Evaluation of geometrically optimized three plane interstitial implants in HDR brachytherapy for cervical cancer

Purpose: The use of interstitial brachytherapy management in cervical cancer is increasing because of more conformal dose delivery to the tumour and less dose to rectum and bladder volumes compare to intracavitary method. The aim of this study is to evaluate the geometrically optimized three plane interstitial implants using dose volume histogram (DVH).
Material and methods: Ten implants were selected for this study for each implant with needles varies from 17 to 21. Post implant 3 mm CT slices were transferred to virtual simulation for contouring skin, target and other critical structures. NUCLETRON PLATO BPS V14.3.5 used for catheters reconstruction and optimization of 3D dose distribution. All three plane implants are creating dose points around the target with 5 mm distance and prescription to the dose points and choose geometric optimization with graphical optimization to fine tune the isodose lines and generate the Cumulative and natural DVH with high resolution (100 000 points around implant). For each patients to quantify the coverage volumes of TV100, TV150,VPTV (Target volume), Natural dose Ratio (NDR) were obtained to calculate Dose Volume Indices (DVI). Which includes Dose homogeneity index (DHI) is fraction of the target receiving a dose in the interval 1.0 to 1.5 times the reference dose (RD) and Dose Non uniformity ratio (DNR) defined by saw & sunthralingam is the ratio of high dose volume receives that 1.5 times the reference dose (RD). Natural Dose Ratio (NDR) is ratio of Natural prescription dose to prescription dose External Volume index (EI) is amount of normal tissue volume that receives a dose equal to or greater than the (RD). Conformal Index (COIN) Baltas et al. [1] is taken into consideration the coverage of target by RD and also unwanted irradiation of critical structures outside the target. COIN is product of C1 is ratio of target volume (TV) covered by RD to TV, C2 is ratio of TV covered by Volume of RD to volume covered RD, Reference Dose (RD) is 100 Formula for indices: DHI = (TV100 – TV150)/TV100, where V100 and V150 represents the volumes encompassed by the 100% and 150% isodose surface, respectively. EI = (V100 – TV100)/VTV, where V100 is volume of 100% of isodose on body section, VTV, total target volume, NR = TV150/TV100, NDR = NPD/PD, COIN = C1XC2, C1 = TV100/VTV, C2 = TV100/V100,
Results and Discussion: In all ten implants volume of the target ranging from 84.4 to 168cc.one is the ideal value for DHI, NDR, COIN. For selected implants the average DHI is 0.850 ±0.014 (0.826-0.871) for Homogenous dose distribution. And NDR is 0.984 ±0.04 (0.912-1.040), COIN is 0.722 ±0.05 (0.794-0.613) and the value closer to zero is ideal implant for DNR and EI. DNR is 0.150 ±0.014 (0.173-0.129) and EI is 0.053 ±0.02 (0.103-0.05).
Conclusions: Results shows that ten implants gives values of COIN, DHI and NDR shows the optimal target coverage and homogenous dose distribution to the target with very optimal NDR values which is ideal for implant that indicate under and over dosage of the implant. So that geometrically optimized interstitial implants deliver highly conformal dose to the target and excellent critical organ sparing.