China
Africa
Explore chapters and articles related to this topic
Quality Assurance of Treatment Delivery
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
Margaret Bidmead, Nathalie Fournier-Bidoz, Ginette Marinello, J.-C. Rosenwald, Helen Mayles
For imaging purposes, some EPID calibration is required. It consists most often in subtracting the noise by acquisition of a dark field and removing the intrinsic fluctuation over the image by acquisition of a flood field (open field image). For dose measurement, it is also necessary to ‘flatten' the dose profiles and to use a specific mode of acquisition avoiding saturation (Van Esch et al. 2004; Berger et al. 2006). After these preliminary steps, the EPID signal (expressed in calibrated units [CU] for the Varian EPIDs) must be correlated with the dose in predefined reference conditions. There are some advantages in taking as reference the dose in water at dmax with the dosimetry reference point (DRP) located at the detector position (Chen et al. 2006; François et al. 2011). This enables the dose to be calculated at other distances by application of the inverse square law and at other depths by application of the tissue maximum ratio (TMR) (see Section 26.2.7). A typical calibration setup is represented in Figure 48.15.
Photon Beam Dosimetry of Conventional Medical Linear Accelerators
Published in Arash Darafsheh, Radiation Therapy Dosimetry: A Practical Handbook, 2021
All quantities defined up to this point deal with how dose changes with jaw collimator settings or with depth on the central axis (CAX). Dose variation away from the CAX is characterized with dose profile measurements and off-axis output factors ratios (OARs). Dose profiles are an important part of TPS beam model commissioning and validation, and characterize all off-axis aspects of the beam with respect to the CAX values like PDD and output factors. The beam profile consists of three main regions: the in-field region, the penumbra, and the umbra. The central region typically refers to the region of the field that is within 80% of the beam width, i.e., for a 10 × 10 cm2 field the central 8 × 8 cm2 is considered the central region (Figure 16.7).
Medical Linear Accelerators
Published in Eric Ford, Primer on Radiation Oncology Physics, 2020
Penumbra is a key concept underlying the properties of radiation therapy beams, and considerations of penumbra drive many of the design choices of medical linacs including the MLC. Penumbra is illustrated in Figure 9.1.10 where it is seen that the dose profile across a beam does not have an extremely sharp falloff at the edge, but rather there is some “smearing” of the dose gradient. This is a penumbra.
3D high resolution clonogenic survival measurement of xrs-5 cells in low-dose region of carbon ion plans
Published in International Journal of Radiation Biology, 2023
Dea Kartini, Olga Sokol, Chutima Talabnin, Chinorat Kobdaj, Marco Durante, Michael Krämer, Martina Fuss
Experiments utilizing 12C ion beams have been carried out at Marburger Ionenstrahl-Therapiezentrum (MIT), Marburg (Germany). Monoenergetic, laterally scanned 12C ions with an energy of 110 MeV/u were used in the survival curve experiment, while actively scanned 12C ion beams with energies ranging from 156.34 to 211.54 MeV/u were used for treatment plan verification and film dosimetry experiments. For the extended target irradiation plan, dosimetric measurements were carried out in a water phantom by means of a pinpoint chamber (PTW type 31015) array. The array block consisted of 24 ionization chambers with an outer radius of 1.45 mm and a sensitive volume of 0.03 cm3 each. Chambers were arranged in 6 staggered rows of 4 chambers each, covering in total 5 cm in z-direction, 4.2 cm laterally, and 3 heights around the isocenter plane, to avoid perturbing the dose measurements downstream. The array block was positioned to obtain the central depth dose distribution as well as lateral dose profiles in several depths. The calibration used was in absorbed dose to water, according to the TRS398 protocol (Andreo et al. 2000).
The γH2AX DSB marker may not be a suitable biodosimeter to measure the biological MRT valley dose
Published in International Journal of Radiation Biology, 2021
Jessica A. Ventura, Jacqueline F. Donoghue, Cameron J. Nowell, Leonie M. Cann, Liam R. J. Day, Lloyd M. L. Smyth, Helen B. Forrester, Peter A. W. Rogers, Jeffrey C. Crosbie
There was a linear increase in Monte Carlo-predicted valley dose as the peak dose increased in both tissue and cell studies (Tables 1 and 2, and Figure 7). The Monte Carlo-predicted PVDR for mouse skin tissue was 107. The total uncertainty (k = 1) of the physical dosimetry for the irradiations was 5.1% and 8.6% for MRT peak and valley doses respectively, and 4.8% for BB. The PVDR for the fibroblast cells was predicted to be 30. This difference in PVDRs is due to the larger field size used (30 mm × 60 mm compared to the 10 mm × 10 mm mouse skin irradiations). This difference is also illustrated in Figure 7, which shows an example of the Monte Carlo simulated microbeam profiles of mouse skin and human fibroblasts irradiated with MRT 50 Gy. These dose profiles are consistent with other reports (Crosbie et al. 2010; Ibahim et al. 2014).
Enhanced response of radioresistant carcinoma cell line to heterogeneous dose distribution of grid; the role of high-dose bystander effect
Published in International Journal of Radiation Biology, 2020
Fatemeh Pakniyat, Hassan Ali Nedaie, Hossein Mozdarani, Aziz Mahmoudzadeh, Mahdieh Salimi, Robert J. Griffin, Somayeh Gholami
Based on the in vitro medium transfer strategy, bystander cells should receive the medium of the directly irradiated cells. Hence, it was vital to expose the cells at the specific dose uniformly, according to the peak to valley dose-profile curve that was created by the clinical Grid (Figure 2) and the medium sharing was then implemented. The dosimetry validation (Gholami et al. 2017) of the cells under aperture and shielded areas of the Grid confirmed that when 10 Gy Grid-irradiation was applied, the aperture center (peak dose regions of dose-profile curve) received 10 Gy at Dmax. Moreover, the block center (valley dose regions of dose-profile curve) received 1 to 2 Gy at Dmax based on the center-to-center distance variations, which was considered as an average dose of 1.5 Gy. Therefore, donors and recipients were uniformly exposed by 10 Gy and 1.5 Gy, respectively. The irradiation was performed using the Varian 2100C linear accelerator (Linac) of 6 MV photon beam, field size of 20 cm × 20 cm at the isocenter and SSD of 100 cm. Subsequently, cell dishes were placed between 2 cm (top) and 6 cm (below) slab phantoms, by considering full scatter conditions. The linac has been calibrated in terms of the IAEA TRS 398 dosimetry protocol.