Commissioning and Clinical Implementation of Varian Surface Applicators

Abstract

To validate the dosimetric performance of Varian surface applicators with the source vertically positioned as well as to develop procedures for clinical implementation. Varian surface applicators with the source vertically positioned are circular or ovoid in shape, ranging in diameter from 10 to 45 mm. The variety of applicator sizes that are available make them clinically advantageous, though the orthogonal source orientation and the steepness of the resulting dose gradient in the region of prescription depth (3-4 mm) presents multiple challenges. In the absence of published guidelines, the following tests were designed for commissioning:1) Verification of functional integrity and physical dimensions by visual inspection and measurement;2) Dosimetric measurements to validate data provided by Varian (based on ionization chamber measurements performed in plastic phantoms during the 1990’s) as well as obtained using the Brachyvision Treatment Planning System equipped with the Acuros BV GBBS algorithm for heterogeneity corrected dose calculation. a) Absolute Dose Measurements Two sets of measurements were performed with the source positioned at -10 mm and -15 mm from the center of the first nominal dwell position. Corrections were applied to account for the electrometer, temperature and pressure, polarity effects, as well as recombination. A solid water phantom was scanned and the Acuros algorithm was used to compute the dose at 5 mm depth and at surface for all applicators, using a resolution of 0.05 cm and assuming medium is water. The measurement results were then compared to the vendor’s data and the Acuros calculated dose.i) Measurements were taken at 5 mm depth in a solid water phantom using a Markus chamber in a 2.5-cm polystyrene slab with a 5-cm solid water slab for backscatter, using a calibration factor for 60Co in water. A previously determined phantom factor was then used to convert the results into dose to water.ii) Measurements at the applicator surface were performed in air using an Exradin A20 ion chamber and a Standard Imaging jig designed for such measurements. The calibration factor for 192Ir was obtained through interpolation between the factors for 137Cs and a 250 kV beam. The corrected reading was converted to dose using the AAPM TG 61 formalism. b) Relative Dose Measurements Measurements using GafChromic film were taken at a depth of 4 mm in solid water with the source positioned at -15 mm from the center of the first nominal dwell position. Dwell times were selected to deliver a dose at depth of 1- 2 Gy. The exposed films were scanned and from the digital dataset profiles were generated for comparison with those predicted by the Acuros algorithm. 1) All applicators were found to be functional with physical dimensions within 1 mm of specifications.2) Dosimetric Measurements (excluding the 10 mm applicator):ai) Measurements taken in solid water at a depth of 5 mm were within 15% of the vendor’s data when using a source at -10 mm position and within 10% when using a source at -15 mm position. These measured data were also within 5% and 4% of the Acuros predictions, respectively.aii) Measurements taken at the surface of each applicator were found to be within 8% of the vendor’s data when using a source at -10 mm position and within 6% when using a source at -15 mm position. These measured data were also within 16% and 5% of the Acuros predictions, respectively. It should also be noted that the vendor’s dataset and the Acuros data at surface were within 15% agreement when using a source at -10 mm position and within 5.5% agreement when using a source at -15 mm position. Measurements taken using the 10 mm applicator showed poor agreement with both the vendor’s data as well as the Acuros calculations.b) The full widths of the measured dose profiles were within 2 mm of those predicted by Acuros at the 90% dose level. Based on the dosimetric results, a quality assurance program and procedures for clinical implementation were developed. Treatment planning will be performed using a single dwell position located at -15 mm from the center of the first nominal dwell position. The 10 mm applicator will not be released for clinical use. A prescription depth of 4 mm is recommended, to ensure full coverage at 3 mm and a minimum dose of 90% of prescribed dose at 4 mm depth. As a conservative approach, the applicator selection will be based on the 90% dose level width at 4 mm depth from Acuros plans.