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Factors Influencing Point A Treatment Volume for Tandem/Ring Applicators

D Wilkinson

D Wilkinson*, M Kolar, Cleveland Clinic Foundation, Cleveland, OH

SU-E-T-225 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To study factors affecting the treatment volume (V100) for tandem/ring HDR brachytherapy. V100 values are used for patient specific treatment plan second checks.

Methods: Treatment plans generated using Oncentra 4.1 (Nucletron/Elekta) for 28 patients involving a 2.6 cm ring with various tandem lengths (4 to 8 cm) were analyzed. All plans were prescribed to point A and no optimization was applied. Original plans were manipulated to alter dwell positions and the position of point A relative to the applicator to simulate gaps between the ring and external os. Dose volume histograms (dvh) were obtained for the treatment volume and in a subset of the study population for bladder and rectum according to GEC-ESTRO guidelines.

Results: Statistically significant differences in V100 were found for two different applicators having differing dwell position loadings. The differences were less than 10%. Increasing the distance from the ring surface to point A by 2 or 5 mm caused an increase in V100 of approximately 6% and 16% respectively for applicators with 4 or 6 cm tandems. The effect on organs at risk (bladder and rectum) according to GEC-ESTRO dvh analysis showed changes of up to 20% with different dwell position ratios.

Conclusion: The use of V100 values for treatment time verification should be supplemented with verification of the V100 itself. Not only does the size of the applicator affect V100 but also tandem/ring dwell positions need to be taken into account.

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