Determination of field size limitations in stereotactic and intensity modulated radiotherapy

Abstract

For dose measurement in small photon fields, different detectors are currently in use: TLD, semiconductor, diamond-detector, film, etc. But for absolute dosimetry, ionization chambers show the most advantages. To meet the basic dosimetrical requirements for lateral electron equilibrium the field size F must not remain under specified values: i.e. 5.2 x 5.2 cm2 for 15 MeV X-bremsstrahlung. As well as increasing the focus-chamber-distance, changing the physical density of the build-up material in the close vicinity of the chamber will be helpful to determine the output factor OF for smaller fields. By means of a correction factor, k(mat), this is taken into account. For a 6 MeV X and a 15 MeV X-bremsstrahlung of linear accelerators the lower limit of the field size F is determined: F > or = 0.8 cm. This value is mainly dependent on the diameter of the focal-spot (phi = 3 mm) of the treatment unit including design characteristics of the treatment head. Beside the dosimetrical aspects, some geometrical parameters have to be considered, when accuracy of dose application should remain on the same level as in medium and large field treatment (4 cm < or = F < or = F(max)). To keep dose-volume errors as low as +/- 10 % (diameter of PTV: 20 mm), the mean total error delta of CT-scanning (delta(p)), planning (delta(pl)), patient positioning (delta(x)), and treatment unit instabilities (delta(m)) should not exceed +/- 0.8 mm.

Evaluation of the Therapeutic Role of the Low Level Laser on the Gamma Irradiated Teeth and the Antimicrobial Salivary Biomarkers in Rat’s Model: an Experimental Study

“…Moreover, irradiated rats revealed a significant depletion in the salivary PH and salivary IgA, compared to control group. Salivary gland injury accompanied with structural modification and functional constraint is a result of radiotherapy in the head and neck area, leading to complications, including mucositis, xerostomia and hyposalivation, where it is not potential to avoid salivary glands from the treatment field (12,13) .…”

Thirty year celebration of journal publications on radiation oncology medical physics

“…Just how small these beams can be with adequate beam coverage and accuracy is an important aspect to investigate. This was the subject of a 2003 report by an overseas contributor, Jenson et al 65 . The authors concluded that the beam size needed to be at least 1 x 1 cm 2 to guarantee sufficient accuracy in the dose calculation.…”

Effect of scatter material on diode detector performance forin vivodosimetry

“…The scatter material made from water-equivalent materials has the disadvantage of being bulky, causing problems with patient comfort and with maintaining contact with the patient's skin (Jornet et al 2004). The volume of the scatter material required can be reduced by increasing the density of the scatter material surrounding the detector by using thin metal caps (Haryanto et al 2002, Jensen et al 2003. Dosimeters with metal caps perform well for medium to large fields; however, the inclusion of these caps results in the overestimation of dose in small fields due to the increased probability of interaction in the cap compared to a water-equivalent material, increasing the amount of scatter reaching the active volume.…”