Dosimetric impact of target definition in brachytherapy for cervical cancer – Computed tomography and trans rectal ultrasound versus magnetic resonance imaging

Highlights

• This study evaluated the dosimetric impact of target volume delineation in Computed Tomography (CT) with assistance from Trans Rectal Ultrasound (TRUS) as compared to Magnetic Resonance Imaging (MRI) image-based brachytherapy for cervical cancer.
• No statistically significant systematic difference was found between MRI and CT for target structures.
• However, considerable variations were seen on individual patient level which needs to be considered during clinical practice, which also needs further investigations.
• Current findings provide useful information to optimally utilize various imaging modalities for brachytherapy planning.
• Although, CT + TRUS based delineation of target volume, appear promising, MRI remains the gold standard.

Abstract

Background and Purpose

Magnetic Resonance Imaging (MRI) based target definition in cervix brachytherapy is limited by its availability, logistics and financial implications, therefore, use of computed tomography (CT) and Trans Rectal UltraSonography (TRUS) has been explored. The current study evaluated the dosimetric impact of CT + TRUS based target volumes as compared to gold standard MRI.

Methods and Materials

Images of patients (n = 21) who underwent TRUS followed by MRI and CT, were delineated with High-Risk Clinical Target Volume in CT (CTV_HR-CT) and in MRI (CTV_HR-MR). CTV_HR-CT was drawn on CT images with TRUS assistance. For each patient, two treatment plans were made, on MRI and CT, followed by fusion and transfer of CTV_HR-MR to the CT images, referred as CTV_HR-MRonCT. The agreement between CTV_HR-MRonCT and CTV_HR-CT was evaluated for dosimetric parameters (D₉₀, D₉₈ and D₅₀; Dose received by 90%, 98% and 50% of the volumes) using Bland-Altman plots, linear regression, and Pearson correlation.

Results

No statistically significant systematic difference was found between MRI and CT. Mean difference (±1.96 SD) of D₉₀, D₉₈ and D₅₀ between CTV_HR-MRonCT and CTV_HR-CT was 2.0, 1.2 and 5.6 Gy respectively. The number of patients who have met the dose constraints of D₉₀ > 85 Gy were 90% and 80% in MR and in CT respectively, others were in the borderline, with a minimum dose of 80 Gy. The mean ± SD dose-difference between MR and CT plans for bladder was significant (5 ± 13 Gy; p = 0.12) for D_0.1cm3, while others were statistically insignificant.

Conclusion

CT + TRUS based delineation of CTV_HR appear promising, provide useful information to optimally utilize for brachytherapy planning, however, MRI remains the gold standard.