X‐ray surface dose measurements using TLD extrapolation

Abstract

Surface dose measurements in therapeutic x-ray beams are of importance in determining the dose to the skin of patients undergoing radiotherapy. Measurements were performed in the 6-MV beam of a medical linear accelerator with LiF thermoluminescence dosimeters (TLD) using a solid water phantom. TLD chips (surface area 3.17 x 3.17 cm2) of three different thicknesses (0.230, 0.099, and 0.038 g/cm2) were used to extrapolate dose readings to an infinitesimally thin layer of LiF. This surface dose was measured for field sizes ranging from 1 x 1 cm2 to 40 x 40 cm2. The surface dose relative to maximum dose was found to be 10.0% for a field size of 5 x 5 cm2, 16.3% for 10 x 10 cm2, and 26.9% for 20 x 20 cm2. Using a 6-mm Perspex block tray in the beam increased the surface dose in these fields to 10.7%, 17.7%, and 34.2% respectively. Due to the small size of the TLD chips, TLD extrapolation is applicable also for intracavity and exit dose determinations. The technique used for in vivo dosimetry could provide clinicians information about the build up of dose up to 1-mm depth in addition to an extrapolated surface dose measurement.

Surface dose measurements with commonly used detectors: a consistent thickness correction method

“…Previously reported extrapolated results for EDR2, EBT2, and TLDs were also in good agreement with our results 7 , 13 , 15 , 16 . The extrapolation method, using a linear fit, is demonstrated in Fig.…”

“…Surface dose overestimation by more than 10% has been reported for TLD chips by both Gerbi and Khan (2) and Kron et al, (7) and for both EDR2 (12) and EBT2 (15) …”

“…In another study extrapolation using three different thickness TLDs showed a slight overestimation of surface dose demonstrating nonlinear TLD response versus TLD thickness 7 , 8 . This is not the case for TLD powder (pTLD), and a monolayer of powder can be used for surface dose measurements (2) .…”

Surface buildup dose dependence on photon field delivery technique for IMRT

“…In the buildup dose region, it is known that the thickness of the TLD chips causes a systematic error in depth dose measurements. This problem was alleviated by using an extrapolation method (LiF, $3.175\times 3.175\hspace{0.17em}{\text{mm}}^2$ chips with three thicknesses, 0.89 mm, 0.38 mm, and 0.15 mm) (21) . The accuracy of this technique was verified using a static open field for comparison of depth doses as measured by the TLD extrapolation method and the Attix chamber.…”

Radiographic film dosimetry for IMRT fields in the near‐surface buildup region

“…The small $\left(3.18\times 3.18\hspace{0.17em}\text{mm}\right)$ size of the TLD is ideal for point dose small field measurements. Since the thickness of the TLD can bias the results in the buildup region, an extrapolation method (15) using TLD chips of 0.89 mm, 0.38 mm and 0.15 mm was used to extrapolate to dose at zero thickness for measurements shallower than depth of maximum dose. The TLD chips were calibrated at a 10 cm depth using a $10\times 10\hspace{0.17em}\text{cm}$ field size to determine individual sensitivity factors.…”