Sci-Thurs PM: Delivery-02: Improving image quality produced by CCD cameras exposed to stray radiation from a medical linac.

PURPOSE: Charge coupled devices (CCDs) are increasingly used in radiation therapy. CCDs are ideal for applications such as two-dimensional dosimetry of scintillator sheets or to read arrays of miniature scintillation detectors. However, CCDs are sensitive to stray radiation. Radiation-induced noise strongly alters images and limits their quantitative analysis. We have characterized radiation-induced noise and developed filtration algorithms to restore image quality. METHOD AND MATERIALS: Two models of CCD cameras were used for measurements in linac environments. Images were acquired with and without radiation. The structure of the transient noise was characterized. Then, four methods of noise filtration were compared: median filtering of a time series of images, uniform median filtering of single images, an adaptive filter with switching mechanism and a modified version of the adaptive filter. RESULTS: The intensity distribution of noisy pixels was similar in both cameras. However, the spatial distribution of the noise was different: the average noise cluster size was 1.2±0.6 and 3.2±2.7 pixels for each of the two cameras. The median of a time series of image resulted in the best filtration and minimal image distortion. For applications where time series is impractical, adaptive filtration must be used to reduce image distortion. CONCLUSION: We have characterized the transient noise produced in CCDs by scattered radiation from a linac and have developed an efficient filtration scheme to remove this noise and restore image quality. Use of our filtration scheme allows detailed quantitative analysis of an image even when subjected to scattered radiation.

Intraoperative radiation therapy using a mobile electron linear accelerator: field matching for large-field electron irradiation.

Intraoperative radiation therapy (IORT) consists of delivering a large, single-fraction dose of radiation to a surgically exposed tumour or tumour bed at the time of surgery. With the availability of a mobile linear accelerator in the OR, IORT procedures have become more feasible for medical centres and more accessible to cancer patients. Often the area requiring irradiation is larger than what the treatment applicators will allow, and therefore, two or more adjoining fields are used. Unfortunately, the divergence and scattering of the electron beams may cause significant dose variations in the region of the field junction. Furthermore, because IORT treatments are delivered in a large single fraction, the effects of underdosing or overdosing could be more critical when compared to fractionated external beam therapy. Proper matching of the fields is therefore an important technical aspect of treatment delivery. We have studied the matching region using the largest flat applicator available for three different possibilities: abutting the fields, leaving a small gap or creating an overlap. Measurements were done using film dosimetry for the available energies of 4, 6, 9 and 12 MeV. Our results show the presence of clinically significant cold spots for the low-energy beams when the fields are either gapped or abutted, suggesting that the fields should be overlapped. No fields should be gapped. The results suggest that an optimal dose distribution may be obtained by overlapping the fields at 4 and 6 MeV and simply abutting the fields at 9 and 12 MeV. However, due to uncertainties in the placement of lead shields during treatment delivery, one may wish to consider overlapping the higher energy fields as well.

Preliminary evaluation of implantable MOSFET radiation dosimeters.

In this paper, we report on measurements performed on a new prototype implantable radiation detector that uses metal-oxide semiconductor field effect transistors (MOSFETs) designed for in vivo dosimetry. The dosimeters, which are encapsulated in hermetically sealed glass cylinders, are used in an unbiased mode during irradiation, unlike other MOSFET detectors previously used in radiotherapy applications. They are powered by radio frequency telemetry for dose measurements, obviating the need for a power supply within each capsule. We have studied the dosimetric characteristics of these MOSFET detectors in vitro under irradiation from a 60Co source. The detectors show a dose reproducibility generally within 5% or better, with the main sources of error being temperature fluctuations occurring between the pre- and post-irradiation measurements as well as detector orientation. A better temperature-controlled environment leads to a reproducibility within 2%. Our preliminary in vitro results show clearly that true non-invasive in vivo dosimetry measurements are feasible and can be performed remotely using telemetric technology.

Non-Fermi-liquid spin dynamics in CeCoGe3-xSi(x) for x = 1.2 and 1.5.

Muon spin relaxation has been measured in CeCoGe3-xSi(x) at the magnetic/nonmagnetic boundary compositions of x = 1.2 and x = 1.5. Both the alloys are found to exhibit an ordered region and a disordered region. At x = 1.2, short-range magnetic ordering is observed below 0.86 K in the ordered region. The disordered region is paramagnetic and the muon spin-lattice relaxation rate lambda2 in this region displays non-Fermi-liquid (NFL) spin dynamics, i.e., the power law lambda2 proportional to T0.72 which shows the formation of Griffiths phase. lambda2 in the x = 1.5 alloy displays logarithmic (NFL) scaling below 1 K, in agreement with the theory of a T = 0 K magnetic transition.