Study of the relationships between brachytherapy plan parameters in an attempt to verify total dwell time in vaginal cylinder applications.

PURPOSE: Source strength (Sk ), sizes of vaginal cylinder applicators (VCA), number of dwell positions (DPs), and the prescribed dose (D) are basic parameters in brachytherapy (BT) treatment planning contributing to total dwell time (TDT). This study was aimed at assessing the relationships between the specified variables in an attempt to verify the TDT in high-dose-rate (HDR) vaginal cylinder applications.

METHODS: One hundred and twenty-one patients treated with Gynesource-Co60 (Bebig, Germany) using VCAs of diameters 20, 25, and 30 mm at University College Hospital, Nigeria, were enrolled in this study. Brachytherapy doses ranging from 3 to 7 Gy were always prescribed to points 5 mm away from the cylinder's surface. Treatment planning was undertaken on HDR-Basic treatment planning system (TPS) which utilizes source step size of 5 mm. Data on the stated parameters related to the first BT fractions of the patients were acquired. With the aid of EViews statistical software, two forms of mathematical models were thereafter developed. The resulting TDTs from the models were compared with the TPS values using Minitab statistical software.

RESULTS: The relationships obtained for the increasing sizes of the VCA were TDT1(min)=2.22+3.17DSk;TDT1(min)=3.52+3.74DSk;TDT1(min)=-1.96+6.91DSkandTDT2(min)=0.50-0.03Sk+0.02D+0.55DPs;TDT2(min)=7.08-0.06Sk+0.02D+0.67DPs;TDT2(min)=7.02-0.11Sk+0.03D+1.25DPs The model-based TDTs correlate with the TPS-calculated values with r1 = 0.80 (P = 0.412) and r2 = 0.97 (P = 0.468).

CONCLUSIONS: The findings of this study could suggest likely variations in the treatment time when certain changes occur in the related parameters. The increasing size of the vaginal cylinder has a positive influence on the brachytherapy treatment time. The latter model has been a useful tool in the verification of the dose delivery time at the first HDR brachytherapy center in Nigeria and West Africa.