Tomas Kron

Dose response of various radiation detectors to synchrotron radiation

Accurate dosimetry is particularly difficult for low- to medium-energy x-rays as various interaction processes with different dependences on material properties determine the dose distribution in tissue and radiation detectors. Monoenergetic x-rays from synchrotron radiation offer the unique opportunity to study the dose response variation with photon energy of radiation detectors without the compounding effect of the spectral distribution of x-rays from conventional sources. The variation of dose response with photon energies between 10 and 99.6 keV was studied for two TLD materials (LiF:Mg,Ti and LiF:Mg,Cu,P), MOSFET semiconductors, radiographic and radiochromic film. The dose response at synchrotron radiation energies was compared with the one for several superficial/orthovoltage radiation qualities (HVL 1.4 mm Al to 4 mm Cu) and megavoltage photons from a medical linear accelerator. A calibrated parallel plate ionization chamber was taken as the reference dosimeter. The variation of response with x-ray energy was modelled using a two-component model that allows determination of the energy for maximum response as well as its magnitude. MOSFET detectors and the radiographic film were found to overrespond to low-energy x-rays by up to a factor of 7 and 12 respectively, while the radiochromic film underestimated the dose by approximately a factor of 2 at 24 keV. The TLDs showed a slight overresponse with LiF:Mg, Cu, P demonstrating better tissue equivalence than LiF:Mg, Ti (maximum deviation from water less than 25%). The results of the present study demonstrate the usefulness of monoenergetic photons for the study of the energy response of radiation detectors. The variations in energy response observed for the MOSFET detectors and GAF chromic film emphasize the need for a correction for individual dosimeters if accurate dosimetry of low- to medium-energy x-rays is attempted.

Dose resolution in radiotherapy polymer gel dosimetry: effect of echo spacing in MRI pulse sequence

In polymer gel dosimetry using magnetic resonance imaging, the uncertainty in absorbed dose is dependent on the experimental determination of T2. The concept of dose resolution (Dpdelta) of polymer gel dosimeters is developed and applied to the uncertainty in dose related to the uncertainty in T2 from a range of T4 encountered in polymer gel dosimetry. Dpdelta is defined as the minimal separation between two absorbed doses such that they may be distinguished with a given level of confidence, p. The minimum detectable dose (MDD) is Dpdelta as the dose approaches zero. Dpdelta and the minimum detectable dose both give a quantifiable indication of the likely practical limitations and usefulness of the dosimeter. Dpdelta of a polyacrylamide polymer gel dosimeter is presented for customized 32-echo and standard multiple-spin-echo sequences on a clinical MRI scanner. In evaluating uncertainties in T2, a parameter of particular significance in the pulse sequence is the echo spacing (ES). For optimal results, ES should be selected to minimize Dpdelta over a range of doses of interest in polymer gel dosimetry.

X-ray surface dose measurements using TLD extrapolation.

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.

Variations in dose response with x-ray energy of LiF:Mg,Cu,P thermoluminescence dosimeters: implications for clinical dosimetry

In many medical procedures where accurate radiation dose measurements are needed, the variation of detector response with x-ray energy is of concern. The response of LiF:Mg,Cu,P TLDs to a range of x-ray energies was analysed in monoenergetic (synchrotron), diagnostic and therapy radiation beams with the aim of implementing this dosimeter into clinical practice where existing dosimetry techniques are limited due to lack of sensitivity or tissue equivalence (e.g. neonatal radiography, mammography and brachytherapy). LiF:Mg,Cu,P TLDs in different forms from two manufacturers (MCP-N: TLD Poland, GR200: SDDML China) were irradiated using x-ray beams covering 10 keV to 18 MVp. Dose readings were compared with an ionization chamber. The effect of different TLD types and annealing cycles on clinical utility was investigated. The measured energy response of LiF:Mg,Cu,P TLDs was fit to a simple model devised by Kron et al (1998 Phys. Med. Biol. 43 3235-59) to describe the variation of TLD response with x-ray energy. If TLDs are handled as recommended in the present paper, the energy response of LiF:Mg,Cu,P deviates by a maximum of 15% from unity and agrees with the model to within 5% or experimental uncertainty between 15 keV and 10 MeV. LiF:Mg,Cu,P TLDs of all forms have consistent and superior energy response compared to the standard material LiF:Mg,Ti and are therefore suitable for a wide range of applications in diagnostic radiology and radiotherapy.

Online Adaptive Radiotherapy for Muscle-Invasive Bladder Cancer: Results of a Pilot Study

PURPOSE To determine the advantages and disadvantages of daily online adaptive image-guided radiotherapy (RT) compared with conventional RT for muscle-invasive bladder cancer. METHODS AND MATERIALS Twenty-seven patients with T2-T4 transitional cell carcinoma of the bladder were treated with daily online adaptive image-guided RT using cone-beam computed tomography (CBCT). From day 1 daily soft tissue-based isocenter positioning was performed using CBCT images acquired before treatment. Using a composite of the initial planning CT and the first five daily CBCT scans, small, medium, and large adaptive plans were created. Each of these adaptive plans used a 0.5-cm clinical target volume (CTV) to planning target volume expansion. For Fractions 8-32, treatment involved daily soft tissue-based isocenter positioning and selection of suitable adaptive plan of the day. Treating radiation therapists completed a credentialing program, and one radiation oncologist performed all the contouring. Comparisons were made between adaptive and conventional treatment on the basis of CTV coverage and normal tissue sparing. RESULTS All 27 patients completed treatment per protocol. Bladder volume decreased with time or fraction number (p < 0.0001). For the adaptive component (Fractions 8-32) the small, medium, large, and conventional plans were used in 9.8%, 49.2%, 39.5%, and 1.5% of fractions, respectively. For the adaptive strategy, 2.7% of occasions resulted in a CTV V95 <99%, compared with 4.8% of occasions for the conventional approach (p = 0.42). Mean volume of normal tissue receiving a dose >45 Gy was 29% (95% confidence interval, 24-35%) less with adaptive RT compared with conventional RT. The mean volume of normal tissue receiving >5 Gy was 15% (95% confidence interval, 11-18%) less with adaptive RT compared with conventional RT. CONCLUSIONS Online adaptive radiotherapy is feasible in an academic radiotherapy center. The volume of normal tissue irradiated can be significantly smaller without reducing CTV coverage.

Fast T1 imaging of dual gel samples for diffusion measurements in NMR dosimetry gels

Diffusion of iron is one of the major problems limiting the usefulness of NMR gel dosimetry. This was studied in dual gel samples using a 4.7T micro-imaging MR scanner and a fast T1 imaging sequence which allowed the acquisition of a 64 x 128 x 8 data sets (phase encoding x frequency encoding x number of inversion times) in less than 15 min. The procedure enabled us to obtain relative relaxation times for any region of interest within the sample. After the two differently doped gels were brought into contact in the dual gel samples (diameter 12 mm), the diffusion could be observed on subsequent images as a function of time. An inverse square root function was used to fit the change of 1/T1 across the junction between the two gel phases. A diffusion constant of 0.014 +/- 0.003 cm2/h was determined for Fe3+ in a typical dosimetry gel (1.5% agarose, 50 mM H2SO4). This could be lowered by adding a chelating agent such as xylenol orange to the gel. It was also found that diffusion was slower in gelatine gels, however these gels tended not to set properly when H2SO4 was added as required for NMR dosimetry. From the present results we propose that a gel consisting of 1.5% agarose (for stability), 3% gelatine and 0.1 mM xylenol orange (to combat diffusion and allow a visual evaluation) is a suitable base for NMR dosimetry gels. The use of a fast T1 imaging sequence reduces acquisition times and therefore the potential impact of diffusion.

Underprediction of human skin erythema at low doses per fraction by the linear quadratic model

BACKGROUND AND PURPOSE The erythematous response of human skin to radiotherapy has proven useful for testing the predictions of the linear quadratic (LQ) model in terms of fractionation sensitivity and repair half time. No formal investigation of the response of human skin to doses less than 2 Gy per fraction has occurred. This study aims to test the validity of the LQ model for human skin at doses ranging from 0.4 to 5.2 Gy per fraction. MATERIALS AND METHODS Complete erythema reaction profiles were obtained using reflectance spectrophotometry in two patient populations: 65 patients treated palliatively with 5, 10, 12 and 20 daily treatment fractions (varying thicknesses of bolus, various body sites) and 52 patients undergoing prostatic irradiation for localised carcinoma of the prostate (no bolus, 30-32 fractions). RESULTS AND CONCLUSIONS Gender, age, site and prior sun exposure influence pre- and post-treatment erythema values independently of dose administered. Out-of-field effects were also noted. The linear quadratic model significantly underpredicted peak erythema values at doses less than 1.5 Gy per fraction. This suggests that either the conventional linear quadratic model does not apply for low doses per fraction in human skin or that erythema is not exclusively initiated by radiation damage to the basal layer. The data are potentially explained by an induced repair model.

Investigation of the tissue equivalence of gels used for NMR dosimetry.

The transition of Fe2+ to Fe3+ in Fricke solution after irradiation results in a change of NMR proton relaxation times in agarose gels which can be used for the dosimetry of ionizing radiation. The main advantage of this system is the possibility of observing dose distributions in 3 dimensions in a medium which is supposed to be tissue equivalent. The aim of the present study was to quantify parameters which determine the tissue equivalence of NMR dosimetry gels. Electron densities and effective atomic numbers were calculated for gels with varying iron, sulphur and agarose concentration. The Hounsfield CT numbers so derived agree well with the CT numbers measured on a clinical CT scanner (effective photon energy 70.7 keV). The Hounsfield CT number of 7.2 +/- 1.5 (n = 9) measured for a 1.5% agarose gel doped with 0.5 mM ammonium ferrous sulphate and 125 mM sulphuric acid compares well with the calculated one of 8 +/- 5. Relaxation times were measured from a series of MR images obtained on a 1.5 T clinical MR scanner. The observed change in 1/T1 of the gel with dose was found to be linear up to 10 Gy (0.084 s-1 Gy-1). No difference in dose response for 10 Gy delivered by four different superficial radiation qualities (HVT = 1.4-7.5 mm Al) could be observed. These findings and the calculated effective atomic number of 7.46 demonstrate the close tissue equivalence of this agarose gel which makes it an ideal tool for the investigation of low energy therapeutic x-rays.

A comparison of methods of cosmetic assessment in breast conservation treatment

Abstract The aim of this study was to compare live and photographic methods of assessing variables which can influence cosmetic outcome following breast conserving treatment. This study was undertaken in 47 patients who had previously received breast conserving surgery, radiotherapy and simultaneous chemotherapy for stage I and II breast cancer and a matched group of patients who had received surgery and radiotherapy alone. The assessment consisted of patient and spouse self-assessment, a live assessment by two trained observers and a photographic assessment by five observers, two trained and three untrained. Patients rated their outcome more favourably than their spouses, and both rated the outcomes above those of the other observers. Quantitative variables such as measurement of nipple retraction were assessed by different observers more consistently than qualitative variables such as overall perception of assessed cosmetic outcome. Upward retraction of the nipple emerged as the most powerful determinant of cosmetic outcome in the eyes of both the patient and the trained observers and was reproducibly measured by both live and photographic techniques. The distinction between post-surgical effects and post-radiation effects was more readily made by live assessment. Photographic assessment is as effective as live assessment in post-surgical cosmetic assessment. It provides reliable information about all of the factors which were important to both the patient and observers in formulating an overall cosmetic outcome score. The effects of surgery, which include nipple retraction, need to be taken into account in future trials of adjuvant therapy in which cosmesis is an important outcome measure. Stratification using upward retraction of the nipple is a possibility.

Uncertainty analysis in polymer gel dosimetry

Verification of advanced radiotherapy treatment modalities requires measurement of three-dimensional absorbed dose distributions with high spatial resolution and precision. Polymer gel dosimeters combined with magnetic resonance imaging may be able to fulfil this requirement. However, verification requires that the uncertainty in the dosimeter is well known. One method of estimating the overall uncertainty in polymer gel dosimeters involves the propagation of the uncertainty in the R2 (nuclear magnetic resonance relaxation rate) map and the uncertainties in the calibration data. This work shows that using this method with current data suggests that the lowest uncertainty currently obtainable is about 3% at 8 Gy and 7% at 2 Gy. Furthermore, the most significant reductions in overall uncertainty will be achieved by reducing the noise in the R2 map.