Volumetric evaluation of an alternative bladder point in brachytherapy for locally advanced cervical cancer

Purpose

To evaluate an alternative dose point, so-called ALG (for Alain Gerbaulet), for the bladder in comparison to the International Commission on Radiation Units and Measurements (ICRU) point and D2cm³ (minimal dose to maximally exposed 2 cm³) in a large cohort of patients with locally advanced cervical cancer treated with external beam radiotherapy followed by image-guided pulsed dose rate brachytherapy.

Methods and materials

For each patient, the ALG point was constructed 1.5 cm above the ICRU bladder, parallel to the tandem (coronal and sagittal planes). The dosimetric data from 162 patients were reviewed.

Results

Average doses to ALG and bladder points were 19.40 Gy?±?7.93 and 17.14?±?8.70, respectively (p?=?0.01). The 2 cm³ bladder dose averaged 24.40?±?6.77 Gy. Ratios between D2cm³ and dose points were 1.37?±?0.46 and 1.68?±?0.74 (p?<?0.001) for ALG and ICRU points, respectively. Both dose points appeared correlated with D2cm³ (p?<?0.001) with coefficients of determination (R²) of 0.331 and 0.399 respectively. The estimated dose to the ICRU point of the rectum was 12.77?±?4.21 and 15.76?±?5.94 for D2cm³ (p?<?0.0001). Both values were significantly correlated (p?<?0.0001, R²?=?0.485).

Conclusion

The ALG point underestimates the D2cm³, but its mean on a large cohort is closer to D2cm³ than the dose to ICRU point. However, it shows great variability between cases and the weak strength of its correlation to D2cm³ indicates that it is not a good surrogate for individual volumetric evaluation of the dose D2cm³.