Validity of transition‐zone dosimetry at high atomic number interfaces in megavoltage photon beams

Abstract

Measurement of dose or dose perturbation factors at high atomic number interfaces are usually performed with a thin-window parallel-plate ion chamber. In a transition region, under nonequilibrium conditions, accuracy of ion chamber readings for the dose measurements has often been questioned. This paper critically analyzes the factors (stopping power ratio and charge collection) for the dose measurements at interfaces. Monte Carlo simulations were performed to investigate the secondary electron spectrum produced by photon beams and to calculate the stopping power ratios at the point of measurement. The validity of dose measurements was studied for the photon beams in the range of Co-60 gamma rays to 24-MV x rays at bone and lead interfaces with polystyrene, using thermoluminescent dosimeters, extrapolation chamber and several types of commercially available parallel-plate ion chambers. It is observed that for energies greater than 10 MV most parallel-plate chambers can be used to measure dose accurately. At lower energies, however significant differences between measured doses with different detectors were noticed. It is suggested that at high-Z interfaces and lower energies, the dose measurements should be performed with ultrathin-window parallel-plate ion chambers or extrapolation chambers.

Influence of backscatter radiation on cranial reconstruction implants

“…It has long been known that when metal inhomogeneities with high atomic numbers are in the path of photons or electron beams, the dose distribution is altered and regions of overdosage or underdosage are produced. [14][15][16][17][18] Molecules with similar atomic numbers have nearly the same dose distribution. Accordingly, titanium (which is atomic number 22) and HAP, which has calcium (atomic number 20) as a primary component of its structure, have nearly the same dose distribution.…”

Physics of Radiotherapy Planning and Delivery

“…Das and colleagues measured dose perturbation factors (DPFs) proximal and distal for simulated bone-tissue interface regions using a parallel plate chamber for both 6 and 24 MV X-ray beams (Das et al 1990;Das and Khan 1989). They reported DPFs of 1.1 for the 6 MV beam and 1.07 for the 24 MV beam at the proximal interface.…”

“…Das and associates measured forward dose perturbation factors (FDPFs) following a 10.5 mm thick stainlesssteel layer simulating a hip prosthesis geometry (Das et al 1990). They measured a dose enhancement of 19% for 24 MV photons, but only 3% for 6 MV photons.…”

Evaluation of Dose Calculation Algorithms of Isogray Treatment Planning System Using Measurement in Heterogeneous Phantom

“…There was a difference (-2.5%) between measurement and calculated dose. The CCC algorithm predicts a dose near to inhomogeneity exactly (25)(26)(27)(28). The CCC algorithm as used for the Monte Carlo calculation method and the results show that it has an accuracy of 1% in complex geometries (29).…”

“…Linthout et al showed the three different calculation algorithms (Clarkson, PB and the Collapsed Cone (CC) algorithms) overestimate the dose in the PTV at the boundary with the low-density tissue, with 12, 10 and 7% for the Clarkson, the PB and the CC algorithm, respectively (24). In another study, Das et al showed that the Collapsed cone algorithm has better accuracy than the PB algorithm in the interface of inhomogeneous media (27). Nisbet et al showed Collapsed cone has better accuracy than the PB model in interface lung/tissue and Collapsed cone and the PB model also have inadequate accuracy in interface bone/tissue (25).…”