizionamento e localizzazione di materiale termoconformabile per l'irradiazione di lesioni cervico facciali ed encefaliche trattate con tecnica stereotassica in frazionamento.

In this work we first analyzed the reliability of materials supplied by the main firms, which must be considered when defining the lesion and planning target volume. The coordinates of specific target landmarks, i.e., bone, calcium deposition or catheters, on baseline CT scans were compared with those measured on control scans. Since the PLATO-SRS system yields 3 coordinates for each target, the final error is calculated by the evaluation of the shift of the different coordinates. The mean error reported for the first material (ORFIT) was 5 mm (13/23 patients). This margin of error is too high and was considered unacceptable; a second material was then tested. The latter, which is not recyclable and softens at higher temperatures, exhibited a mean error of 2.5 mm (10/23 patients), which allows daily repositioning of greater reliability. Treatment was repeated in 4/20 arcs in all, which number depended mainly on fraction size. Correct lesion location and patient positioning were allowed by laser landmarks on the mask made directly on the treatment couch. The laser too was checked before each treatment session, i.e., preferentially at the end of an ordinary working day.