Comparison of Air-Gaps Effect in a Small Cavity on Dose Calculation for 6 MV Linac.

BACKGROUND: Many authors stated that cavities or air-gaps were the main challenge of dose calculation for head and neck with flattening filter medical linear accelerator (Linac) irradiation. OBJECTIVE: The study aimed to evaluate the effect of air-gap dose calculation on flattening-filter-free (FFF) small field irradiation. MATERIAL AND METHODS: In this comparative study, we did the experimental and Monte Carlo (MC) simulation to evaluate the presence of heterogeneities in radiotherapy. We simulated the dose distribution on virtual phantom and the patient's CT image to determine the air-gap effect of open small field and modulated photon beam, respectively. The dose ratio of air-gaps to tissue-equivalent was calculated both in Analytical Anisotropic Algorithm (AAA) and MC. RESULTS: We found that the dose ratio of air to tissue-equivalent tends to decrease with a larger field size. This correlation was linear with a slope of -0.198±0.001 and -0.161±0.014 for both AAA and MC, respectively. On the other hand, the dose ratio below the air-gap was field size-dependent. The AAA to MC dose calculation as the impact of air-gap thickness and field size varied from 1.57% to 5.35% after the gap. Besides, patient's skin and oral cavity on head and neck case received a large dose discrepancy according to this study. CONCLUSION: The dose air to tissue-equivalent ratio decreased with smaller air gaps and larger field sizes. Dose correction for AAA calculation of open small field size should be considered after small air-gaps. However, delivered beam from others gantry angle reduced this effect on clinical case.

RADIATION DOSE FROM DENTAL RADIOGRAPHY IN INDONESIA: A FIVE-YEAR SURVEY.

Dose metrics on intraoral and panoramic dental radiography were analyzed to preliminarily indicate dose trend of dental radiology equipment in Indonesia. Measurements were performed in 71 healthcare institutions involving a total of 92 dental X-ray devices. Imaging modes are categorized into 'low dose' and 'high dose' based on the task. The 75th percentile for intraoral IAK was 2.8 and 4.3 mGy for 'low' and 'high' dose modes, respectively while for panoramic devices the 75th percentile of the KAP for 'low' and 'high' dose modes were 85.2 and 192.4 mGy cm2, respectively. Results compared with local regulatory recommendation for intraoral units indicated that 38.8 and 6.3% of measured dose modes on analogue and digital devices, respectively, were above the recommended range. The dose trend can be used as reference for local regulations concerning patient dose in dental radiography and preliminary value prior to the establishment of national diagnostic reference level.

Brief histories of medical physics in Asia-Oceania.

The history of medical physics in Asia-Oceania goes back to the late nineteenth century when X-ray imaging was introduced, although medical physicists were not appointed until much later. Medical physics developed very quickly in some countries, but in others the socio-economic situation as such prevented it being established for many years. In others, the political situation and war has impeded its development. In many countries their medical physics history has not been well recorded and there is a danger that it will be lost to future generations. In this paper, brief histories of the development of medical physics in most countries in Asia-Oceania are presented by a large number of authors to serve as a record. The histories are necessarily brief; otherwise the paper would quickly turn into a book of hundreds of pages. The emphasis in each history as recorded here varies as the focus and culture of the countries as well as the length of their histories varies considerably.

Validation for 2D/3D registration. I: A new gold standard data set.

PURPOSE: In this article, the authors propose a new gold standard data set for the validation of two-dimensional/three-dimensional (2D/3D) and 3D/3D image registration algorithms. METHODS: A gold standard data set was produced using a fresh cadaver pig head with attached fiducial markers. The authors used several imaging modalities common in diagnostic imaging or radiotherapy, which include 64-slice computed tomography (CT), magnetic resonance imaging using T1, T2, and proton density sequences, and cone beam CT imaging data. Radiographic data were acquired using kilovoltage and megavoltage imaging techniques. The image information reflects both anatomy and reliable fiducial marker information and improves over existing data sets by the level of anatomical detail, image data quality, and soft-tissue content. The markers on the 3D and 2D image data were segmented using ANALYZE 10.0 (AnalyzeDirect, Inc., Kansas City, KN) and an in-house software. RESULTS: The projection distance errors and the expected target registration errors over all the image data sets were found to be less than 2.71 and 1.88 mm, respectively. CONCLUSIONS: The gold standard data set, obtained with state-of-the-art imaging technology, has the potential to improve the validation of 2D/3D and 3D/3D registration algorithms for image guided therapy.

Efficient implementation of the rank correlation merit function for 2D/3D registration.

A growing number of clinical applications using 2D/3D registration have been presented recently. Usually, a digitally reconstructed radiograph is compared iteratively to an x-ray image of the known projection geometry until a match is achieved, thus providing six degrees of freedom of rigid motion which can be used for patient setup in image-guided radiation therapy or computer-assisted interventions. Recently, stochastic rank correlation, a merit function based on Spearman's rank correlation coefficient, was presented as a merit function especially suitable for 2D/3D registration. The advantage of this measure is its robustness against variations in image histogram content and its wide convergence range. The considerable computational expense of computing an ordered rank list is avoided here by comparing randomly chosen subsets of the DRR and reference x-ray. In this work, we show that it is possible to omit the sorting step and to compute the rank correlation coefficient of the full image content as fast as conventional merit functions. Our evaluation of a well-calibrated cadaver phantom also confirms that rank correlation-type merit functions give the most accurate results if large differences in the histogram content for the DRR and the x-ray image are present.