Comparative Study of Fast Superposition, Superposition Algorithm in Intensity Modulated Radiotherapy Techniques for Prostate Cancer

This study aims to compare the dosimetric coverage of the Planning Target Volume (PTV) and the dose to main organs at risk (OARs) between two different algorithms fast superposition (FSUP), superposition (SUP) using in-tensity-modulated radiotherapy (IMRT) techniques for prostate cancer. Ten patients with prostate cancer were selected for this study. For each patient, IMRT plans were created with 6 MV photon beam quality using CMS XiO treatment planning system. The delivery of IMRT was carried out using the step and shoot techniques. The dose coverage for each patient was designated to an ICRU report 62 reference point in the PTV, medium coverage of the planned target volume to be 95% of the prescribed dose while the maximum dose in the target volume to be not greater than 107% of the prescribed dose. A hypofractionated prescription dose of 70 Gy/28# at 2.5 Gy per fraction was used. Besides we compared the number of MUs and OARs dose to D15%, D25%, D35%, D50% on both algorithm planning rithm showed better results in the IMRT plan compared to the fast superposition algorithm.


Introduction
Intensity Modulated Radiation Therapy (IMRT) techniques can provide better dose distribution for prostate cancer than manual 3DCRT planning. Intensitymodulated radiation therapy (IMRT) is one of the techniques which can deliver a radiation dose to the tumor in the form of sliding window (SW) or step and shoot (SS) methods [1]. IMRT increases the volume of normal tissue exposed to radiation but can reduce the total dose received by the organ a trisk [2], permits tumor dose escalation, thereby yielding higher rates of local tumor control [3] [4] [5]. Several single-institution series have reported are ductioninlate toxicity with the introduction of IMRT compared to 3DCRT, even with dose escalation [6].
The accuracy of dose calculation had been ameliorated by shifting from homogeneity corrections over pencil beam algorithms to point kernel-dependent Convolution/Superposition calculation algorithm [7].
In our study, Fast superposition, Superposition algorithms were used for each patient IMRT plan.
The purpose of this study was to compare the dosimetric coverage of the Planning Target Volume (PTV), the dose to main organs at risks (OARs) such as the rectum, bladder, femoral head, dose homogeneity index, conformity index and number of MUs between two different algorithms using intensity-modulated radiotherapy (IMRT) techniques for prostate cancer.

Methods and Material
Ten patients undergoing radical radiation treatment with histologically confirmed and clinically staged localized prostate cancer were selected for this study.
The age of the patients ranged from 60 to 75 years. All patients were immobilized using the thermoplastic pelvic mask in the supine position with special instruction to keep their rectum empty and bladder comfortably fill at the time of CT simulation. Planning CT images with 3 mm thickness were acquired from the level of L2 -3 to the ischialtuberosity. All CT images of the patient were transferred to the Elekta focal-SIM contouring workstation via DICOM for contouring.
Planning target volume and OARs such as the rectum, Bladder, Right & Left femoral head were delineated as per RTOG guidelines [8].
The seven field (25˚, 75˚, 130˚, 180˚, 230˚, 280˚, 335˚) IMRT plans were created using CMS XiO treatment planning system with 6MV photon beam and FSUP, SUP algorithm for each patient. Thus a total of 20 plans were generated using Step & Shoot IMRT treatment delivery techniques with 80 leaf multileaf collimator and leaf width of 1 cm at the isocenter. A hypofractionated prescription dose of 70 Gy/28# at 2.5 Gy per fraction was used. Treatment planning was performed to achieve at least 95% of PTV volume receiving 95% of the Prescription dose and with less than 2% of PTV volume receiving < 107% of the prescribed dose.
For each plan, a dose-volume histogram (DVH) was generated using CMS XiO TPS. D max , D mean , and D min were recorded for PTV. HI, CI and the number of MUs were computed in all patients.
1) D max : the absolute maximum dose received by any point in the OARs or PTV (inGy).
2) D min : the absolute minimum dose received by any point in the OARs or PTV (inGy).
3) D mean : the absolute mean dose received by the OARs or PTV (inGy). 4) D15%: the absolute dose received by the 15% of the OARs volume (inGy). 5) D25%: the absolute dose received by the 25% of the OARs volume (inGy). 6) D35%: the absolute dose received by the 35% of the OARs volume (inGy). 7) D50%: the absolute dose received by the 50% of the OARs volume (inGy). Dosimetric plan evaluation parameters: To evaluate the dosimetric parameters, cumulative dose-volume histogram (DVHs) were calculated for each algorithm IMRT plan. To evaluate the target dose of each algorithm IMRT plan, homogeneity index (HI), conformity index (CI).
The homogeneity index (HI) was calculated according to the following formula [9]: where, D2, D98, and D50 represent the dose to 2%, 98%, and 50% volume for the PTV, respectively. Values of HI closer to 0 indicate greater dose homogeneity within the volume of PTV, while large values indicate more heterogeneous dose distribution.
The conformity index (CI) was calculated according to the RTOG protocol formula [10]: where, V RI : the volume of reference isodoseonbody, TV: Total PTV Volume.
The closer value of CI to 1.0 the better the dose conformity. Statistical analysis: The difference between the two algorithm planning was compared using mean statistics for their radiated OARs volumes and PTV coverage as the main parameters. A paired t-test was used to verify the significance of the differences of the treatment plans, the p-value of ≤0.05 was taken into account as a significant difference.

Results and Discussion
Comparison between the FSUP and SUP algorithms according to D max , D mean and D min doses for PTV: However, the SP algorithm dose for maximum, mean, minimum is less than that of the FSP algorithm IMRT plan. Statistically, a significant difference was observed for both algorithm (p = 0.004, p = 0.000, p = 0.007) Figures 1-3 shows the maximum value of average D max , D mean , and D min gets the lowest doses with the SUP algorithm compared the FSUP algorithm.
Comparison between the FSUP and SUP algorithms for PTV, HI, CI, MUs: Compared to mean dosimetric parameters 95% PTV volume received a dose to FSP algorithm was 66.63 Gy and 66.45 Gy for SUP algorithm IMRT plans. However, a statistically significant difference was observed for both Plans (p = 0.0005) as shown in Table 2 & Figure 4 shows a comparison between two algorithms.     Compared to mean dosimetric parameters 107% prescription dose received volume to FSUP algorithm was 1.755% and 1.139% for SUP algorithm IMRT plans (p = 0.0565). However, statistically not significant difference was observed in 107% PTV volume for both algorithms as shown in Table 2 and Figure 5. Figure 6 shows a comparison between two algorithms according to the average conformity indexes for PTV of prostate cancer patients with the IMRT plan.   Conformity index PTV was 0.9496 for FSUP verses 0.9473 for SUP (p = 0.370) are not significant differences were observed in both algorithms in the IMRT plan as shown in Table 2. SUP algorithm shows the minimum value of average HI (closer value to zero) and statistically significant differences were observed in both algorithms (p = 0.001) as shown in Table 2 and Figure 7.
Compare to mean value MU/CC was 2.199 in FSUP and 2.152 in the SUP IMRT plan (p = 0.215). However, statistically not significant difference was observed as shown in Table 2 & the number of monitor unit/CC gets lower with the SUP algorithm compared to the FSUP algorithm as shown in Figure 8.
Comparison between the FSUP and SUP algorithms for OARs (Rectum): Figure 9 & Table 3 shows the doses to 15%, 25%, 35%, 50% of the rectum volumes in two algorithms were 69.27 Gy, 66.17 Gy, 62.31 Gy, 55.67 Gy for FSP and 69.07 Gy, 65.99 Gy, 62.13 Gy, 55.49 Gy for SP algorithm for 7 field IMRT Plans. As we can see the dose of 15%, 25%, 50% rectum volumes in the SP algorithm IMRT plan are less than that of the FSP algorithm.
Comparison between the FSUP and SUP algorithms for OARs (Femoral Head): Figure 11, Figure 12 & Table 5 shows the maximum and mean dose of right Figure 11. Comparison between two algorithms according to maximum& mean dose for Lt. femoral head.

Discussion
In this study, adosimetric case study was performed for prostate cancer, and two algorithms (FSUP and SUP) IMRT plan were compared using the DVHs generated in the TPS. Rectum, which is extremely close to the PTV, is the most important organ at risk in prostate cancer. The table shows the dose to rectum D15, D25% is significantly reduced in SUP algorithm IMRT plan. There is no significant difference was observed in both algorithm doses to rectum volume of D35%, D50%.
The bladder is another important organ that should be protected in prostate cancer treatment. SUP IMRT plan performed better plan quality on bladder sparing than FSUP IMRT plan at the dose of bladder volume D15%, D25%, D35%, D50%. However, we found that there was a significant variation between FSUP and SUP algorithm IMRT plan in bladder irradiated volume.
For delivery efficiency, our study showed that there is no significant difference in MU/CC for both algorithms.

Conclusion
We compared Fast superposition, Superposition algorithms (CMS, Xio treatment planning System) using seven field IMRT technique for prostate cancer. Significant variation was observed in maximum, mean, minimum doses of PTV for both algorithms. The superposition algorithm showed excellent results for prostate cancer. In our study, we recommend using of superposition algorithm with IMRT techniques in the treatment planning of prostate cancer. This recommendation is based on the better conformation of all PTV parameters and the sparing of the rectum, bladder, femoral head normal tissue.

Ethical Approval
This article does not contain any studies with human participants or animals performed by any of the authors.