PO-341 INTRANASAL MOLD HDR BRACHYTHERAPY FOR NASAL VESTIBULE CANCER: ANALYSIS OF HETEROGENEITY CORRECTION

World Congress of Brachytherapy 2012 PO-340 DOSIMETRIC IMPACT OF NOT CORRECTING FOR THE DISTAL SHIFT REPORTED IN VARIAN TANDEM AND RING (T&R) APPLICATORS O. Craciunescu1, J. Sánchez Mazón2, L. Lan3, J. Maurer1, B. Steffey1, J. Cai1, J. Adamson1, J. Chino1 1 Duke University Medical Center, Radiation Oncology, Durham NC, USA 2 Hospital Universitario Marqués de Valdecilla, Servicio de Radiofísica y Protección Radiológica, Santander, Spain 3 Duke University Medical Center, Department of Biostatistics and Bioinformatics, Durham NC, USA Purpose/Objective: Studies have shown that source stop positions within Varian’s HDR CT/MR compatible ring applicators can deviate from the intended positions by several millimeters, and that a corrective shift has to be applied to limit the effect of these inherent offsets. The purpose of this study was to investigate the dosimetric impact of NOT applying this correction. Materials and Methods: Twenty-seven HDR T&R clinical plans (cP) from six different patients treated for cervical cancer in our clinic were used in this study. Both CT and MR were acquired for each plan. The dose per fraction was 5.5 Gy. GEC-ESTRO guidelines were followed for HRCTV and organs at risk (OARs): bladder, rectum, sigmoid. ICRU 38 points A were also defined. The planning was done in BrachyVision with a hybrid volume optimization method based on HRCTV and ABS recommended reference lines. Depending on the shape of the HRCTV, the posterior ring dwell positions were allowed in the optimization for 5 patients. For 1 patient, a standard ring loading that mimicks ovoid placement was used. To show the dosimetric impact of NOT applying distal corrections, plans were generated by shifting the dwells in the ring in the opposite direction incrementally by 1- 5 mm. The plans were calculated using the TG43 formalism and ACUROS™, a model-based analytical solver that accounts for inhomogeneities. To establish clinical significance, the mean percent difference between all metrics in each of the shifted plans and the cP were calculated. The Wilcoxon signed-rank test was used to determine if the differences have any statistical significance, for all cases together, but also separating patients depending on the ring dwells activated. Results: For brevity, only D2cc results for OARs for all 27 plans are presented. Ring loading strategy did not impact the dosmetric results. The mean percent differences between the largest shift (5mm) and the cP for the D2cc bladder, rectum and sigmoid were 1.6, 1.7, 2.5% for TG43, and 1.2, 1.8 and 1.4 % for ACUROS. The Wilcoxon rank sum p-values were > 0.05 for all the OAR metrics, shift amounts, and method of calculation. For A_RT, A_LT and HRCTV D90, the mean percent differences were 0.8, 0.5 and 0.6 %, and 0.6, 0.8 and 0.9 % for TG43 and ACUROS, respectively. However, the Wilcoxon signedrank p-values for Point A_RT were < 0.05 for both TG43 and ACUROS, for all shifts. Only when using ACUROS, the p-values for HRCTV D90 were < 0.05 for all shifts. Conclusions: The dosimetric impact of NOT applying corrective shifts <= 5mm when using Varian rings does not significally affect any OAR. For HRCTV D90 metric, the results depend on the method used for calculation, TG43 being more forgiving, as it does not account for the attenuation of the ring and cap, nor the potential air pockets around the ring. NOT applying the correction affects the dose to point A_RT the most. However, points A play less of a role in a volume-based treatment planning paradigm. PO-341 INTRANASAL MOLD HDR BRACHYTHERAPY FOR NASAL VESTIBULE CANCER: ANALYSIS OF HETEROGENEITY CORRECTION E.F. Crandley1, B. Libby1, K.A. Reardon1, K. Ding1, A. Martof2, S.H. Benedict1, P.W. Read1 1 University of Virginia, Radiation Oncology, Charlottesville VA, USA 2 University of Virginia, Dentistry, Charlottesville VA, USA Purpose/Objective: As an alternative to external beam radiation therapy, surgery, or interstitial brachytherapy, custom intranasal molds were designed for patients with superficial nasal vestibule cancers with the intent to deliver conformal HDR brachytherapy while sparing adjacent mucosal tissue and critical structures. Plans were calculated with and without heterogeneity correction for comparison.

S 137 Materials and Methods: A soft polyether dental impression material (Impregum™; 3M™ ESPE™, St. Paul, MN) was used to develop a custom mold of the nasal cavity with approximately 1 cm spacing of the HDR catheters. A total of 2-4 catheters were used. One patient was treated with a bite block. CT simulation was obtained and HDR treatment planning assuming a homogeneous medium was used with manual optimization of target volume coverage. Calculations with heterogeneity correction were performed with Acuros™ (Varian Medical Systems, Palo Alto, CA). All patients were treated to 32 Gy / 8 fractions delivered twice daily with at least 6 hours between fractions over 4 consecutive days. Viscous lidocaine was used for topical anesthesia and lubrication of the applicator for placement. Results: Three patients were treated (2 squamous cell carcinoma, 1 basal cell carcinoma) with target volumes ranging 0.8 – 8.7 cm3. The average maximum dose for the course of treatment to avoidance structures with homogeneous planning was 2.7 Gy (range 1.9 – 4.0 Gy) for the lens, 3.7 Gy (2.3 – 5.5) for the eyes, 1.8 Gy (1.1 – 2.3) for the optic nerves, 12.2 Gy (9.1 – 18.9) for the hard palate, 4.0 Gy (3.4 – 4.4) for the tongue, 17.3 cGy (7.3 – 37.1) for the maxillary teeth, and 13.9 Gy (range 6.8 – 23.2) for the upper lip. With heterogeneity correction, the average absolute difference in dose to avoidance structures was -4.8 cGy/fraction (-2.5 to -19.6 cGy) and the average relative difference was -11.3% (-1.1 to -22.9%) when compared to homogeneous planning. Calculation with heterogeneity correction resulted in minimal decrease in target volume coverage by the prescription isodose line (mean -0.8%, range -0.2 to -1.35%). The fabrication and daily placement of the nasal molds was tolerated well by all patients. Conclusions: Intracavitary mold HDR brachytherapy is a minimally invasive strategy for delivery of radiation therapy to targets in the nasal vestibule. 3D planning with dose volume histogram analysis shows acceptable dose to adjacent mucosal and critical structures. Homogeneous calculations per TG 43 overestimates target volume coverage and the dose to mucosal and critical structures when compared to calculations with heterogeneity correction; however, the absolute difference in this analysis was not clinically significant. Further follow up is needed to assess late toxicity and local control using this technique. PO-342 MEASUREMENTS OF AN ELECTRONIC BRACHYTHERAPY SOURCE WITH A BRACHYTHERAPY DOSE VERIFICATION PHANTOM T. Pike1, L. DeWerd1, J. Micka1 1 Univ. of Wisconsin School of Medicine and Public H, Medical Physics, Madison WI, USA Purpose/Objective: This investigation presents work with a unique phantom to perform measurements of the TG-43 parameters of the Xoft Axxent (an iCad company, Sunnyvale, CA) electronic brachytherapy source. This prototype brachytherapy dose verification phantom, designed in collaboration with Standard Imaging (SI) (Middleton, WI) allowed for measurements of azimuthal and polar anisotropy, and for dose-rate constant (DRC) measurements. The recommended medium for TG-43 measurements is liquid water, which is difficult to work with when using brachytherapy sources and certain dosimeters. This phantom provides a straightforward and practical methodology for measurement of the TG-43 parameters. It is constructed from PMMA which is used as a 'standard material.' Materials and Methods: The brachytherapy dose verification phantom consisted of a main outer body that accommodated three specialized inserts: a radiochromic film insert (Figure 1a), a polar TLD insert (Figure 1b), and an SI Exradin A16 microchamber insert. Measurements of azimuthal anisotropy were performed with Gafchromic EBT radiochromic film (International Specialty Products, Wayne, NJ) and TLD microcubes. Azimuthal film measurements were done at 2 cm and TLD measurements were done at 3 cm in the main outer body with acrylic plugs that accommodated TLD microcubes. Polar anisotropy measurements were performed at 1 cm and 2 cm and also at multiple angles around the source to obtain a threedimensional dose distribution with TLD microcubes. An air-kerma calibrated SI Exradin A16 microionization chamber was used to measure the dose-rate constant. Monte Carlo simulations were performed with the user code egs_chamber from EGSnrc to obtain a correction factor to obtain dose to water from the A16 chamber