In vivo dosimetry with optically stimulated luminescent dosimeters, OSLDs, compared to diodes; the effects of buildup cap thickness and fabrication material

Journal Article · Sat Oct 15 00:00:00 EDT 2011 · Medical Physics

DOI:https://doi.org/10.1118/1.3633939· OSTI ID:22098636

Jursinic, Paul A.; Yahnke, Clifford J. ^[1]

---

1. West Michigan Cancer Center, 200 North Park St. Kalamazoo, Michigan 49007 (United States)

Purpose: For external beam in vivo measurements, the dosimeter is normally placed on the patient's skin, and the dose to a point of interest inside the patient is derived from surface measurements. In order to obtain accurate and reliable measurements, which correlate with the dose values predicted by a treatment planning system, a dosimeter needs to be at a point of electronic equilibrium. This equilibrium is accomplished by adding material (buildup) above the detector. This paper examines the use of buildup caps in a clinical setting for two common detector types: OSLDs and diodes. Clinically built buildup-caps and commercially available hemispherical caps are investigated. The effects of buildup cap thickness and fabrication material on field-size correction factors, C{sub FS}, are reported, and differences between the effects of thickness and fabrication material are explained based on physical parameters. Methods: Measurements are made on solid water phantoms for 6 and 15 MV x-ray beams. Two types of dosimeters are used: OSLDs, InLight/OSL Nanodot dosimeters (Landauer, Inc., Glenwood, IL) and a P-type surface diode (Standard Imaging, Madison, WI). Buildup caps for these detectors were fabricated out of M3, a water-equivalent material, and sheet-metal stock of Al, Cu, and Pb. Also, commercially available hemispherical buildup caps made of plastic water and brass (Landauer, Inc., Glenwood, IL) were used with Nanodots. OSLDs were read with an InLight microStar reader (Landauer, Inc., Glenwood, IL). Dose calculations were carried out with the XiO treatment planning system (CMS/Elekta, Stockholm) with tissue heterogeneity corrections. Results: For OSLDs and diodes, when measurements are made with no buildup cap a change in C{sub FS} of 200% occurs for a field-size change from 3 cm x 3 cm to 30 cm x 30 cm. The change in C{sub FS} is reduced to about 4% when a buildup cap with wall thickness equal to the depth of maximum dose is used. Buildup caps with larger wall thickness do not cause further reduction in C{sub FS}. The buildup cap fabrication material has little or no effect on C{sub FS}. The perturbation to the delivered dose caused by placing a detector with a buildup cap on the surface of a patient is measured to be 4%-7%. A comparison between calculated dose and dose measured with a Nanodot and a diode for 6 and 15 MV x-rays is made. When C{sub FS} factors are carefully determined and applied to measurements made on a phantom, the differences between measured and calculated doses were found to be between {+-}1.3%. Conclusions: OSLDs and diodes with appropriate buildup caps can be used to measure dose on the surface of a patient and predict the delivered dose to depth dmax in a range of {+-}1.3% for 100 cGy. The buildup cap: can be fabricated from any material examined in this work, is best with wall thickness dmax, and causes a perturbation to the delivered dose of 4%-7% when the wall thickness is dmax. OSLDs and diodes with buildup caps can both give accurate measurements of delivered dose.

• OSTI ID: 22098636
• Journal Information: Medical Physics, Vol. 38, Issue 10; Other Information: (c) 2011 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
62 RADIOLOGY AND NUCLEAR MEDICINE
COMPARATIVE EVALUATIONS
DOSIMETRY
IN VIVO
LUMINESCENCE
PATIENTS
PERTURBATION THEORY
PHANTOMS
PLANNING
QUANTUM DOTS
RADIATION DOSES
RADIATION MONITORING
RADIATION PROTECTION
RADIOTHERAPY
SKIN
THERMOLUMINESCENT DOSEMETERS
X RADIATION