Prospective Validation of A High Dimensional Shape Model for Organ Motion in Intact Cervical Cancer

Publication date: Available online 22 August 2016
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): Casey W. Williamson, Garrett Green, Sonal S. Noticewala, Nan Li, Hanjie Shen, Florin Vaida, Loren K. Mell
Purpose/Objective(s)Validated models are needed to justify strategies to define planning target volumes (PTVs) for intact cervical cancer used in clinical practice. Our objective was to independently validate a previously published shape model, using data collected prospectively from clinical trials.Materials/MethodsWe analyzed 42 patients with intact cervical cancer treated with daily fractionated pelvic IMRT and concurrent chemotherapy on one of two prospective clinical trials. We collected on-line cone beam computed tomography (CBCT) scans before each fraction. Clinical target volume (CTV) structures from the planning CT were cast onto each CBCT after rigid registration and manually redrawn to account for organ motion and deformation. We applied the 95% isodose cloud from the planning CT to each CBCT and computed any CTV volume outside the 95% isodose cloud. The primary aim was to determine the proportion of CTVs that were encompassed within the 95% isodose volume. A one-sample t-test was used to test the hypothesis that the probability of complete coverage was different than 95%. We used mixed-effects logistic regression to assess effects of time and patient variability.ResultsThe 95% isodose line completely encompassed 92.3% of all CTVs (95% CI 88.3-96.4), not significantly different from the 95% probability anticipated a priori (p=0.19). The overall proportion of missed CTV was small: the grand mean of covered CTV was 99.9%, and 95.2% of misses were located in the anterior body of the uterus. Time did not affect coverage probability (p=0.71).ConclusionWith the clinical implementation of a previously proposed PTV definition strategy based on a shape model for intact cervical cancer, the probability of CTV coverage was high and the volume of CTV missed was low. This PTV expansion strategy is acceptable for clinical trials and practice; however, we recommend daily image guidance to avoid systematic large misses in select patients.

Teaser

We sought to validate a strategy for planning target volume definition in patients with intact cervical cancer, based on a previously published shape model. Using daily cone-beam computed tomography imaging from patients treated with intensity modulated radiation therapy, we found that 92.3% of target volumes were entirely encompassed within the 95% isodose structure, which was not significantly lower than our hypothesized probability of 95.0% (p=0.19). Therefore, we consider this expansion strategy to be valid.


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