James E. Rodgers

Radiochromic film dosimetry: Recommendations of AAPM Radiation Therapy Committee Task Group 55

Recommendations of the American Association of Physicists in Medicine (AAPM) for the radiochromic film dosimetry are presented. These guidelines were prepared by a task group of the AAPM Radiation Therapy Committee and have been reviewed and approved by the AAPM Science Council.

Investigation of neutron-induced damage in DNA by atomic force microscopy: experimental evidence of clustered DNA lesions.

Using atomic force microscopy (AFM), we have investigated neutron-induced DNA double-strand breaks in plasmids in aqueous solution. AFM permits direct measurement of individual DNA molecules with an accuracy of a few nanometers. Furthermore, the analysis of the DNA fragment size distribution is non-parametric, whereas other methods are dependent on the model. Neutron irradiation of DNA results in the generation of many short fragments, an observation not made for damage induced by low-LET radiation. These data provide clear experimental evidence for the existence of clustered DNA double-strand breaks and demonstrate that short DNA fragments may be produced by such radiations in the absence of a nucleosomal DNA structure.

Spatial Distribution of Radiation-Induced Double-Strand Breaks in Plasmid DNA as Resolved by Atomic Force Microscopy

Abstract Pang, D., Rodgers, J. E., Berman, B. L., Chasovskikh, S. and Dritschilo, A. Spatial Distribution of Radiation-Induced Double-Strand Breaks in Plasmid DNA as Resolved by Atomic Force Microscopy. Radiat. Res. 164, 755–765 (2005). Atomic force microscopy (AFM) has been used to directly visualize, size and compare the DNA fragments resulting from exposure to low- and high-LET radiation. Double-stranded pUC-19 plasmid (“naked”) DNA samples were irradiated by electron-beam or reactor neutron fluxes with doses ranging from 0.9 to 10 kGy. AFM scanning in the tapping mode was used to image and measure the DNA fragment lengths (ranging from a few bp up to 2864 bp long). Double-strand break (DSB) distributions resulting from high-LET neutron and lower-LET electron irradiation revealed a distinct difference between the effects of these two types of radiation: Low-LET radiation-induced DSBs are distributed more uniformly along the DNA, whereas a much larger proportion of neutron-induced DSBs are distributed locally and densely. Furthermore, comparisons with predictions of a random DSB model of radiation damage show that neutron-induced DSBs deviate more from the model than do electron-induced DSBs. In summary, our high-resolution AFM measurements of radiation-induced DNA fragment-length distributions reveal an increased number of very short fragments and hence clustering of DSBs induced by the high-LET neutron radiation compared with low-LET electron radiation and a random DSB model prediction.

Dose-volume and complication in interstitial implants for breast carcinoma

A common radiotherapeutic technique for treating breast cancer is the combination of external beam radiation with an interstitial iridium-192 boost. When smaller tumors (T1 and T2) are treated using this technique, the soft tissue complication rate is small. However, with treatment of more advanced stages of disease, where large volumes of breast tissue must be treated to high radiation doses, the incidence of complication increases. This paper investigates the dose and volume relationships for breast tissue treated by interstitial technique and correlates this to the risk of soft tissue radiation injury. A method of analysis of interstitial radiation implants suitable for intra- or inter-institutional clinical evaluations is offered. The records of 111 patients treated at Georgetown University Hospital, were retrospectively analyzed and the five who had experienced radiation-related complications were compared to 51 randomly selected patients experiencing no complications. The volumes of tissue enclosed by selected isodose surfaces were calculated and used to determine a relationship between these dose-volumes and the probability of complication. The mean volume at specified dose levels between 10 Gy and 50 Gy was significantly higher (p less than .05) for the patients developing complications than those in whom no complications were seen. Using the 20 Gy isodose surface as defining our usual treated volume, a complication probability versus dose-volume curve was developed using a linear logistic model. The curve fitted the data closely (p less than .006) suggesting that, for our cases, the calculated treatment volume (within the 20 Gy isodose surface) can be used to effectively separate patients into groups that have different probabilities of developing complications. We propose this method as a basis for specification of dose and volume which can be used for clinical risk assessment, and for intra- and inter-institutional comparison.

Atomic force microscope imaging of DNA and DNA repair proteins: Applications in radiobiological research

By using the atomic force microscope (AFM), three-dimensional structures of biological specimens may be imaged at nanometer resolution. Furthermore, samples can be imaged in air or in fluid environments. The tapping mode of AFM operation for imaging has offered a significant advance in visualizing soft biological structures, such as DNA, proteins, and membranes. Here, we review the principles underlying the application of this instrument to radiation biological investigations. We focus on examples of proteins involved in the processes of repair of damaged DNA, including poly(ADP-ribose) polymerase, Ku protein, and DNA protein kinase. Novel observations on the character of DNA damage and repair have been addressed by direct visualization of DNA and protein-DNA interactions, such as the observation that the Ku protein is capable of physically joining DNA fragments in vitro. The AFM offers a powerful tool for investigating biologically important molecular interactions that are relevant to DNA damage and repair processes.

Wedge factor dependence on depth and field size for various beam energies using symmetric and half‐collimated asymmetric jaw settings

The depth- and field-size dependence of the in-phantom wedge factor have been determined for a Cobalt-60 (Co-60) teletherapy unit and four medical linear accelerators with 4-, 6-, 10-, and 18-MV x-ray beams containing 15 degrees-60 degrees (nominal) lead, brass, and steel wedge filters. Measurements were made with ionization chambers in solid water or water with a source-skin distance of 80 or 100 cm. Field sizes varied from 4 x 4 cm up to a maximum allowable size for each wedge filter. Measurements were performed for symmetric and half-collimated asymmetric fields at depth of maximum dose, 5- and 10-cm depths. For half-collimated fields, wedge factor reference points were located at a fixed off-axis distance from the collimators rotational axis. These systematic measurements on wedges indicate that the wedge factor dependence on depth and field size is a function of beam energy as well as the design of the treatment head and wedge filters. Significance of the results reported herein are discussed for the most commonly used treatment depths and field sizes with various beam energies and wedge filters.

Effects of beam spoiler on radiation dose for head and neck irradiation with 10-MV photon beam

PURPOSE To determine the effects of a lucite beam spoiler on the dose distribution to points inside and outside the primary beam for head and neck irradiation with a 10-MV photon beam. METHODS AND MATERIALS Build-up and depth-dose measurements were performed with a parallel-plate ionization chamber for 5 x 5, 10 x 10, and 15 x 15-cm field sizes using lucite spoilers with two different thicknesses at two different lucite-to-skin distances (LSD) for a 10-MV x-ray beam. Corrections were applied to account for finite chamber size. Beam profiles and isodose curves were obtained at several depths using film dosimetry. Beam uniformity was determined from uniformity indices. Peripheral doses (PD) were measured at the surface and at 1.5- and 2.5-cm depths using film dosimetry and a parallel-plate ionization chamber. Measurement points were positioned at the edge of a 10 x 10-cm field and at distances extending to 5.0 cm away. The treatment planning data for the 10-MV x-ray beam were modified to account for the effects of the beam spoiler when treating head and neck patients. RESULTS The spoiler increased the surface and build-up dose and shifted the depth of maximum dose toward the surface. With a 10-MV x-ray beam and a 1.2-cm-thick lucite at 15 cm LSD, a build-up dose similar to a 6-MV x-ray beam was achieved. The beam uniformity was altered at shallow depths. The peripheral dose was enhanced particularly at the surface and at the points close to the beam edge. The effects of the beam spoiler on beam profile and PD were reduced with increasing depths. CONCLUSION The lucite spoiler allowed use of a 10-MV x-ray beam for head and neck treatment by yielding a build-up dose similar to that of a 6-MV x-ray beam while maintaining skin sparing. The increase in PD was at superficial depths and was reduced at points away from the edge; therefore, it is clinically nonsignificant. Spoiling the 10-MV x-ray beam resulted in treatment plans that maintained dose homogeneity without the consequence of increased skin reaction or treatment volume underdose for regions near the skin surface.

Scatter fractions from linear accelerators with x-ray energies from 6 to 24 MV.

Computation of shielding requirements for a linear accelerator must take into account the amount of radiation scattered from the patient to areas outside the primary beam. Currently, the most frequently used data are from NCRP 49 that only includes data for x-ray energies up to 6 MV and angles from 30 degrees to 135 degrees. In this work we have determined by Monte Carlo simulation the scattered fractions of dose for a wide range of energies and angles of clinical significance including 6, 10, 18, and 24 MV and scattering angles from 10 degrees to 150 degrees. Calculations were made for a 400 cm2 circular field size impinging onto a spherical phantom. Scattered fractions of dose were determined at 1 m from the phantom. Angles from 10 degrees to 30 degrees are of concern for higher energies where the scatter is primarily in the forward direction. An error in scatter fraction may result in too little secondary shielding near the junction with the primary barrier. The Monte Carlo code ITS (Version 3.0) developed at Sandia National Laboratory and NIST was used to simulate scatter from the patient to the barrier. Of significance was the variation of calculated scattered dose with depth of measurement within the barrier indicating that accurate values may be difficult to obtain. Mean energies of scatter x-ray spectra are presented.

Digital image processing of radiation therapy portal films

Digital image processing has the potential to enhance and improve several functions of a modern radiation oncology department. These functions may include improving perception of information for low contrast films, electronic transfer of images to remote facilities and back, and reducing storage space requirements for archiving once treatment is finished. This paper gives an overview of the digitization process and of image processing fundamentals. The clinical evaluation of digitized portal films is also discussed. The authors conclude that digitizing low contrast radiation therapy portal films is feasible with present technology and will produce images acceptable for routine clinical use in most instances. The role of image enhancement is less well established and remains investigational.

Dosimetric parameters of a modified set of wedges for use with asymmetric fields of a 6 MV linear accelerator

A set of standard wedge filters has been modified for use with half-collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half-collimated field coverage (up to 20 x 30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off-axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.