Patricia Ostwald

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.

ASSESSMENT OF MUCOSAL UNDERDOSING IN LARYNX IRRADIATION

PURPOSE Mucosal underdosing as a result of electron disequilibrium at the air cavity may affect local recurrence rates for T1 and T2 larynx cancers. Secondary build-up properties of high-energy beams have been demonstrated in a slab phantom. It was the aim of this investigation to determine whether significant surface underdosing exists for the mucosa under clinical conditions. METHODS AND MATERIALS Measurements were made using a thermoluminescent dosimetry (TLD) extrapolation technique in an anatomic larynx phantom. The larynx phantom was constructed using tissue and cartilage equivalent material, based on patient cross-sectional anatomy. Three different thicknesses of LiF ribbons, 0.14, 0.39, and 0.89 mm, were placed reproducibly at 12 different positions at the anterior, posterior, and lateral walls on the endolarynx surface. Measured doses were plotted and an extrapolation was made back to the mucosal depth to obtain the dose received at each of the positions. Results were obtained for two different field configurations, opposed laterals and oblique fields, for 6-MV X rays and opposed lateral fields from a telecesium unit. In addition, the larynx surface doses of field sizes from 4 x 6 cm2 to 7 x 6 cm2 were investigated. RESULTS Surface underdosing was observed owing to the secondary build-up and build-down effect of the air cavity, and the dose measured for the three extrapolation TLDs at any position varied by up to 18%. An average variation of 6% was observed. The surface underdosing was most apparent for the 6-MV opposed lateral beam technique, where mucosa doses down to 76% of the prescribed dose were observed. Mucosal underdosing at the measurement positions was less marked with oblique techniques, telecesium treatment, and increasing field size. CONCLUSION Because of underdosing, some surface positions receive < 80% of the prescribed dose. This may contribute to the potential for higher recurrence rates observed with high-energy photons.

Clinical use of carbon-loaded thermoluminescent dosimeters for skin dose determination

PURPOSE Carbon-loaded thermoluminescent dosimeters (TLDs) are designed for surface/skin dose measurements. Following 4 years in clinical use at the Mater Hospital, the accuracy and clinical usefulness of the carbon-loaded TLDs was assessed. METHODS AND MATERIALS Teflon-based carbon-loaded lithium fluoride (LiF) disks with a diameter of 13 mm were used in the present study. The TLDs were compared with ion chamber readings and TLD extrapolation to determine the effective depth of the TLD measurement. In vivo measurements were made on patients receiving open-field treatments to the chest, abdomen, and groin. Skin entry dose or entry and exit dose were assessed in comparison with doses estimated from phantom measurements. RESULTS The effective depth of measurement in a 6 MV therapeutic x-ray beam was found to be about 0.10 mm using TLD extrapolation as a comparison. Entrance surface dose measurements made on a solid water phantom agreed well with ion chamber and TLD extrapolation measurements, and black TLDs provide a more accurate exit dose than the other methods. Under clinical conditions, the black TLDs have an accuracy of +/- 5% (+/- 2 SD). The dose predicted from black TLD readings correlate with observed skin reactions as assessed with reflectance spectroscopy. CONCLUSION In vivo dosimetry with carbon-loaded TLDs proved to be a useful tool in assessing the dose delivered to the basal cell layer in the skin of patients undergoing radiotherapy.

Surface dose measurements for highly oblique electron beams

Clinical applications of electrons may involve oblique incidence of beams, and although dose variations for angles up to 60 degrees from normal incidence are well documented, no results are available for highly oblique beams. Surface dose measurements in highly oblique beams were made using parallel-plate ion chambers and both standard LiF:Mg, Ti and carbon-loaded LiF Thermoluminescent Dosimeters (TLD). Obliquity factors (OBF) or surface dose at an oblique angle divided by the surface dose at perpendicular incidence, were obtained for electron energies between 4 and 20 MeV. Measurements were performed on a flat solid water phantom without a collimator at 100 cm SSD. Comparisons were also made to collimated beams. The OBFs of surface doses plotted against the angle of incidence increased to a maximum dose followed by a rapid dropoff in dose. The increase in OBF was more rapid for higher energies. The maximum OBF occurred at larger angles for higher-energy beams and ranged from 73 degrees for 4 MeV to 84 degrees for 20 MeV. At the dose maximum, OBFs were between 130% and 160% of direct beam doses, yielding surface doses of up to 150% of Dmax for the 20 MeV beam. At 2 mm depth the dose ratio was found to increase initially with angle and then decrease as Dmax moved closer to the surface. A higher maximum dose was measured at 2 mm depth than at the surface. A comparison of ion chamber types showed that a chamber with a small electrode spacing and large guard ring is required for oblique dose measurement. A semiempirical equation was used to model the dose increase at the surface with different energy electron beams.

Response of human hair cortical cells to fractionated radiotherapy

Hair cortical cell counting (HCCC) represents a non-invasive, in-vivo measure of cell kill in the human integument. Sixty-six patients undergoing conventionally fractionated, external beam radiotherapy for early stage carcinoma of the prostate had groin hair samples counted. This technique is a sensitive and reproducible measure of radiation effect and may have applicability as an in-vivo prediction tool or in the field of biological dosimetry. A repopulative follicular response occurring at 3-4 weeks may explain flattening of the dose response curve.

A comparison of three electron planning algorithms for a 16 MeV electron beam

PURPOSE We report results of a comparison of three electron planning algorithms, an Age-Diffusion Pencil beam algorithm and two (2-D) and three dimensional (3-D) Hogstrom pencil beam algorithms, using simple 2 x 2 cm air and hard bone inhomogeneities and a complex anthropomorphic head and neck phantom. METHODS AND MATERIALS The simple inhomogeneities have variable dimensions outside the plane of calculation to test the effects of out of plane scattering on 2-D algorithms, compared with dose measured by film below the inhomogeneity in the dose fall-off range. Comparisons are also made of a parotid treatment field for 16 MeV electrons, and the dose measured by high sensitivity thermoluminescent dosimeters in the head and neck phantom. RESULTS Behind the simple inhomogeneities, the electron algorithms are found to underestimate the dose behind the air cavity by up to 40% and overestimated the dose behind bone by up to 30%. In the head phantom, the presence of inhomogeneities also presents problems for the algorithms, with overestimations of dose of up to 20% found behind bone-tissue interfaces, apparently due to shielding by high density bone. Overestimations of up to 17% are also found beside interfaces parallel to the beam. Underestimations of dose of up to 10% are found on the beam-side of interfaces, due to under-prediction of backscattered electrons. All three investigated algorithms underestimate the dose by up to 20% behind extreme surface curvature. One algorithm is found to underestimate the dose in the falloff region while another overestimates the dose around the 90% isodose. CONCLUSION Clinicians should be aware of the limitations of their planning systems.

VERIFICATION OF SURFACE DOSE ON PATIENTS UNDERGOING LOW TO MEDIUM ENERGY X-RAY THERAPY

About 5% of patients still undergo cancer treatment with superficial (peak energy < or = 120 kVp) X-ray radiation. Dosimetry of these beams is difficult since the maximum dose is delivered at the surface and backscatter contributes significantly to the dose. This is particularly a problem in the difficult geometries encountered in superficial treatments in the head and neck area. It has recently been shown that surface dose measurements in mega-voltage X-ray beams can be performed using TLD (Thermoluminescence Dosimetry) extrapolation. In this technique, LiF TLD chips with a surface area of 3.15 x 3.15 cm2 and three different thicknesses (0.230, 0.099, and 0.038 g/cm2) are used together in the same radiation beam which allows the extrapolation of the measured dose back to the true surface. The energy response curve of the three thicknesses of LiF chips was measured for the energy range of 60kVp, HVL 1.6 mm Al to 300kVp, 4 mm Cu. LiF was found to over respond by a factor of 1.7 at 60kVp HVL 1.6 mm Al with respect to a 6MV photon beam. A feasibility study was carried out on three patients undergoing treatment at 120kVp. Because of the small field sizes involved it was necessary to limit irradiation to one or two chips at a time. The dose fall off in the first millimetre of tissue could be clearly detected. TLD extrapolation, in low to medium energy beams, was found to be useful to assess the dose of patients undergoing treatment for superficial lesions.

Variation in calculated effective source-surface distances with depth.

Effective source-surface distances (ESSD) are assessed at the depth of maximum dose in electron beams. This study investigated the variation of the ESSD with the depth of measurement. The dose was measured with the range of SSDs 100-130 cm, using a water-equivalent parallel-plate ion chamber in solid water. ESSDs were calculated for electron beams in the energy range 4-20 MeV and were found to vary with depth. The surface ESSD varied from 68 cm for 4 MeV to 82 cm for 16 MeV, but increased with depth to a maximum value, which was found at approximately half the practical range (Rp), at 0.3Rp for 4 MeV and at 0.6Rp for 20 MeV. Beyond this depth the ESSD decreased towards the end of the practical range. Without an electron applicator, the ESSD was higher at the surface. For smaller field sizes, the depth of the maximum ESSD increased towards Rp, and ESSD values increased. The 20 MeV beam in the 6 cm x 6 cm2 field showed a difference of 31 cm between the surface ESSD and the maximum ESSD. The ESSD calculated at the maximum dose depth (Dmax) may be used with reasonable accuracy for calculation of the dose in the therapeutic range, except at larger SSDs or when high-energy beams are used in small fields Depth-dose distributions under these conditions should be compared with measured results.

Dosimetry of high energy electron therapy to the parotid region

Well-known inadequacies in currently available electron planning systems, and two cases of temporal lobe necrosis following electron therapy of the parotid stimulated a comprehensive head and neck phantom dosimetric study of the use of high energy electrons for parotid treatments. A typical electron field employed for the treatment of parotid malignancy was examined in an anthropomorphic head phantom from which air cavities had been excavated. Thermoluminescent dosimeter measurements were compared with predicted point doses obtained from a Theraplan Treatment planning system (V05). Data was examined for three different electron energies: 12, 16 and 20 MeV and with the addition of contoured bolus for 20 MeV. A number of significant discrepancies between the measured and predicted dose were observed. Measured doses were seen to exceed predicted doses by up to 23% in the temporal lobe. Further under-predictions of dose were found behind the mandible and in the nasal cavity. Over-predictions of dose by the planning algorithm of up to 22% were observed beside the oropharynx. Some of these discrepancies were found to relate to Theraplan under-estimation of the dose in the fall-off region. Other errors are attributable to the difficulties in predicting dose at density interfaces. Localised over- and under-predictions of this magnitude must be accounted for by the clinician prescribing treatment in terms of possible late effects on the temporal lobe and, in particular, the nominated dose specification point.

Determination of the dose distribution of therapeutic electrons at interfaces

The variation of dose close to tissue/inhomogeneity or tissue/air interfaces was investigated in slab and anthropomorphic phantoms for electron beams between 4 and 20 MeV. Effective source-surface distance (ESSD), beam obliquity, backscatter from internal interfaces, and dose to the sidewalls of air cavities were investigated using an Attix ion chamber, different thicknesses of thermoluminescent dosimeters, and EGS4 and MCNP Monte Carlo simulations. Surface dose was found to vary due to the SSD and beam obliquity. Relative surface dose decreased as the SSD increased due to the effect of low energy scattering from the cone and ESSD determined at the surface differed from the ESSD measured at d max . The difference was found to be most significant for the combination of high energy beams and small field sizes. Surface dose increased with angle for obliquely incident beams from approximately 30° up to a maximum, which was found at approximately 80°, and then decreased rapidly. This dose maximum angle increased with increasing electron beam energy for both surface and near surface depths. Dose variations at internal tissue/inhomogeneity interfaces are due to disequilibrium of electron scatter. The backscatter of electrons (EBF) from inhomogeneities was found to be dependent on the effective atomic number and relative electron density (ρ e ) of the scatterer. Where ρ e =0.03, dose upstream of the scatterer was reduced due to a lack of backscattered electrons. EBF was dependent on the mean energy of the beam incident on the interface, but was independent of the shape of the energy spectrum. The energy of backscattered electrons at the interface was generally low, but increased with increasing atomic number of the inhomogeneity and was related to the upstream extent of backscattered electrons. Dose variation along the side wall of a cavity was dependent on the angular distribution of the beam entering the cavity and dimensions of the cavity.