Evaluation of dosimetric characteristics of a grid block fabricated for mega-voltage grid therapy purposes

In conventional radiation therapy, regarding normal tissue tolerance, the treatment of bulk tumors is one of the remaining challenges. Grid Radiation Therapy [GRT] is a technique to deliver high doses, approximately 15 - 20 Gy per fraction, to several small volumes located in a large radiation field. This can be performed using a grid block. The current work has concentrated on the dosimetric characteristics of a designed mega-voltage grid, used for a unique treatment modality. All measurements performed using a Neptune linear accelerator [9 MV photon beam]. A square 16 x 16 array grid block was designed and fabricated. Several dosimetric characteristics including: depth dose, Valley To Peak [VTP] ratio, and grid out-put factor were evaluated using a calibrated diode dosimeter for a range of radiation fields. The percent depth dose curves, measured in the presence of grid block, lie within those measured for the corresponding open field and a narrow beam. At the D[max], the VTP ratio was found to be within 17% - 28%, while these ranges between 23% - 35% at a depth of 10 cm. The grid out -put factor found to be 0.78 and it slightly decreases with increasing of radiation field size. The VTP ratio found to be dependent strongly on the grid design and manufacturing properties. However, other parameters such as radiation field size and the depth of measurement should also be addressed as important factors. The measured dosimetric characteristics of grid block indicate that the mega-voltage grid therapy can be applied as a possible clinical modality for palliative cases

[Accuracy of cone beam computed tomography, intraoral radiography and periodontal probing for periodontal bone defects measurement]

Cone beam computed tomography [CBCT] produces high-quality data about diagnosis and periodontal treatment. To date, there is not enough research regarding periodontal bone measurement using CBCT. The aim of this study was to compare the accuracy of CBCT in measuring periodontal defects to that of intraoral radiography and probing methods. Two-hundred and eighteen artificial osseous defects [buccal and lingual infrabony, interproximal, horizontal, crater, dehiscence and fenestration defects] were created on 13 mandibles of dry skulls. The mandibles were put into a plexiglass box full of water to simulate soft tissue. CBCT images, radiographic images taken with parallel technique and direct measurements using a WHO periodontal probe were recorded and compared to a standard reference [digital caliper]. Inter and intra observe consistencies were assessed using Intra class correlation coefficient and pearson correlation. Inter and intra observer consistencies were high for CBCT and probing methods [ICC-Intra class correlation coefficient>88%], but moderate for intraoral radiography [ICC-Intra class correlation coefficient>54%]. There were not any significant differences between observers for all techniques [P>0/05]. According to paired T-test analysis, mean difference for CBCT technique [0.01 mm] was lower than that for probing [0.04 mm] and radiography [0.62 mm]. CBCT was able to measure all kinds of lesions, but radiography could not measure defects in the buccal and lingual sites. All three modalities are useful for identifying periodontal defects. Compared to probing and radiography, the CBCT technique has the most accuracy in measuring periodontal defects