Subhash C. Sharma

A comparison of surface doses for two immobilizing systems

To achieve accurate and reproducible treatments, many types of immobilizing systems have been used. Two of these systems: Alpha Cradle and VacFix, conform around the patient to improve day-to-day treatment repositioning. The Alpha Cradle system has been in clinical use for several years. The system involves two polyurethane chemicals which upon mixing in a latex bag will expand and harden around the patient. The second system (VacFix) uses a 0.15 mm thick plastic bag loosely filled with 1 mm polysterol spheres. The patient lays on the bag in the treatment position and the air is evacuated from the bag. This system retains its shape for the entire treatment and can be re-used. In some patient set-ups, it is possible that the beam is modified as it passes through these devices, causing a potential change in the target dose and an increase in the patient surface dose. CT scans of the Alpha Cradle and VacFix systems were performed and relative densities were calculated and compared to air. Material densities and surface dose data for a Cobalt-60 teletherapy unit and a dual photon linear accelerator are presented.

Clinical considerations in the use of missing tissue compensators for head and neck cases

The irregular shape or contour of the patients surface in the treatment field can alter the dose distribution resulting in non-uniformity of dose in the treatment volume. Missing tissue compensators have been most commonly used to improve this non-uniformity, especially in head & neck, breast, lung and supraclavicular regions. Two or three dimensional compensators have been typically designed to make the dose uniform at a specific depth. This compensation shifts the dose distribution within the treatment volume so that some structures may be under or over compensated. This study will examine how various sites in head and neck cases are affected by compensators. We have also analyzed the uncertainty in compensated dose due to the daily variations in patient repositioning. Computer isodose plans using Cobalt-60 gamma rays and 6 and 18 MV x-rays were generated using coronal contours. Results show that the dose uniformity is improved for the treatment sites, especially for the thinner sites, like the larynx and the anterior cervical neck nodes. Finally, patient movement or positioning errors of +/-1.0 cm will cause a change in dose distribution.

Border separation for adjacent orthogonal fields

Field border separations for adjacent orthogonal fields can be calculated geometrically, given the validity of some important assumptions such as beam alignment and field uniformity. Thermoluminescent dosimetry (TLD) measurements were used to investigate dose uniformity across field junctions as a function of field separation and, in particular, to review the CCSG recommendation for the treatment of medulloblastoma with separate head and spine fields.

Electron beam effective source surface distances for a high energy linear accelerator

The design of the Varian Clinac 1800 linear accelerator electron applicator system does not allow clearance for all head and neck patients to be treated at the standard calibration distance of 100 cm. Discrepancies have been found between dose calculations using the inverse square law for extended distances and their measured data. A 4 X 4 cm2 applicator at an energy of 9 MeV, for example, had dose differences of 13 and 23% at distances of 105 and 110 cm SSD. Because of these discrepancies, effective source surface distances (SSDeff) were determined for all the standard electron energies and applicators of a Clinac 1800. These effective source surface distances ranged from 41.6 cm to 92.6 cm for the 4 X 4 cm2 cone/6 MeV electron beam through the 25 X 25 cm2 cone/20 MeV electron beam. A summary of these distances and an analysis of the clinical use of both a best fit SSDeff and a common SSDeff for patient dosimetry calculations is presented.

A new design of Delclos dome cylinders using standard cs-137 sources

Surface dose rates around the currently-marketed Delclos uterine-vaginal afterloading dome (hemispherical) cylinders were calculated and measured for linear standard 3M cesium tube sources. Measurements were carried out using thin thermoluminescent lithium fluoride Chips on the surface of the cylinder and calculations at the same points were generated using a treatment planning computer. Wide surface dose variations were found for 2 to 3.5 cm diameter cylinders, but relatively small variation for 4 to 4.5 cm diameter cylinders. Attempting to achieve a uniform dose distribution around the entire dome surface of the cylinder, we have developed a new ellipsoidal design for the dome component that better conforms to the shape of the isodoses arising from the distal-most source. Thermoluminescent dosimetry indicates that the surface doses for the newly constructed cylinders are quite uniform, with variation within +/- 5%. The effect on surface dose is discussed when the ellipsoidal dome cylinder in combination with vaginal cylinders is used and multiple sources are laid end to end to treat the added areas of the vaginal wall.

Clinical considerations in the use of missing tissue compensators for thoracic cases

The irregular shape or contour of the patients surface in the treatment field can alter the dose distribution resulting in non-uniformity of dose in the treatment volume. Missing tissue compensators have been most commonly used to improve this non-uniformity, especially in head and neck, breast, lung and supraclavicular regions. Two or three dimensional compensators have been typically designed to make the dose uniform at a specific depth. This compensation shifts the dose distribution within the treatment volume so that some structures may be under or over compensated. This study will examine how various thoracic sites are affected by compensators, with and without heterogeneity corrections. We have also analyzed the uncertainty in compensated dose due to the daily variations in patient repositioning. Computer isodose plans using Cobalt-60 gamma rays and 6 and 18 MV X-rays were generated using sagittal contours. Results show that superficial sites, like the supraclavicular nodes and the spinal cord, do not receive as consistent a dose compensation as the midplane sites. In addition, the compensated lung dose increases. Finally, patient movement or positioning errors of +/- 1.0 cm causes only a slight change in dose distribution.

Strontium-90 eye application

Details of the Strontium-90 Eye Application including equipment, irradiation technique, radiation safety and data are presented.

Irradiating with a Prosthesis or Metal Pin in the Radiation Treatment Field

This is a case study of a patient who had a liposarcoma of the tibia and also had a titanium pin directly in the treatment field. A total midline dose of 63 Gy was prescribed. The effects of the pin in the treated area was examined. Four different methods were used to examine and verify the delivered dose: (1) Treatment planning computerized isodose curves, (2) A parallel plate ion chamber and polystyrene phantom, (3) TLD, and (4) Film dosimetry. The results from all four methods of dose examination are compiled and are used to make a decision on what to do about the dose non-uniformity due to the dense titanium pin. If a prosthesis or metal pin is in the treatment area, one must determine the composition of the pin. Then one should verify or examine the results of the dose perturbations produced. One may or may not want to change the original dose prescription but the dose throughout the irradiated volume will be changed due to the metal pin. It is important to evaluate these effects, as discussed in this article, to accurately deliver a prescribed dose.

Photon beam attenuation for a patient support assembly during arc therapy for a medical linear accelerator.

Arc therapy is one of the treatment techniques for small, centrally located deep-seated tumors. However, care must be taken to remove any components that would interfere with the beam as the gantry rotates around the patient. One such component that may interfere with the beam is the patient support assembly (PSA) or treatment table. Beam attenuation factors due to the presence of the couch side-rails and the centerspine bar of the PSA are presented for both 6 MV and 18 MV photon beams of a Clinac 1800 during a 360 degrees rotation. Dose perturbations arising from these obstructions are displayed using an Alderson Rando phantom. A method is described to calculate the start and stop angles for the largest unobstructed arc for a given field size (FS), centerspine bar to the isocenter height (H), centerspine bar width (W), and SAD. As an illustration, for an SAD of 100 cm and a W of 4 cm, the start and stop angles for arc therapy with a FS of 10 x 10 cm2 and an H of 12.7 cm would be 29 degrees and 331 degrees.

Procedures for interstitial radioactive gold grains

Detailed implantation procedures and safety guidelines for use of interstitial radioactive gold grains are presented. These guidelines have been found to be of practical value for personnel involved with the implant to ensure compliance with regulations but are not necessarily the only procedures which could be utilized. Topics covered include: Description of Grains and Implantation, Management and Planning of Au-198, Source Logging, Source Transportation, Source Accounting During and After Implant, Room Monitoring, Recording, Dosimetry Films, Nursing Procedures, Levels in Patients Containing Radioactivity, and Patient Discharge of Radioactive Patients.