Patient selection
Twenty patients of diagnosed cases of carcinoma cervix with and without high-Z implant each were taken up for the study. The patients were selected in such a way that all were having prosthesis implant in their right-side femoral bone. The material of prosthesis for the selected patients was “titanium” (composition: carbon 0.08%, oxygen 0.13%, iron 0.25%, aluminum 5.6–6.5%, vanadium 3.5–4.5%, and titanium 88.5–91.0%; average electron density 3.74 relative to water; diameter of femoral heads of prosthesis ranging from 40 to 54 mm). All these information are required for executing the dose computations with precision. Bhushan et al. [5] reported that dose perturbation changes with a factor for different energies also. Patients had already completed the total course of treatment. Plans were re-optimized and calculated for collapse-cone-convolution (CCC) algorithm.
Simulation and target delineation
Patients were taken for mold room procedure and thermoplastic cast were made for immobilization. Radio-opaque fiducial markers were used on pelvic region at the level of pubic symphysis to set the reference position. Computed tomographic (CT) scans were acquired with slice thickness of 3 mm. We have used Siemens’s CT unit (model: Somatom Sensation Open) of our radiotherapy department for the above procedure. Full bladder protocol was followed for all the patients for simulation as well as for the whole course of treatment. Scans were acquired from the level of L2 vertebra to 5 cm below the ischial tuberosity. These scans were transferred in DICOM (Digital Imaging and Communication in Medicine) format to the contouring workstations. MONACOSIM (Contouring station; Elekta Medical Solutions) was used for delineation of planning target volume (PTV) and other critical structures by a qualified radiation oncologist.
Clinical target volume (CTV) included the lymphonodal regions (presacral, obturator, and iliac region), uterus, adnexa, and vagina. A margin of 5 mm was given to CTV to generate PTV. The organs at risk (OARs) delineated were the bladder, rectum, small bowel, and femoral heads. Bladder was contoured from apex to dome while the rectum was delineated from the anorectal junction, defined by where the levator muscles fuse with the external sphincter muscles, to the recto-sigmoid junction. Bilateral femoral heads were contoured along with the proximal femur inferiorly from the lowest level of ischial tuberosity and superiorly to the top of the ball of the femur including the trochanters as shown in Fig. 1 [6].
Treatment planning
Isocentric plans were generated using MONACO (Elekta Medical Solutions) treatment planning system (TPS). Plans were made for a prescribed dose of 45 Gy in 25 fractions with a dose of 1.8 Gy per fraction. Planner medical physicist used the gantry angles as 0°, 180°, 90°, and 270° with collimator and couch angle 0°. Beam arrangement was kept identical in all the plans to analyze the effect of calculation algorithms. MONACO facilitates the planner to use different calculation algorithms like collapse-cone-convolution (CCC) and Monte-Carlo (MC) for plan calculation. Our planning goal was to achieve 98% prescription dose to 100% of PTV volume, i.e., the PTV should be covered with at least 98% of prescribed dose. The nearby critical structures were kept as low as possible and the “hot spots, i.e., dose more than 110% of prescribed dose” were allowed within the PTV only.
Calculation algorithms (pencil beam convolution, collapse-cone-convolution, and Monte-Carlo)
Pencil beam convolution (PBC) calculation depends on energy fluence and incorporates head scatter modeling. It depends on a two-dimensional pencil beam convolution for volume reconciliation. Inhomogeneities are taken care of by a proportional way length amendment for the essential portion commitment and a one-dimensional convolution along fan lines with an exponential for dispersed radiation.
Collapse-cone convolution (CCC) calculation algorithm is a methodology wherein a ray-trace procedure through the irradiated object is used to get the TERMA (total energy released per unit mass) at all points in the dose calculation matrix. The TERMA is divided into an essential part (collision kerma) and a scatter part.
Monte-Carlo (MC) calculation from Elekta (MONACO/MC) depends on a fluence model utilizing virtual energy fluence (VEF) model, while the dose distribution inside the patient is determined by the photon voxel MC calculation (XVMC).
Evaluation parameters
PTV was evaluated for D98% (dose received by 98% volume of PTV), D2% (dose received by 2% volume of PTV), D50% (dose received by 50% volume of PTV), Dmax (maximum dose received by PTV), Dmean (average dose received by PTV), V107% (percentage volume of PTV receiving 107% of prescription dose), and V110% (percentage volume of PTV receiving 110% of prescription dose).
Bladder and rectum were evaluated for Dmax (maximum dose received by respective organ), Dmean (mean dose received by respective organ), D2cc (dose received by 2 cc volume of respective organ), V45Gy (percentage volume of respective organ receiving 45Gy), and V50Gy (percentage volume of respective organ receiving 50Gy).
Parameters evaluated for bowel were V5Gy (percentage volume of particular organ receiving 5Gy), V15Gy (percentage volume of particular organ receiving 15Gy), V30Gy (percentage volume of particular organ receiving 30Gy), and Dmean (mean dose received by particular organ). Mean dose to femoral heads and total monitor units (TMU) were also evaluated.
Dose-volume histogram (differential and cumulative)
Dose-volume histograms (DVH) are the best method to modify treatment procedures to diminish the intense and long term radiation related toxicities. There are two forms of dose-volume histograms categorized as differential DVH and cumulative DVH.
Differential DVH is a plot of volume receiving a dose inside a predetermined dose interval (or dose bin) as a function of dose. A differential DVH shows dose variation inside a given structure. A differential DVH resembles a typical histogram. The structure volumes are given on the vertical axis, and the bin doses are given on the horizontal axis. The height of each bar of a differential DVH shows the volume of structure accepting a dose given by the bin.
Cumulative DVH is a plot of the volume of a given structure accepting a specific dose or higher as a function of dose. The volume is given on the vertical axis, and the bin doses are on the horizontal axis similarly as the differential DVH, in any case, the height of each bar represents the volume of structure accepting more noteworthy than or equivalent to that dose. Since 100% of the volume consistently gets in any event zero portion, all DVHs start at 100% on the vertical axis. With very small bin sizes, cumulative DVHs resemble a smooth line diagram. The cumulative DVH is utilized most normally in radiation treatment. Cumulative DVHs generally incorporate the target volume and nearby critical structures to the treatment volume.
Despite the fact that DVHs are significant for assessment of dose distribution, they offer no spatial information and do not show where inside a structure the dose is delivered. Dose-volume histograms are ought to be utilized related to 3D dose distribution to see where the dose is actually received.
Conformity indices
The conformity index is a tool which essentially evaluates the level of consistency between isodoses, tumor shapes, and organs at risk contours by geometric intersection techniques. An analysis of conformity indices for isodoses more prominent than the 95% isodose would give data about the level of homogeneity of irradiation of the target. Conformity index (CI) was calculated using following formulae:
Conformity number (CN) [7]: (TVRI/TV)*(TVRI/VRI)
where TVRI: target volume covered by the reference isodose (98%)
TV: target volume
VRI: volume of reference isodose, i.e., 98%
Conformity index RTOG (CIRTOG) [8]: VRI/TV
Healthy tissue conformity index (CIHT) [9]: TVRI/VRI
Conformity number (CN) incorporates the fraction of target inclusion and the portion of volume of healthy tissues receiving a dose more prominent or equivalent to the prescription dose. The CN index ranges from 0 to 1. The estimation of CIRTOG equivalent to 1 compares to perfect adaptation. If there should be an occurrence of significant worth more prominent than 1 shows the consideration of solid tissues in the irradiated volume. CIHT measures the extent of the volume of the reference isodose containing the objective volume for example in a roundabout way the volume of healthy tissue remembered for the reference isodose. It ranges from 0 to 1 (from “no spatial concordance” to “flawless compliance”). As indicated by Lomax and Scheib, irradiation is considered as conformal if and only if this index is equivalent to or more prominent than 0.6.
Homogeneity indices
HIICRU: (D2 − D98)/D50 [10, 11]
HI: D2/D98 [12]
HIRTOG : Imax/RI [8]
where Imax: maximum isodose in the target
RI: reference isodose
HIICRU was mentioned in ICRU Report No. 83 and was defined by the ratio of difference of D2% and D98% and dose received by 50% of PTV. A value of HI close to 0 is considered as homogeneous plan. As per the literature published by Semenenko et al. [12], the ideal value of HI is 1, and it increases as the plan becomes less homogeneous. As per recommendations of RTOG, if the “homogeneity index” (HIRTOG) is less or equal to 2, treatment is considered to comply with the protocol. If the index varies between 2 and 2.5, the protocol is considered to be minorly violated, but when the index exceeds 2.5, the protocol is considered as major violation and not acceptable.
Data and statistical analysis
DVH was plotted by the TPS for target volumes and different organs at risk. The subjective examination of target coverage was finished utilizing axial, sagittal, and coronal slices. For factual examination, the two-tailed paired t test was performed to compare the results with the statistical significance level set at P ≤ 0.05, between with and without prosthesis plans. Data analysis was performed by the Statistical Package of Social Sciences software (SPSS version 23.0), designed in Chicago, USA.