T Gray*, S Cherian, S Amarnath, B Guo, P Xia, D Wilkinson, Department of Radiation Oncology, Cleveland Clinic Foundation, Cleveland, OH

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  T Gray

PO-GePV-T-54 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To evaluate the effect of changes in HDR tandem and ring applicator position on doses to nearby organs at risk.

Methods: CT imaging studies (45) and dosimetry of 10 women ages 35 to 86 undergoing tandem/ring based HDR for cervical cancer were available for this retrospective study. Changes in the applicator position (translation and rotation) from the first fraction were estimated using image fusion in MIM software. Dosimetric data were calculated in MIM using the original (first fraction) dose distribution applied to organs at risk (rectum and bladder) via deformable registration of the original contours. Differences in translation and rotation between the planning CT and subsequent individual fractions were recorded and plotted against dose changes between each fraction of treatment and the first fraction. Percent differences in positional shifts vs dosimetric changes were calculated and correlation between dose parameters and measured positional shifts was determined.

Results: Dosimetric parameters for rectum and bladder (V50 and D2cc) calculated using Oncentra and MIM gave similar results within 1% for the original plan (Fraction 1 of 5). Average percent difference between planned dose and fractional doses obtained through deformable registration were 12%, 16%, 4% and 49% for bladder D2cc, Bladder V50, Rectum D2cc and Rectum D50, respectively. Correlation between dose parameters and measured positional changes for bladder D2cc and displacement and bladder D50 and sagittal plane rotation gave an r2 of 0.3. Large displacements resulting in large dosimetric differences were seen for two patients, with maximum dose difference of 55 Gy was seen for Bladder V50.

Conclusion: We have quantified the effects of applicator positional changes on dose differences for bladder and rectum. Even large changes in applicator positioning were not correlated with significant changes in dose, however, significant dose changes are seen with tandem and ring displacements in specific directions and distances.

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