[en] Contact radiotherapy uses small field sizes and very short SSDs to deliver low-energy X-rays very close to the position of a tumour. We present a Monte Carlo simulation of the Papillon contact radiotherapy machine, and in particular the backscatter factor, to investigate whether the factors given in current Codes of practice for kV dosimetry remain valid under these conditions

[en] Monte-Carlo simulation is playing an ever increasing role in the radiotherapy dosimetry. The different applications of Monte-Carlo stimulation on radiotherapy physics are discussed

[en] Purpose: To assess whether incorporation of measurements of surviving fraction at 2 Gy (SF₂) and colony-forming efficiency (CFE) into a tumor control probability (tcp) model increases their prognostic significance. Methods and Materials: Measurements of SF₂ and CFE were available from a study on carcinoma of the cervix treated with radiation alone. These measurements, as well as tumor volume, dose, and treatment time, were incorporated into a Poisson tcp model (tcp_α,ρ). Regression analysis was performed to assess the prognostic power of tcp_α,ρ vs. the use of either tcp models with biologic parameters fixed to best-fit estimates (but incorporating individual dose, volume, and treatment time) or the use of SF₂ and CFE measurements alone. Results: In a univariate regression analysis of 44 patients, tcp_α,ρ was a better prognostic factor for both local control and survival (p<0.001 and p=0.049, respectively) than SF₂ alone (p=0.009 for local control, p=0.29 for survival) or CFE alone (p=0.015 for local control, p=0.38 for survival). In multivariate analysis, tcp_α,ρ emerged as the most important prognostic factor for local control (p<0.001, relative risk of 2.81). After allowing for tcp_α,ρ, CFE was still a significant independent prognostic factor for local control, whereas SF₂ was not. The sensitivities of tcp_α,ρ and SF₂ as predictive tests for local control were 87% and 65%, respectively. Specificities were 70% and 77%, respectively. Conclusions: A Poisson tcp model incorporating individual SF₂, CFE, dose, tumor volume, and treatment time was found to be the best independent prognostic factor for local control and survival in cervical carcinoma patients

[en] EGS4 Monte Carlo simulations have been performed to examine general cavity theory for a number of TLD cavity materials irradiated in megavoltage photon and electron beams. The TLD materials were LiF, Li₂B₄O₇, CaF₂ and aSO₄ irradiated in Perspex, water, Al, Cu and Pb phantoms. For megavoltage Photon beams, this has been done by determining the dose component (1-d) resulting from photon interactions in the cavity compared with the dose component resulting from photon interactions in the phantom material (d) by Monte Carlo simulations and analytical techniques. The results indicate that the Burlin exponential attenuation technique can overestimate the dose contribution from photon interactions in a 1 mm thick LiF cavity by up to 100% compared with the Monte Carlo results for LiF TLDs irradiated in a water or Perspex phantom. However, there is agreement to within 1% in the quality dependence factor, determined from Burlin's cavity theory, Monte Carlo simulations and experimental measurements for LiF and Li₂B₄O₇ TLDs irradiated in a Perspex or a water phantom. The agreement was within 3% for aF₂ TLDs. However there was disagreement between Monte Carlo simulations and Burlin's theory of 6 and 12% for LiF TLDs irradiated in copper and lead phantoms respectively. The adaptation of Burlin's photon cavity theory and other modifications to this photon general cavity theory for electrons have been shown to be seriously flawed. (author)

[en] Kerma, collision kerma and absorbed dose in media irradiated by megavoltage photons are analysed with respect to energy conservation. The user-code DOSRZnrc was employed to compute absorbed dose D, kerma K and a special form of kerma, K _n_c_p_t, obtained by setting the charged-particle transport energy cut-off very high, thereby preventing the generation of ‘secondary bremsstrahlung’ along the charged-particle paths. The user-code FLURZnrc was employed to compute photon fluence, differential in energy, from which collision kerma, K _c_o_l and K were derived. The ratios K/D, K _n_c_p_t/D and K _c_o_l/D have thereby been determined over a very large volumes of water, aluminium and copper irradiated by broad, parallel beams of 0.1 to 25 MeV monoenergetic photons, and 6, 10 and 15 MV ‘clinical’ radiotherapy qualities. Concerning depth-dependence, the ‘area under the kerma, K, curve’ exceeded that under the dose curve, demonstrating that kerma does not conserve energy when computed over a large volume. This is due to the ‘double counting’ of the energy of the secondary bremsstrahlung photons, this energy being (implicitly) included in the kerma ‘liberated’ in the irradiated medium, at the same time as this secondary bremsstrahlung is included in the photon fluence which gives rise to kerma elsewhere in the medium. For 25 MeV photons this ‘violation’ amounts to 8.6%, 14.2% and 25.5% in large volumes of water, aluminium and copper respectively but only 0.6% for a ‘clinical’ 6 MV beam in water. By contrast, K _c_o_l/D and K _n_c_p_t/D, also computed over very large phantoms of the same three media, for the same beam qualities, are equal to unity within (very low) statistical uncertainties, demonstrating that collision kerma and the special type of kerma, K _n_c_p_t, do conserve energy over a large volume. A comparison of photon fluence spectra for the 25 MeV beam at a depth of  ≈51 g cm"−"2 for both very high and very low charged-particle transport cut-offs reveals the considerable contribution to the total photon fluence by secondary bremsstrahlung in the latter case. Finally, a correction to the ‘kerma integral’ has been formulated to account for the energy transferred to charged particles by photons with initial energies below the Monte-Carlo photon transport cut-off PCUT; for 25 MeV photons this ‘photon track end’ correction is negligible for all PCUT below 10 keV. (paper)

[en] In a previous study, the dependence of the therapeutic ratio on the number of fractions (n), including both acute and chronic hypoxia, was investigated for homogeneously irradiated tumors. The present study further develops the model to include simultaneous dose-boosting to the hypoxic tumour subvolumes. The acutely hypoxic (ah) tumor subvolume was partitioned into a large number (10²-10³) of oxygenation subvolumes, modelled through rectangular pO₂(t) waves all with the same frequency and fractional time spent below the hypoxic threshold, but with randomly distributed phases. Three quite different assumptions were considered for the effect of prolonged hypoxia on the radiosensitivity (α) of the chronically hypoxic (ch) clonogens, ranging from equal radiosensitivity to that of the ah-cells to an even greater radiosensitivity than that of the well-oxygenated (ox) cells. The linear-quadratic model, including tumor repopulation, intertumor α-heterogeneity, and dependence of the oxygen enhancement ratio on the dose per fraction, was adopted for tumor control probability (TCP) computation. To include a consideration of therapeutic ratio, lung irradiation was considered and the mean normalized total lung dose (NTD_L) was used as a risk indicator. For those 1(fr/d)·5(d/w) schedules yielding 50% TCP with homogeneous irradiation (our reference benchmark), we estimated the gain in TCP and the corresponding NTD_L from dose boosting only the ch-subvolume, both the ah- and the ch-subvolumes, or 50% of the pretreatment tumor volume without specific targeting to tumor hypoxia. For two of the three assumptions for the radiosensitivity of the ch-clonogens, dose-boosting the ch-subvolume was associated with a substantial gain in TCP, and with a trend including minima in NTD_L, for severely hypofractionated schedules only, whereas when dose-boosting both the ah- and the ch-subvolumes a substantial gain in TCP was always obtained for multifractionated schedules. By contrast, the ''blind'' dose-boosting strategy was generally inferior, although an appreciable gain in TCP for severely hypofractionated schedules was obtained. In conclusion, a strategy of dose-boosting tumor hypoxia, guided by nuclear medicine techniques that substantially map chronic hypoxia, is expected to yield optimal gains in TCP via severely hypofractionated delivery

[en] A trial of nonescalated conformal versus conventional radiotherapy treatment of prostate cancer has been carried out at the Royal Marsden NHS Trust (RMH) and Institute of Cancer Research (ICR), demonstrating a significant reduction in the rate of rectal bleeding reported for patients treated using the conformal technique. The relationship between planned rectal dose-distributions and incidences of bleeding has been analyzed, showing that the rate of bleeding falls significantly as the extent of the rectal wall receiving a planned dose-level of more than 57 Gy is reduced. Dose-distributions delivered to the rectal wall over the course of radiotherapy treatment inevitably differ from planned distributions, due to sources of uncertainty such as patient setup error, rectal wall movement and variation in the absolute rectal wall surface area. In this paper estimates of the differences between planned and treated rectal dose-distribution parameters are obtained for the RMH/ICR nonescalated conformal technique, working from a distribution of setup errors observed during the RMH/ICR trial, movement data supplied by Lebesque and colleagues derived from repeat CT scans, and estimates of rectal circumference variations extracted from the literature. Setup errors and wall movement are found to cause only limited systematic differences between mean treated and planned rectal dose-distribution parameter values, but introduce considerable uncertainties into the treated values of some dose-distribution parameters: setup errors lead to 22% and 9% relative uncertainties in the highly dosed fraction of the rectal wall and the wall average dose, respectively, with wall movement leading to 21% and 9% relative uncertainties. Estimates obtained from the literature of the uncertainty in the absolute surface area of the distensible rectal wall are of the order of 13%-18%. In a subsequent paper the impact of these uncertainties on analyses of the relationship between incidences of bleeding and planned rectal dose-distributions is explored

[en] The impact of density and atomic composition on the dosimetric response of various detectors in small photon radiation fields is characterized using a ‘density-correction’ factor, F_detector, defined as the ratio of Monte Carlo calculated doses delivered to water and detector voxels located on-axis, 5 cm deep in a water phantom with a SSD of 100 cm. The variation of F_detector with field size has been computed for detector voxels of various materials and densities. For ion chambers and solid-state detectors, the well-known variation of F_detector at small field sizes is shown to be due to differences between the densities of detector active volumes and water, rather than differences in atomic number. However, associated changes in the measured shapes of small-field profiles offset these variations in F_detector, so that integral doses measured using the different detectors are quite similar, at least for slit fields. Since changes in F_detector with field size arise primarily from differences between the densities of the detector materials and water, ideal small-field relative dosimeters should have small active volumes and water-like density. (paper)

[en] Cavity theory is fundamental to understanding and predicting dosimeter response. Conventional cavity theories have been shown to be consistent with one another by deriving the electron (+positron) and photon fluence spectra with the FLURZnrc user-code (EGSnrc Monte-Carlo system) in large volumes under quasi-CPE for photon beams of 1 MeV and 10 MeV in three materials (water, aluminium and copper) and then using these fluence spectra to evaluate and then inter-compare the Bragg–Gray, Spencer–Attix and ‘large photon’ ‘cavity integrals’. The behaviour of the ‘Spencer–Attix dose’ (aka restricted cema), D _S_-_A(▵), in a 1-MeV photon field in water has been investigated for a wide range of values of the cavity-size parameter ▵: D _S_-_A(▵) decreases far below the Monte-Carlo dose (D _M_C) for ▵ greater than  ≈  30 keV due to secondary electrons with starting energies below ▵ not being ‘counted’. We show that for a quasi-scatter-free geometry (D _S_-_A(▵)/D _M_C) is closely equal to the proportion of energy transferred to Compton electrons with initial (kinetic) energies above ▵, derived from the Klein–Nishina (K–N) differential cross section. (D _S_-_A(▵)/D _M_C) can be used to estimate the maximum size of a detector behaving as a Bragg–Gray cavity in a photon-irradiated medium as a function of photon-beam quality (under quasi CPE) e.g. a typical air-filled ion chamber is ‘Bragg–Gray’ at (monoenergetic) beam energies  ⩾260 keV. Finally, by varying the density of a silicon cavity (of 2.26 mm diameter and 2.0 mm thickness) in water, the response of different cavity ‘sizes’ was simulated; the Monte-Carlo-derived ratio D _w/D _S_i for 6 MV and 15 MV photons varied from very close to the Spencer–Attix value at ‘gas’ densities, agreed well with Burlin cavity theory as ρ increased, and approached large photon behaviour for ρ  ≈  10 g cm"−"3. The estimate of ▵ for the Si cavity was improved by incorporating a Monte-Carlo-derived correction for electron ‘detours’. Excellent agreement was obtained between the Burlin ‘d’ factor for the Si cavity and D _S_-_A(▵)/D _M_C at different (detour-corrected) ▵, thereby suggesting a further application for the D _S_-_A(▵)/D _M_C ratio. (paper)

[en] Background. The current rationale for severely hypofractionated schedules (3-5 fractions) used in stereotactic-body-radiotherapy (SBRT) of non-small-cell lung cancer (NSCLC) is the small size of the irradiated volumes. Being the dose prescribed to the 60-80% isodose line enclosing the PTV, a non-homogeneous tumour-dose-delivery results which might impact on tumour hypoxia. A comparison between homogeneous and SBRT-like non-homogeneous tumour-dose-delivery is then proposed here, using severe hypofractionation on large tumour volumes where both dose prescription strategies are applicable. Materials and methods. For iso-NTCP hypofractionated schedules (1f/d*5d/w) with respect to standard fractionation (d=2Gy), computed from the individual DVHs for lungs, oesophagus, heart and spinal cord (Lyman-Kutcher-Burman NTCP-model), TCP values were calculated (a-averaged Poissonian-LQ model) for homogeneous and SBRT-like non-homogeneous plans both with and without tumour hypoxia. Two different estimates of the oxygen-enhancement-ratio (OER) in combination with two distinct assumptions on the kinetics of reoxygenation were considered. Homogeneous and SBRT-like non-homogeneous plans were finally compared in terms of therapeutic ratio (TR), as the product of TCP and the four (1-NTCPi) values. Results. For severe hypofractionation (3-5 fractions) and for any of the hypotheses on the kinetics of reoxygenation and the OER, there was a significant difference between the computed TRs with or without inclusion of tumour hypoxia (anova, p=0.01) for homogeneous tumour-dose-delivery, but no significant difference for the SBRT-like non-homogeneous one. Further, a significantly increased mean TR for the group of SBRT-like non-homogeneous plans resulted (t-test, p=0.05) with respect to the group with homogeneous target-dose-coverage. Conclusions. SBRT-like dose-boosting seems to counterbalance the loss of reoxygenation within a few fractions. For SBRT it then seems that, in addition to the high level of dose-sparing to the adjacent normal tissues, when severe hypofractionation is adopted it is probably the intrinsic ability of stereotactic techniques to perform intra-tumour simultaneous dose-boosting which yields the reported high clinical efficacy