No abstract

Purpose Non-melanoma skin cancers of the face are at high-risk for local recurrence and metastatic spread. While surgical interventions such as Mohs microsurgery are considered the standard of care, this modality has the potential for high rates of toxicity in sensitive areas of the face. Catheter flap high-dose-rate (HDR) brachytherapy has shown promising results, with high rates of local control and acceptable cosmetic outcomes. Material and methods Patients with non-melanoma skin cancers (NMSC) located on the face were treated with 40 Gy in 8 fractions, given twice weekly via catheter flap HDR brachytherapy. Clinical target volume (CTV) included the visible tumor plus a margin of 5 mm in all directions, with no additional planning target volume (PTV) margin. Results Fifty patients with 53 lesions on the face were included, with a median follow-up of 15 months. All were considered high-risk based on NCCN guidelines. Median tumor size and thickness were 18 mm and 5 mm, respectively. Median PTV volume and D 90 were 1.7 cc and 92%, respectively. Estimated rate of local control at twelve months was 92%. Three patients (5%) experienced acute grade 2 toxicity. Two patients (4%) continued to suffer from chronic grade 1 skin toxicity at 12 months post-radiotherapy (RT), with an additional two patients (4%) experiencing chronic grade 2 skin toxicity. Forty-nine lesions (92%) were found to have a good or excellent cosmetic outcome with complete tumor remission. Conclusions CT-based flap applicator brachytherapy is a valid treatment option for patients with NMSC of the face. This modality offers high rates of local control with acceptable cosmetic outcomes and low rates of toxicity.

Electron-beam characteristics of a Philips SL25 linear accelerator have been studied. Central-axis percentage depth doses, cross-beam profiles and beam output factors of 6-, 10-, and 20-MeV beams, selected from the available energy range of 4 to 22 MeV, are reported in this paper. The main thrust of this work is to determine the systematic variation of beam characteristics, especially the output factor, with standard cone sizes and cerrobend beam-shaping cutouts down to a field size of 2 X 2 cm Output factors for the standard cones (open field) are energy dependent in a complex manner, increasing with the cone size for the 6-MeV beam whereas decreasing for 10- and 20-MeV beams. The output factor falls below unity at lower energies (6 and 10 MeV) for fields with at least one side smaller than 6 cm, and stays nearly constant for the 20-MeV beam. Measured output factors of small fields are least squares fitted by a second-order polynomial function. Output factors for small rectangular fields have been derived from the one-dimensional and square-root formulas, and the equivalent-square method. Only the one-dimensional formula predicts the measured output factors of highly elongated fields to within +/- 1% experimental uncertainties. Different cones with the same size electron cutout show a varied dose response, primarily due to variation in scattered electron contamination from the cones.

Abstruct-A small plastic scintillator bonded to an optical fiber has several characteristics that make it promising as a brachytherapy dosimeter. In these dosimeters, scintillation light represents signal, whereas Cerenkov and luminescence light from the optical fiber stem is noise that must be subtracted. The dosimeter accuracy can be improved by optically filtering part of the fiber stem light. Spectral measurements were performed to guide the choice of scintillator, fiber, and filter. Spectral signatures and total luminescence of three scintillators and five different silica optical fibers, excited by a 8 Ci I9'Ir source, were measured. The total radiation-induced light from the various optical fibers differed by up to a factor of 5.6. The percentage of fiber-produced light due to luminescence varied between 15 and 79%. A fiber with weak emission was used in the dosimeter with BC408S, a scintillator with minimum emission wavelength of 400 nm. A 400-nm cutoff UV filter gave a factor of two increase in signal-to-noise. The dosimeter response was linear for dose rates varying by at least three orders of magnitude, representing source-to-probe distances of 0.2-10 cm. Measurement errors of the dosimeter compare favorably with other brachytherapy dosimeters.

An energy constancy checking method is presented which involves a specially designed wedge-shaped solid phantom in combination with a multiple channel ionization chamber array known as the Thebes device. Once the phantom/beam scanner combination is set up, measurements for all electron energies can be made and evaluated without re-entering the treatment room. This is also valid for the readjustment of beam energies which are found to deviate from required settings. The immediate presentation of the measurements is in the form of crossplots which resemble depth dose profiles. The evaluation of the measured data can be performed using a hand-held calculator, but processing of the measured signals through a PC-type computer is advisable. The method is insensitive to usual fluctuations in beam flatness. The sensitivity and reproducibility of the method are more than adequate. The method may also be used in modified form for photon beams.

A simple method has been developed to reproduce depth ionization data of electron beams for energy determination. The method utilizes a simple set of equipment, a combination of a specially designed wedge-shaped polystyrene phantom and a linear array of detectors, to collect the necessary data. The wedge-shaped phantom provides varying depths to various detectors in the array. The ionization readings received from the detectors were corrected for off-axis ratio and plotted against corresponding ray-line depths to produce depth ionization curves. The instrument setup was fast and simple. The relevant data, for a high-energy linear accelerator with multiple electron energies, were collected in minutes. The depths of 80% and 50% ionization determined by this method were found to differ by 2 and 3 mm, respectively, at the most, with those determined by a conventional method.

To evaluate the dosimetric accuracy of EBT3 film calibrated with a 6 MV beam for high dose rate brachytherapy and propose a novel method for direct film calibration with an Ir-192 source. Methods: The 6 MV calibration was performed in water on a linear accelerator (linac). The Ir-192 calibration was accomplished by irradiating the film wrapped around a cylinder applicator with an Ir-192 source. All films were scanned 1-day post-irradiation to acquire calibration curves for all three (red, blue, and green) channels. The Ir-192 calibration films were also used for single-dose comparison. Moreover, an independent test film under a H.A.M. applicator was irradiated and the 2D dose distribution was obtained separately for each calibration using the red channel data. Gamma analysis and point-by-point profile comparison were performed to evaluate the performance of both calibrations. The uncertainty budget for each calibration system was analyzed. Results: The red channel had the best performance for both calibration systems in the single-dose comparison. We found a significant 4.89% difference from the reference for doses <250 cGy using the 6 MV calibration, while the difference was only 0.87% for doses >600 cGy. Gamma analysis of the 2D dose distribution showed the Ir-192 calibration had a higher passing rate of 91.9% for the 1 mm/2% criterion, compared to 83.5% for the 6 MV calibration. Most failing points were in the low-dose region (<200 cGy). The point-by-point profile comparison reported a discrepancy of 2%-3.6% between the Ir-192 and 6 MV calibrations in this low-dose region. The linac-and Ir-192-based dosimetry systems had an uncertainty of 4.1% (k = 2) and 5.66% (k = 2), respectively. Conclusions: Direct calibration of EBT3 films with an Ir-192 source is feasible and reliable, while the dosimetric accuracy of 6 MV calibration depends on the dose range. The Ir-192 calibration should be used when the measurement dose range is below 250 cGy.

No abstract