|Year : 2019 | Volume
| Issue : 2 | Page : 28-37
Clinical dosimetric impact of AAA and Acuros XB on high-density metallic implants in case of carcinoma cervix
Manindra Bhushan1, Girigesh Yadav2, Deepak Tripathi3, Lalit Kumar4, Vimal Kishore5, Rahul Lal Chowdhary2, Gourav Kumar2, Soumya Datta2, Swarupa Mitra2, Munish Gairola2
1 Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, New Delhi; Department of Physics, Amity School of Applied Sciences, Amity University (AUUP), Noida, Uttar Pradesh, India
2 Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, India
3 Department of Physics, Amity School of Applied Sciences, Amity University (AUUP), Noida, Uttar Pradesh, India
4 Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, New Delhi; Department of Applied Science and Humanities, Dr. APJ Abdul Kalam Technical University, Lucknow, Uttar Pradesh, India
5 Department of Applied Science and Humanities, Bundelkhand Institute of Engineering and Technology, Jhansi, Uttar Pradesh, India
|Date of Web Publication||18-Sep-2019|
Mr. Manindra Bhushan
Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, Sector-5, Rohini, New Delhi - 110 085
Source of Support: None, Conflict of Interest: None
Background: Metallic implant in radiotherapy leads to difficulty in tumor target and critical organ delineation. Four-field box technique is conventional approach to treat pelvic malignancies. Aim of the Study: The aim of study is to evaluate the dosimetric impact of calculation algorithms in the treatment of carcinoma cervix with metallic implants. Materials and Methods: A paraffin wax-coated iron rod was used to evaluate the beam characteristics under the influence of metallic implant. Beam characteristics such as tissue phantom ratio (TPR20,10) were measured and analyzed. 15 patients with and without metallic prosthesis of carcinoma cervix were compared in the study. Planning was done for the prescription dose of 45 Gy/25 fractions. Plans were calculated using AAA algorithm and recalculated using Acuros XB (AXB) and pencil beam convolution algorithms for the same monitor units. RTOG and Quantec Protocol were used for plan evaluation. Results: Transmission and TPR20,10increases with field size and beam energy. Surface dose Dsalso increases with field size. D98%and D2%of planning target volume showed a significant difference for AAA versus AXB. 4FN (AAA) are significantly better for all the 4F plans, calculated by three algorithms in case of V15Gyof small bowel. Analyzed data indicated the significant attenuation caused by high-Z material. Analyzed value of conformity index showed that value of index comes >1 in all the cases. Conclusion: The results indicate that when creating treatment plans for cervical cancer lesions with metallic prosthesis, the AAA algorithm would be a more appropriate choice.
Keywords: AAA, Acuros XB, carcinoma cervix, metallic implant, prosthesis
|How to cite this article:|
Bhushan M, Yadav G, Tripathi D, Kumar L, Kishore V, Chowdhary RL, Kumar G, Datta S, Mitra S, Gairola M. Clinical dosimetric impact of AAA and Acuros XB on high-density metallic implants in case of carcinoma cervix. Oncol J India 2019;3:28-37
|How to cite this URL:|
Bhushan M, Yadav G, Tripathi D, Kumar L, Kishore V, Chowdhary RL, Kumar G, Datta S, Mitra S, Gairola M. Clinical dosimetric impact of AAA and Acuros XB on high-density metallic implants in case of carcinoma cervix. Oncol J India [serial online] 2019 [cited 2019 Dec 12];3:28-37. Available from: http://www.ojionline.org/text.asp?2019/3/2/28/266978
| Introduction|| |
Hip fractures are common cause of morbidity and mortality in women of older age group. In a published Swedish study, data showed that significant deaths occurred due to hip fractures in the people of the age 50 years or above.
It is known that the radiation delivered for treating the patients may lead to bone damage and further can result as fracture. Although the data related to radiation-induced fractures are scarce, it is a thing not to be ignored off.
It has been a conventional approach to use four-field box technique to treat pelvic malignancies before the evolution of intensity-modulated radiation therapy (IMRT). In IMRT, a dedicated planning system is used to optimize the beam intensities to conform the planning target volume (PTV) precisely while minimizing the dose to critical structures. IMRT has become widely accepted due to these features.,
Metallic hip prostheses are made up of high-Z materials. Due to scattering of radiation signals in computed tomography (CT) images, these metallic hips produce streak artifacts [Figure 1]. This leads to difficulty in tumor target and critical organ delineation. In addition to this, the commercially available planning systems do not calculate accurately in the near and far regions of metallic hip implants, due to deviation from beam modeling parameters. However, in the presence of metallic hip prosthesis, the dose distribution alters due to beam attenuation and inhomogeneity.,, The technique using intensity modulation may be a better alternative to treat such patients.
|Figure 1: Computed tomographic image showing artifacts due to phantom metal implant|
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The report of AAPM Task Group 63 reveals the fact that around 1%–4% of radiotherapy patient population is having metallic implant which is of high-Z materials. Cobalt–Chromium–Molybdenum alloy is considered to be the best choice for hip prostheses due to its feature of fatigue and corrosion resistance and mechanical strength. As an alternative, prostheses made up of stainless steel and titanium is also commercially available and widely acceptable in clinics.,
Many a time, the actual geometry of the prostheses is not known which will further overdose or underdose the target. These practical difficulties lead to compromise target treatment.
Catlı and Tanir published a study showing evaluation of Eclipse TPS for dose distribution of the patient having hip prostheses. They showed that the significant changes of dose occurred due to radiation scattering for high-Z material of hip prostheses. They observed that the dose difference depends on density and type of prostheses material as well as photon energy used for treatment planning. It was found that the metallic prostheses create significant dose perturbation, and pencil beam convolution (PBC) algorithm cannot predict the dose accurately.
Availability of Monte Carlo (MC) algorithm for dose calculation in such patient's treatment planning has shown a significant improvement and can be used as gold standard.,
Ojala et al. analyzed the accuracy of Acuros XB (AXB) algorithm and compared with the MC simulation for the volumetric modulated arc therapy and found a good agreement between two algorithms. Their study showed that the AXB calculated the dose in the vicinity and inside the prostheses of high-Z material accurately.
IMRT has become popular after advancement of technology for treating patients having deadly malignancies; but, generating a plan for patient having metallic implant is still a challenge. The high atomic number material not only creates streak artifacts in CT images but also significantly attenuates the laterally placed fields. Fattahi and Ostapiak found that intrafraction motion error can be about 5cGy per mm for 2 Gy fraction dose for the match planes.
Radiotherapy planning for the patients having metallic implants is complicated due to few reasons. The metallic hip prostheses create severe streak artifacts that create difficulty in target and critical organ delineation. Second, the electron density near or within the prosthesis deviates significantly from the original values, resulting in dose calculation error. In addition, the approximate percentage of attenuation is around 10%–64% for the beam passing through the prostheses, requires accuracy in modulation. However, the complication of target delineation can be overcome by fusing magnetic resonance imaging with CT images. However, the other two difficulties can be avoided by choosing suitable beam angles to ignore metallic prostheses in dose calculation. It has been shown that the dose to a point, distal to prosthesis, for a single megavoltage photon beam attenuates significantly and reduction in dose appears as 10%–40% as compared to homogeneous medium.
It has been found by many researchers that the dose calculation algorithms such as PBC, Collapsed Cone (CC), and AAA behave differently in the presence of complex geometry with inhomogeneity along the beam path.,,,, In fact, the dose calculation algorithms, i.e., AAA and CC are basically the convolution/superposition algorithms which takes care the primary and secondary dose kernel, derived by MC simulation.
AAA is considered superior as it considers the dose inhomogeneity correction in beam direction (the direction of dose deposition) as well as in the lateral direction, including the electron density-based photon scatter of beam., Liu et al. compared the dosimetric parameters of treatment plans of carcinoma lung patients, calculated by AAA and AXB calculation algorithms. They found that the AXB achieves low conformity and higher heterogeneity for the target when compared to AAA.
Kroon et al. evaluated the plans of lung cancer patients of Stage I and Stage III and found that the PTV doses differ around −12.3% and −0.8% in respective stages with the dose calculated by AXB, as compared to AAA. The PTV mean dose was lower by −4.9% when calculated by AXB and was compared with AAA plans.
Rana et al. also published that the mean doses and maximum doses to PTV were lowered by −0.3% and −4.3%, respectively, when calculated by AXB algorithm. Khan et al. found that the monitor units (MUs) required delivering the same prescribed dose were 2% higher for AXB calculated plans when compared with AAA calculated plans.
The aim of the study is to evaluate the dosimetric impact of calculation algorithms in the presence of metallic hip implants in the treatment of carcinoma cervix.
| Materials and Methods|| |
Phantom metal implant
Density of steel and iron are approximately similar (d = 7.87 g/cm3 and d = 7.81 g/cm3 respectively). Since stainless steel is used as clinical prosthesis, we used iron metal to fabricate the similar implant. To evaluate the beam characteristics under the influence of metallic implant, an indigenously made phantom was used. The cylindrical phantom was made using iron rod and covered with paraffin wax coating. Paraffin wax was used to simulate the effect of adjacent tissues. The dimension of the phantom is shown in [Figure 2].
The measurements were carried out on Varian Clinac-iX Linear Accelerator (Varian Medical Systems, Palo Alto, CA, USA) using phantom metal implant (PMI). PMI was placed on a Perspex tray in the central axis of the 6MV beam at the minimum possible distance from the X-ray source. The beam profiles were taken for three field size, i.e., 5 cm × 5 cm, 10 cm × 10 cm, and 20 cm × 20 cm for two depths as dmax(1.5 cm) and 10 cm. Percentage depth dose was also measured for above field sizes. All the measured profiles and dose curve were compared with normal 6MV and 15MV beam, and data were tabulated. Experimental setup is shown in [Figure 3].
Tissue Phantom Ratio20,10
Measurements were performed using CC13 chamber (IBA Dosimetry, Germany) having active volume 0.13 cc. Chamber was placed inside a 30 cm × 30 cm × 30 cm water phantom at the two different depths, i.e., 20 cm and 10 cm, keeping source-to-axis distance at 100 cm. Chamber was exposed for 100 MUs, and the ratio was established for the charge collected, for the above respective depths. Data were gathered for with and without PMI as shown in [Figure 4] and [Figure 5] for 6MV and 15MV, respectively.
Surface dose (Ds)
Parallel plate chamber 40 (IBA Dosimetry, Germany) was used to measure the surface doses. Chamber was kept at the surface with source-to-surface distance (SSD) as 100 cm along with a water phantom (dimension 30 cm × 30 cm × 30 cm). Charge was collected for different field sizes when exposed for 100 MUs, with and without PMI and shown in [Figure 6].
Percentage depth dose
The measurements were carried out using CC13 thimble chamber (IBA Dosimetry, Germany), by placing at the central axis with SSD at 100 cm, inside a Radiation Field Analyzer, Blue Phantom (IBA Dosimetry, Germany). The data were captured up to the depth of 31 cm inside the water. Measurements were done for small field size 5 cm × 5 cm, reference field size 10 cm × 10 cm, and large field size 20 cm × 20 cm for with and without PMI. Software OmniPro Accept v7.0 (IBA Dosimetry, Germany) was used for analysis [Table 1].
|Table 1: Variation of percentage depth dose due to phantom metal implant|
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In-air measurements were carried out to measure the attenuation, caused by metallic implant. Farmer chamber FC-65 (IBA Dosimetry, Germany) with a buildup cap was used to assess the effect of transmission through PMI. Chamber was kept in air at source-to-detector distance 100 cm and exposed for 100 MUs for different field sizes. Measurements were performed for with and without PMI, and ratio was established to get the transmission factor [Figure 7] and [Figure 8].
Beam profiles were measured using Radiation Field Analyzer, Blue Phantom (IBA Dosimetry, Germany). CC13 thimble chamber (IBA Dosimetry, Germany) and IC15 ion chamber (IBA Dosimetry, Germany) were used for relative dosimetry by placing the CC13 at SSD at 100 cm. Inline and crossline profiles were taken for small field size 5 cm × 5 cm, reference field size 10 cm × 10 cm, and large field size 20 cm × 20 cm for with and without PMI. OmniPro Accept v7.0 (IBA Dosimetry, Germany) Software was used for analysis [Figure 9] and [Table 2].
|Table 2: Analysis of beam profile parameters and variation due to phantom metal implant|
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Patient selection and simulation
15 patients of carcinoma cervix having metallic hip implant were included in the study and compared with similar another 15 patients without hip prosthesis of cervical cancer. Before 30 min of CT scan, the patient was asked to drink water to fulfill the bladder protocol. The patient was immobilized using Orfit Cast and AIO. CT scan was taken in the supine position. In-room lasers were used to align the patient, and the position was verified by using a topograph. The slices acquired were of 3 mm thickness, and the acquired CT data series was sent to TPS using digital communication. The target volumes and critical structures were contoured by oncologist as per guidelines of RTOG, and the planning was done by medical physicist.
PTV and nearby critical structures were drawn for all the patients on their respective tomographic images using SomaVision workstation (Varian Medical System, Palo Alto, California, USA). Clinical target volume (CTV) included lymphonodal region, uterus, adnexa, and vagina. Lymphonodal region included common iliac, external iliac, internal iliac, presacral, and obturator. PTV was generated by giving 5.0 mm margin to CTV. Critical structures such as bladder, rectum, femoral heads, and bowel loop were also delineated. RTOG guidelines were followed in delineating the contours.,
The planning was done for the prescription dose of 45 Gy/25 fractions as 1.8 Gy per fraction for all the patients. For each patient, the planner has made fix isocentric plans with the same isocenter in the present study. Varian Clinac-iX linear accelerator (Varian Medical System, Palo Alto, California, USA), equipped with millennium-multileaf collimator (m-MLC) was used for the treatment delivery. M-MLC is characterized by special resolution of 40 mm × 5.0 mm central leaves and 20 mm × 10.0 mm in outer leaves.
Conventional approach of treating the target by 4-field box technique has an advantage that it reduces the chances of tumor missing in the field border areas.
Plans were made using gantry angles 0, 180, 90, and 270 with collimator 0 and couch 0. Plans were calculated using AAA algorithm and recalculated using AXB and PBC algorithms for the same MUs as obtained in AAA calculated plans.
This exercise was done to evaluate the effect of different algorithms on the dose calculations irrespective of MUs.
The protocol given by Quantec et al. was used to evaluate the doses to PTV and critical structures. Using cumulative dose-volume-histogram, the following parameters were evaluated:
- PTV: D98%, D95%, D50%, D2%, D107%, D110%, conformity index (CI98%), homogeneity index (HI)
- Bladder: D50Gy, Dmax, Dmean, D2cc
- Rectum: D50Gy, Dmax, Dmean, D2cc
- Bowel: D5 Gy, D30 Gy, Dmean, V15 Gy, V45 Gy
- Left femoral head: D50 Gy, Dmean
- Right Femoral Head: D50 Gy, Dmean.
Here, Dx% denotes the dose received by “x”% of the target volume and DyGy denotes the percentage volume of the organ receiving “y”Gy dose. D2cc is the dose received by 2cc volume of the organ. However, VzGy is the absolute volume of the structure covered by “z”Gy dose. Dmax and Dmean are the maximum and mean dose of the structure, respectively.
The CI corresponds to the dose coverage. CI equal to 1 is an ideal condition, and in case, CI > 1 indicates that irradiated volume exceeds the target volume and covers part of the healthy tissues. It may be defined as:,,,
CI98= Volume of 98% isodose/PTV volume
The value of CI less than one indicates partial irradiation of the target volume. It is difficult to achieve the ideal value; RTOG define a range of CI values to judge the quality of conformity, as discussed below:
- CI: Between 1.0 and 2.0, the treatment is well with the protocol
- CI: Between 2.0–2.5 and 0.9–1.0, minor deviation from the protocol
- CI > 2.5 and <0.9, severe deviation from the protocol.
Dose index (HI) is an objective tool to analyze the uniformity of dose distribution in the target volume. It may be defined as:,
HI = (D2%− D98%)/D50%
Data were analyzed using paired-t test using SPSS software 20.0 (SPSS Inc., Chicago, IL, USA). P < 0.05 was considered as statistically significant.
| Results|| |
For phantom metal implant
Few most significant parameters related to beam characteristics were evaluated. Transmission through the prosthesis material increases with field size, irrespective of beam energy. Tissue phantom ratio (TPR20,10) is important parameter related to beam quality. TPR20,10 increases with field size and beam energy. Surface dose Ds also increases with field size.
For clinical cases
Four-field plan (4F) was made for patients having hip prosthesis, and another 4-field plan (4FN) was related to the patients without hip prosthesis. D98% of PTV was 48.41 (SD: 1.004), 48.07 (SD: 1.005), 52.69 (SD: 1.099), 49.01 (SD: 0.852), 48.54 (SD: 0.796), and 53.37 (SD: 0.721) Gy for 4FN (AAA), 4FN (AXB), 4FN (PBC), 4F (AAA), 4F (AXB), and 4F (PBC), respectively. Analysis showed a significant difference for AAA and AXB (P = 0.001 and P = 0.009) for both the techniques [Table 3].
|Table 3: Evaluated parameters for target coverage and OARs w. r. t. algorithms and their P values|
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Similarly, to evaluate the maximum dose to PTV, D2% were 55.44 (SD: 1.77), 55.29 (SD: 1.76), 59.88 (SD: 1.93), 56.59 (SD: 1.71), 56.23 (SD: 1.82), and 60.89 (SD: 1.84) Gy for 4FN (AAA), 4FN (AXB), 4FN (PBC), 4F (AAA), 4F (AXB), and 4F (PBC), respectively. Data showed a significant difference in D2% for AAA versus AXB (P = 0.003) for 4FN as well as for AAA versus PBC (P = 0.001) for 4F treatment plans [Figure 10].
|Figure 10: Planning target volume coverage showing 95% of prescription dose|
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Percentage volume of bladder receiving 50 Gy (D50Gy) were 80.16 (SD: 12.51), 74.56 (SD: 16.56), 87.07 (SD: 10.65), 93.81 (SD: 6.78), 92.81 (SD: 7.53), and 96.54 (SD: 3.75) % for 4FN (AAA), 4FN (AXB), 4FN (PBC), 4F (AAA), 4F (AXB), and 4F (PBC), respectively. The data showed a significant difference of AAA for 4FN versus 4F plans (P = 0.022). Similarly, the data indicate a significant difference for AAA and PBC in 4FN treatment technique (P = 0.018).
The maximum dose to rectum were 51.57 (SD: 0.92), 51.61 (SD: 1.02), 55.95 (SD: 1.09), 53.13 (SD: 1.88), 52.99 (SD: 1.66), and 57.43 (SD: 1.58) Gy for 4FN (AAA), 4FN (AXB), 4FN (PBC), 4F (AAA), 4F (AXB), and 4F (PBC), respectively. Analysis showed that the 4FN (AAA) has edge over 4F (AXB) with significant difference in the values (P = 0.031). However, there is significant difference between AXB and PBC calculation for 4FN plans (P = 0.001).
The mean doses to bowel were 23.88 (SD: 5.72), 23.71 (SD: 5.69), 25.43 (SD: 6.27), 24.67 (SD: 5.65), 24.28 (SD: 5.53), and 26.37 (SD: 5.98) for 4FN (AAA), 4FN (AXB), 4FN (PBC), 4F (AAA), 4F (AXB), and 4F (PBC), respectively. The data showed that the AXB has an edge over AAA calculated 4F plans (P = 0.015). However, 4FN (AAA) are significantly better for all the 4F plans, calculated by three algorithms (P = 0.005, P = 0.005 and P = 0.004) in case of V15Gy[Figure 11].
| Discussion|| |
Analyzed data indicated the significant attenuation caused by high-Z material. Increased transmission with field size and beam energy may be due to added contribution of scatter beam component into the primary dose. It is clear that prosthetic implant dramatically increases the value of TPR20,10, i.e., beam quality. This increases the average energy of beam, and finally, the whole dose distribution got affected. Variation of surface dose is due to implanted material as the prosthesis attenuates the beam significantly.
The use of prosthesis and implants are rapidly increasing in the form of variety of materials like plastic, metals, and ceramics. These materials present in the vicinity of tissues affect the dose delivered by radiotherapy. It is important to assess the effect of these devices on treatment plan, especially when the accurate dimension and description of the implant is not known. Of all these materials, metals are the matter of most concern.
Any perturbation to the implant will depend on composition of the prosthesis and the beam characteristics like beam energy. Having hip prosthesis in the radiotherapy planning for the treatment of pelvic tumors has always been a difficult task due to the dose alteration by the metallic prosthesis.
Hudson et al. demonstrated that the shadowing by the prosthesis may be an important factor for dose difference in radiotherapy plan. Biggs and Russell described that a hollow prosthesis with 5 cm diameter and 3 mm wallthickness may underdose the target by 2% for 25 MV X-rays and overdose the tumor by 2% for 10MV X-rays and 5% for Co-60 beam. Sibata et al. found that the algorithm used in the planning may be used to predict the dose distribution under a femoral hip prosthesis within acceptable uncertainty. It was also found that up to 50% attenuation may occur for a solid implant.
In this study, we compared the dosimetric quality of 4-field plans for cervical cancer patients with metallic hip prosthesis, and the dosimetric evaluation was done by applying equal MUs for all the calculation algorithms. In this study, we demonstrated the dosimetric quality of box technique using four fields to treat the patients of carcinoma cervix with a single metallic hip prosthesis.
Analyzed value of CI showed that value of index comes >1 in all the cases. Even though it is within the tolerance protocol, higher value indicates obvious radiation of healthy tissues due to box technique [Figure 12].
Fragoso et al. also observed a marked reduction in calculated dose to the PTV when PBC-based plans were re-calculated with the MC-based algorithm. The minimum PTV doses shown with the PBC and MC were 97% and 58%, respectively. The 80% and 95% isodose lines covering the intermediate target volume and PTV shown in a lung patient study, when calculated with PBC, did not encompass the PTV when recalculated with MC. A similar trend was also found in our study.
D50 as well as mean dose to the bladder and rectum are significantly lower for the patients without hip prosthesis, calculated by AAA and AXB. Lloyd and Ansbacher also concluded that AXB algorithm is a beneficial dose calculation tool for treatments with high-Z materials, as its accuracy is equal to MC, yet fast enough for clinical application.
Mean dose to bowel were significantly better for AXB calculated plans when compared with AAA and PBC. Erlanson M et al. also demonstrated that there were small deviations in dose distributions calculated by the AXB algorithm, on average within 2.5%, when compared to the MC model. Oinam and Singh found that for the surface areas of 1, 50, and 100 cm2, PBC overestimates doses as compared to AAA calculated value in the range of 1.34%–3.62% at 0.6 cm depth. In high-dose buildup region, AAA calculated doses were lower by an average of − 7.56% (SD: 4.73%), while PBC was overestimated by 3.75% (SD: 5.7%). They concluded that AAA calculated the dose more accurately than PBC in clinically important depths. Furthermore, AAA is considered more appropriate in our study due to the findings such as dose coverage of PTV and lesser dose to critical structures as well as on the basis of related studies given in the references.
| Conclusion|| |
The results from this study indicate that when creating treatment plans for cervical cancer lesions with metallic prosthesis, the AAA algorithm, a scatter-based dose model with increased accuracy in scattered dose calculation, would be a more appropriate choice. If one decides to use the AAA calculation algorithm, the prescribed dose should be adjusted down by 10%–14% to maintain equivalence to plans generated by the PBC algorithm.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12]
[Table 1], [Table 2], [Table 3]