Research in Radiation Physics

Evaluation of Dosimetry at the Lung Tumor Interface

Ramaswamy Sadagopan, M.S.

The dosimetry of photon beam in heterogeneous media such as lung and implications of lung tumor motion have been reported. However, there are relatively fewer reports on the dosimetry at the periphery of lung tumors and especially, the variation of peripheral tumor dose with respect to the incident photon beam angle has not been reported. We aim to study the variation of dose within the finger like extensions of a solid tumor surrounded by lung tissue due to 6 and 18 MV photon beams. The study will compare doses measured at various locations inside a lung tumor and its fingerlike extensions using TLDs and film, against the doses calculated by the MCNPX, Monte Carlo method and other calculation algorithms that are widely employed by the clinical treatment planning systems.

A special lung phantom that consists of 3-cm diameter acrylic sphere (representing solid tumor) in the middle of a block of cork representing lung was designed for this purpose. TLDs will be placed at various locations within the sphere and just outside the sphere. The TLDS outside the will simulate the finger like extension of some lung tumors. A set of thin acrylic rods is attached to each side of the sphere. On the outsides cork phantom handles and protractor are attached to the plastic tubes to facilitate the rotation sphere. Two 2.8-cm thick acrylic plates which simulate the chest wall sandwich the entire cork phantom.  

The TLDS inside the phantoms will be exposed to 6 and 18 MV beams. Square beam of cross-section and 100cm SSD are used. The field size chosen is 10x10-cmsq so that the dose at the periphery of the acrylic sphere is not influenced by the beam penumbra. The exposures and calculations will be repeated at every 300 of the fingerlike extension with respect to the incident beam angle. The measured and calculated doses are normalized to the dose at the center of the sphere and results will be tabulated and discussed.

Respiratory Correlated Radiation Therapy

George Starkschall, Ph.D.

My present research interests are in studying respiratory motion during the delivery of radiation therapy and assessing methods for reducing the effects of motion. In order to account for the motion of lung tumors during respiration, radiation fields treating these tumors traditionally have included large margins around the tumor. Because of these margins, excessive uninvolved lung tissue is often irradiated to unacceptably high dose levels. By controlling the amount of tumor motion during irradiation, along with precisely localizing the lung tumor, we hope to reduce the treatment field margins, allowing for more lung sparing and, potentially, higher radiation doses to the tumor.

We have been collaborating with Philips Medical Systems in the development of a process for acquiring moving three-dimensional CT images of lung during respiration. With the use of a high-speed CT scanner, we acquire images of sections of the patient during several phases of a respiratory cycle. With the aid of a respiratory monitor, we combine multiple sets of images acquired at common phases over several respiratory cycles. The resulting “four-dimensional” CT image data sets have provided us with a significant amount of information regarding the actual motion of lung tumors during respiration.

A related area of interest is the use of implanted fiducials to monitor respiratory motion. Present technologies use an external monitor, typically motion of the anterior abdominal surface, as a surrogate for tumor motion. More precise localization of the tumor during respiration can be achieved by means of one or more small gold markers implanted in the vicinity of the tumor. We are presently involved in a patient protocol to assess the safety and reliability of these implanted markers and their effectiveness in monitoring tumor motion during respiration.