Dosimetric parameters

The equieffective dose in 2 Gy (EQD₂) of 2 cm³ of bladder, rectum, sigmoid, D₉₀ CTV_HR, and dose homogeneity index (DHI), dose non-uniformity ratio (DNR), coverage index (CI), conformal index (COIN), and overdose volume index (OI) were calculated by using following formulas [14,15,16]:

Equivalent dose in 2 Gy (EQD₂) = [nd (1 + d/α/β)]/ [1 + (2/α/β)], where n is the number of fractions, d is the dose per fraction, α/β ratio with 3 for normal tissue and 10 for tumor.

Dose homogeneity index: DHI = (V₁₀₀ – V₁₅₀)/V₁₀₀, where V₁₀₀ and V₁₅₀ are the volume of the CTV_HR receiving 100% and 150% of the prescribed dose, respectively.

Dose non-uniformity ratio: DNR = V₁₅₀/V₁₀₀.

Conformal index: COIN = C₁ × C₂, where C₁ = V₁₀₀ and C₂ is the volume receiving 100% of the prescribed dose, which is outside the CTV_HR.

Coverage index: CI = V₁₀₀/CTV_HR, where CI is the ratio of the volume of CTV_HR receiving 100% of the prescribed dose to the total volume of CTV_HR.

Overdose volume index: OI = V₂₀₀/V₁₀₀, where OI is the ratio of volume of CTV_HR receiving 200% of the prescribed dose to the volume of CTV_HR receiving 100% of the prescribed dose.

Descriptive statistics for dosimetric parameters were expressed as mean ± standard deviation and median with range for continuous characteristics using Microsoft Excel.

The disease-free survival was calculated in months from the day of completion of brachytherapy till the day of diagnosis of the recurrence in patients who had a recurrence or the last follow-up for patients who did not had a recurrence. The local control rate at 2 years was reported in percentage as follows:

Local control rate = (N₁/N₂) × 100, where N₁ is the number of patients who did not had any local disease at 2 years, N₂ is the total number of patients included in the study.