Comparative analysis of CT-based 3D volumetric dosimetry and radiography-based 2D dosimetry for high-dose-rate intracavitary applications

Abstract

image-guided planning (Plato). Treatments were delivered using a microselectron HDR unit. For all 30 implants, ICRU-rectal/bladder point doses, the maximal dose and doses to contiguous 1, 2, and 5 cm of rectum and bladder were calculated. Results: Doses resulting from CT-dosimetry were higher than ICRU point estimates. For bladder, the mean CT-maximum dose was 95.2% of the prescribed dose (SD 18.9) compared with 63.9% (SD 17.2) using ICRU point (p 5 0.066). Median bladder volume that received higher than ICRU point estimated dose was 6.98 cm. The ICRU reference point and contiguous 5 cm bladder doses were not significantly different (p 5 0.94). For rectum, the mean CT-maximum dose was 77.5% of the prescribed dose (SD 18.6) compared with 58.8% (SD 16.9) to ICRU rectal point (p!0.0001). Median rectal volume that received higher than the ICRU point dose was 2.67 cm. The ICRU reference and contiguous 2 cm rectal doses were not significantly different (p 5 0.24). Bladder and rectal doses did not correlate with point A dose (mean 5 8.05 Gy, p 5 0.06). Median rectal BED (including EB RT) was 126.7 Gy3 based on CT-dosimetry compared to 100.1 Gy3 using ICRU rectal dose. In our followup (median 15 months, range 10–24), no Grade 3 or higher rectal and bladder complications were observed. Conclusions: In cervix cancer interstitial HDR brachytherapy, ICRU points for rectal and bladder dose consistently and significantly underestimated the delivered dose, as compared to CT planned DVHs. Despite the fact that in our relatively small series there was no increase in Grade 3 or higher rectal or bladder toxicity, median rectal BED as estimated by DVHs is extremely close to complication inducing threshold (125 Gy3). We strongly advocate CT-derived DVHs be routinely used for estimating rectal and bladder doses in pelvic interstitial HDR brachytherapy.