Do HDR Tandem and Ring Interfractional Displacements Predict the Need to Re-Plan?

Abstract

<h3>Purpose/Objective(s)</h3> To evaluate the effect of changes in HDR tandem and ring applicator position on doses to nearby organs at risk. <h3>Materials/Methods</h3> CT imaging studies (29) and dosimetry of 6 women ages 44 to 86 undergoing tandem/ring based HDR for cervical cancer were used retrospectively. Measures of applicator positional changes (translation and rotation) relative to the pelvis were obtained using image fusion in a commercial image analysis software. Dosimetric data were calculated in the commercial image analysis software using the original (first fraction) dose distribution applied to organs at risk (rectum and bladder) via deformable registration of the original contours. The new contours were examined visually for appropriateness. 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 original (first) fraction. Percent differences in positional shifts vs dosimetric changes were calculated and a correlation coefficient between dose parameters and measured positional shifts of the applicator was determined. <h3>Results</h3> Dosimetric parameters for rectum and bladder (D_2cc and V₅₀) calculated using a commercial treatment planning system for HDR brachytherapy and commercial image analysis software gave similar results within 1% of the original plan (Fraction 1 of 5). A summary of positional changes and average and maximum dose changes for each dosimetric endpoint is found in Table 1. Average percent difference between planned dose and fractional doses obtained through deformable registration were 12%, 16%, 4% and 49% for bladder D_2cc, Bladder V₅₀, Rectum D_2cc and Rectum D₅₀, respectively. Correlation between dose parameters and measured positional changes for bladder D₅₀ and displacement and bladder D₅₀ and sagittal plane rotation gave an r² of 0.3. Applicator displacement of approximately 1.6 times greater than average applicator displacement across all plans studied have shown a significant increase in bladder dose. Bladder dose increased by a maximum of 30 Gy and 54.5 Gy for bladder D_2cc and bladder V₅₀, respectively for the patient with the largest catheter shifts. <h3>Conclusion</h3> We have quantified the effects of applicator positional changes on dose differences for bladder and rectum. Even large changes did not result in significant changes in dose to organs at risk, meaning adaptive re-planning is not required in most cases below a certain displacement threshold.