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 Table of Contents    
ORIGINAL ARTICLE
Year : 2022  |  Volume : 47  |  Issue : 3  |  Page : 262-269
 

To study the impact of different optimization methods on intensity-modulated radiotherapy and volumetric-modulated arc therapy plans for hip prosthesis patients


1 Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh; Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, India
2 Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh, India
3 Radiological Physics and Internal Dosimetry Group, Institute of Nuclear Medicine and Allied Sciences, Defence Research and Development Organisation, Delhi, India
4 Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, New Delhi, India

Date of Submission17-Feb-2022
Date of Decision12-May-2022
Date of Acceptance26-May-2022
Date of Web Publication8-Nov-2022

Correspondence Address:
Mr. Pawan Kumar Singh
Amity Institute of Applied Sciences, Amity University, Noida, Uttar Pradesh; Department of Radiation Oncology, Division of Medical Physics, Rajiv Gandhi Cancer Institute and Research Centre, New Delhi
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jmp.jmp_14_22

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   Abstract 

Purpose: To study the impact of different optimization methods in dealing with metallic hip implant using intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) techniques. Materials and Methods: A cohort of 16 patients having metallic implants was selected for the study. Three sets of IMRT and VMAT plans were generated. Set 1 IMRT (IM_Base), VMAT (VM_Base) without any restrictions on beam entry and exit, set 2 (IM_ENT and VM_ENT) optimizer restricts the beam entry and set 3 (IM_EXT+ENT), neither entry nor exit doses were allowed toward the metallic implant. Results: There was no significant difference in target (D95%) and organ-at-risk doses between IM_Base and IM_ENT. There were significant (P = 0.002) improvements in planning target volume (PTV) V95% and homogeneity from IM_EXT+ENT to IM_ENT. There was no significant difference in plan quality between VM_Base and VM_ENT. There were significant (P = 0.005) improvements in PTV, V95%, homogeneity from VM_EXT+ENT to VM_ENT. V40Gy, V30Gy for bladder, rectum, bowel, and bowel maximum dose decreases significantly (P < 0.005) in IM_ENT compared to IM_EXT+ENT, but not significant for VMAT plans. Similarly, there was a significant decrease in dose spill outside target (P < 0.05) comparing 40%, 50%, 60%, and 70% dose spills for IM_ENT compared to IM_EXT+ENT, but variations among VMAT plans are insignificant. VMAT plans were always superior to IMRT plans for the same optimization methods. Conclusion: The best approach is to plan hip prosthesis cases with blocked entry of radiation beam for IMRT and VMAT. The VMAT plans had more volumetric coverage, fewer hotspots, and lesser heterogeneity.


Keywords: Artifacts, intensity-modulated radiotherapy, metallic implants, optimization, volumetric-modulated arc therapy


How to cite this article:
Singh PK, Tripathi D, Singh S, Bhushan M, Kumar L, Raman K, Barik S, Kumar G, Shukla SK, Gairola M. To study the impact of different optimization methods on intensity-modulated radiotherapy and volumetric-modulated arc therapy plans for hip prosthesis patients. J Med Phys 2022;47:262-9

How to cite this URL:
Singh PK, Tripathi D, Singh S, Bhushan M, Kumar L, Raman K, Barik S, Kumar G, Shukla SK, Gairola M. To study the impact of different optimization methods on intensity-modulated radiotherapy and volumetric-modulated arc therapy plans for hip prosthesis patients. J Med Phys [serial online] 2022 [cited 2022 Nov 29];47:262-9. Available from: https://www.jmp.org.in/text.asp?2022/47/3/262/360588



   Introduction Top


According to GLOBOCAN-GCO data, 20 million new cancer cases were reported in year 2020 and 20% of cases were from the pelvic region.[1] The prostate, colorectal, and cervix malignancies were the common sites in this region, and bone loss is common for both men and women of older age. Osteoporosis increases for older persons of both genders. Patients who undergo hormonal therapy face a significantly higher rate of bone loss; it may go up to 4%–5% for male prostate patients put on androgen deprivation therapy.[2],[3] These patients may suffer a bone fracture because of these reasons and to stabilize their bone metallic inserts were implanted in the desired region. Therefore, the number of patients having metallic implants is also increasing.

Patients with metal implants pose a challenge in the treatment planning process for radiation oncologists and physicists. The radiotherapy (RT) starts with the simulation of a patient and computed tomography (CT) images are standard 3-dimensional (3D) imaging procedure for RT patients. The presence of metal in the treatment region changes the attenuation profiles of photon beams passing through it[4] because of these profiles and scattered photons different artifacts are produced in the CT images.[5],[6],[7] A few artifacts seen during CT procedures are streaking, shading, rings, and distortions. The actual electron density (ED) and observed ED varies in the surroundings of high Z metallic implants.[7] The perturbations cause error in organ and target delineations.[8] This further leads to underdosing of the target and overdosing of organs at risk (OAR).[9],[10]

According to AAPM TG-63, the treatment beams should be avoided from the implanted region.[11] The photons passing through the implant reduce the intensity and increase the dose at the exit interface due to an increase in secondary electron generation, leading to a complete change in the depth dose profile of the photon beam. If the exit doses are allowed in the implanted region, the backscatter of photon and electron causes a higher dose disposition at the interface.[12] Gullane did measurements using a wall-less ionization chamber (IC) at the interfaces of implant and tissue for stainless steel and titanium. They measured for both single and parallel-opposed beams and reported that interface doses increase by as much as 50% at the proximal surface of the metallic inhomogeneity.[13]

Previous researchers have studied the different planning strategies in patients having metallic implants in the femur. To evaluate the dosimetric impact on the planning target volume (PTV), the OARs, they used skip arcs in volumetric-modulated arc therapy (VMAT), block field in intensity modulated radiation therapy (IMRT) and used hard constraints during the optimization in their studies.[14] Prabhakar et al. used VMAT techniques with hard constraints for an avoidance structure created around prosthesis to restrict doses in the surrounding of the prosthesis.[15] They found this method simple and effective for prostate patients having prosthesis. Kung et al. compared 5-field and 6-field 3D conformal radiation therapy plans with 9-field IMRT plans. They have recommended 9-field IMRT plans for prostate patients having metallic prosthetic implants.[16]

This study planned to evaluate two standard treatment planning techniques, i.e., IMRT and VMAT, with different optimization methods.[17],[18] In the first method, there was not any restriction on entry or exit of the radiation beam. The method was a standard method used for plan optimization of pelvis patient. The second method restricts the entry of radiation beams through the implanted material shown in [Figure 1]a, and in the third method both entry and exit of the radiation beam was blocked toward implanted material shown in [Figure 1]b. This study uses fixed field IMRT and VMAT template-based treatment planning approaches, which will help researchers use knowledge-based planning in these patients. The methods were implemented to study the implication on plan quality and their feasibility to use in the clinic. The plans were created and exposed on a similar geometry pelvis phantom to validate the methods.[19]
Figure 1: (a) Describes the optimization geometry for a treatment field; Beam 1 was directly facing the implant and the entry of beam is blocked through it, but there was no restriction on Beam2 path and in (b). Both entry and exit of the Beam was blocked

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   Materials and Methods Top


Simulation and contouring

Patients having metallic implants in the femur were simulated on a Somatom Sensation Open CT simulator (Siemens Healthineer, Germany) (maximum HU = 3000) using 5 mm slice thickness in head first supine position. All 16 patients (right hip implant = 9, left hip implant = 5, and bilateral hip implant = 2) suffering from carcinoma of cervix. The material used for was “titanium”(composition: titanium 88.5%–91.0%; aluminum 5.6%–6.5%, vanadium 3.5%–4.5%, iron 0.25%, oxygen 0.13% and carbon 0.08%, average ED 3.74 relative to water; diameter of femoral heads of prosthesis ranging from 40 to 54 mm.[20] The thermoplastic cast was made for every patient. The acquired images were transferred to Varian Soma vision contouring stations. The contouring was done using EMBRACE II Guideline.[21] The PTV, clinical target volume, bladder, rectum, and bowel structures were drawn. Implanted material was also contoured to mark the avoidance zones.

Treatment planning

For each patient, both IMRT and VMAT plans were created. Seven field IMRT plans with gantry angles 60°, 100°, 135°, 180°, 230°, 265°, 300° were created. Similarly, VMAT plans were created with two complete arcs (181°–179°) Clockwise and (179°–181°) counterclockwise. All beams were co-planar. Plans were created on Varian Clinac iX 2300CD linear accelerator using the beam energy of 6 MV, machine had Millennium 120 multiple leaf collimator which offers 0.5 cm leaf resolution at isocenter for the central 20 cm of the 40 cm × 40 cm field. Plans were optimized for 45 Gy in 25 fractions keeping the OAR constraints as per the departmental protocol. IM stands for IMRT plans; VM stands for VMAT plans. Three sets of plans were optimized. In set one, there were no restrictions on beam path and the plans were named as IM_Base and VM_ Base; in set two the entry of the beam through the implanted material was blocked and denoted as IM_ENT and VM_ENT; in third set both entry and exit were blocked through implanted material named as IM_EXT+ENT and VM_EXT+ENT. All plans were optimized using photon optimizer in Eclipse 15.1 treatment planning systems (TPS) (Maximum, HU = 6000, Relative ED = 3.920) (Varian Medical System, Palo Alto, CA) and calculated with anisotropic analytical algorithm, using 2.5 mm calculation grid spacing.

Plan evaluation and data analysis

All the plans were re-normalized to receive 95% of the PTV to the prescription isodose of 45 Gy. To compare the plans we used V95% (volume of 95% isodose), D95% (dose to the 95% isodose), D98%, D2%, Dmean, dose homogeneity index (DHI), V107% and conformity parameters for PTV. For bowel, bladder and rectum V40Gy, V30Gy were used. Bowel Dmax was also monitored to check the variation in OAR doses. For dose spillage outside the target we extracted the volumes of dose receiving 30%, 40%, 50%, 60% and 70% for structure Body-PTV. All the parameters were extracted using dose-volume histogram Metric Package of R Software (R version 3.6.1) [R Foundation for Statistical Computing, Austria].[22]

We performed statistical analysis using Microsoft office and Python Software (Spyder IDE version 5.1.5) [Python. Scotts Valley, CA: CreateSpace][23] Paired t-test with P ≤ 0.05 was considered significant. DHI was calculated according to ICRU 83.[24] Conformity index was calculated using the RTOG formula.[25]



TVRI volume covered by 95% Isodose, TV is total volume of PTV.

Verification

Pelvis phantom (solid water, RW3 whitepolystyrene material by ScanditronixWellhofer) was used to verify the methods used. The scan was done on CT Simulator with CC 13 (ScanditronixWellhofer) and CC 0.01 IC. These chamber inserts were created to represent the target (CC 13) and implanted region (CC 0.01). The slice thickness was kept 1 mm to avoid delineation errors for chamber volume. The dummy target PTV, bladder, rectum, dummy implant, and femoral heads were created.[26] The chamber volumes were assigned HU value 0. [Figure 2] shows the phantom, dummy target, OARs, and dose distribution.
Figure 2: (a) The phantom and chamber inserts. (b) the topographic view of chambers placed, (c) the contours drawn on the phantom and (d) the dose distribution of 95% isodose

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   Results Top


[Table 1] shows all the dosimetric parameters for IMRT plans, IM_Base, IM_ENT and IM_ENT+EXT planning techniques. All plans were normalized to receive the prescription dose. Results were reported as mean ± standard deviation.
Table 1: Dosimetric parameters for all intensity-modulated radiotherapy plans

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Comparing the V95%, D98% and conformity parameters of PTV, there was no significant variation for IM_Base and IM_ENT plans. However, there were significant increase in D mean (P = 0.028), D2% (P = 0.009), V107% (P = 0.050) and also increase in homogeneity of IM_ENT plans DHI (P = 0.010). When we compared the IM_Base plans with IM_ENT+EXT, there was significant variation V95% (P = 0.001), D98% (P = 0.005), D mean (P = 0.002), D2% (P = 0.002), DHI (P = 0.002) and V107% (P = 0.005) except the conformity of plans. The IM_Base plans were better compared to IM_ENT+EXT. Again comparing the IM_ENT plans with IM_ENT+EXT, there was significant variation V95%, D98%, D mean (P = 0.006), D2%, DHI and V107% (P = 0.005) except the conformity of the plans. The IM_ENT plans have increased V95%, D98%, DHI and decreased values of Dmean, V107%, D2% than the IM_ENT+EXT plans.

For OAR, comparing V30Gy and V40Gy for rectum and bowel, also bowel Dmax, there was no significant variation between the IM_BASE and IM_ENT plans. There was a significant increase (P = 0.003) in V30Gy from the IM_Base to IM_ENT plans for the bladder. There was no significant difference in 30%, 40%, 50%, 60%, and 70% spill outside target volume between both the plans.

Comparing IM_BASE plans with IM_ENT+EXT for V30Gy, V40Gy, all values increase significantly (P < 0.05) for bladder and rectum. Although there was significant variation in bowel V30Gy (P = 0.037) and Dmax having (P = 0.002), there was no significant variation in bowel V40Gy parameter. There was no significant difference in 30% spill outside target volume, but there was significant increase in 40% (P = 0.004), 50% (P = 0.001), 60% (P = 0.004), and 70%, spill outside target volume between both the plans.

There was no significant variation for V30Gy, V40Gy for rectum and bowel between the IM_ENT and IM_ENT+EXT plans. There was no significant variation in bowel Dmax. There was a significant increase in V30Gy (P = 0.003) and V40Gy (P = 0.049) from the IM_ENT to IM_ENT+EXT plans for the bladder. There was no significant difference in 30%, 40%, and 50% spill outside target volume, but there was significant increase in 60% (P = 0.004), and 70% (P = 0.002), spill outside target volume between IM_ENT to IM_ENT+EXT plans.

[Table 2] shows all the dosimetric parameters for VMAT plans, VM_Base, VM_ENT, and VM_ENT+EXT planning techniques. Comparing the V95%, Dmean, D2%, DHI, V107%, D98% and conformity parameters of PTV, there was no significant variation for VM_Base and VM_ENT plans. When we compared the VM_Base plans with VM_ENT+EXT, there was significant variation V95%, D98%, Dmean, D2%, DHI and V107% (P = 0.001) except the conformity of plans. Again comparing the VM_ENT plans with VM_ENT+EXT, there was significant variation V95%, D98%, (P = 0.001), Dmean (P = 0.006), D2% (P = 0.002), DHI (P = 0.001) and V107% (P = 0.004) except the conformity of plans. The VM_ENT plans have higher values of V95%, D98%, DHI and lower values of Dmean, V107%, D2% than the VM_ENT+EXT plans.
Table 2: Dosimetric parameters for all volumetric modulated arc therapy plans

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Comparing V30Gy, V40Gy for the rectum, bladder, and bowel, there was no significant variation between the VM_BASE and VM_ENT plans. There was no significant variation in Bowel Dmax. There was no significant difference in 30%, 40%, 50%, 60%, and 70% spill outside the target volume between both the plans. Comparing VM_BASE plans with VM_ENT+EXT for V30Gy, V40Gy all values increase significantly (P < 0.05) for bladder and rectum. There was no significant variation in bowel V30Gy and V40 Gy but Dmax increased significantly. There was no significant difference in 30% spill outside target volume, but there was significant increase in 40% (P = 0.001), 50%, 60% and 70% (P = 0.002), spill outside target volume between both the plans.

There was no significant variation for V30Gy, V40Gy for rectum and bowel between the VM_ENT and VM_ENT+EXT plans. Bladder V40Gy was not changing significantly, but bladder V30Gy (P = 0.021) and bowel Dmax (P = 0.010) increased significantly from the VM_ENT to VM_ENT+EXT plans. There was no significant difference in 30%, 40%, 50%, 60%, and 70% spill outside target volume between VM_ENT to VM_ENT+EXT plans.

[Table 3] shows all the dosimetric parameters for IMRT versus VMAT plans. When we compared the base plans for both the techniques IMRT and VMAT, VMAT plans were better than in terms of volumetric coverage (P = 0.025), D2% (P = 0.090) and conformity (P = 0.030). There were no significant differences in Dmean, D98%, DHI, and V107% between IM_Base and VM_Base. There was no significant variation for V30Gy, V40Gy for bladder, bowel, and bowel Dmax. However, the variations are significant in V30Gy (P = 0.060) V40Gy (P = 0.009) for rectum between both plans. There was no significant difference in 30%, 60%, and 70% spill outside target volume, but there was significant increase in 40% (P = 0.040), and 50% (P = 0.010) spill outside target volume between IM_Base to VM_Base plans.
Table 3: Comparision of intensity-modulated radiotherapy versus volumetric.modulated arc therapy dosimetric parameters

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VMAT plans were found better in terms of volumetric coverage (P = 0.005), Dmean (P = 0.040), V107% (P = 0.024) and conformity (P = 0.006). The variations were also significant in V40Gy (P = 0.001) for bladder and V40Gy (P = 0.018) for rectum between IM_ENT and VM_ENT plans. However, there were no significant differences in D98%, D2%, DHI for PTV, V30Gy for bladder and bowel, bowel Dmax, 30%, 70% spill outside target volume but there was significant increase in 40% (P = 0.003), and 50% (P = 0.010), 60% (P = 0.037) spill outside target volume between IM_ENT to VM_ENT plans.

The comparison of IM_ENT+EXT and VM_ENT+EXT plans shows that VMAT plans were better in volumetric coverage (P = 0.009), D98% (P = 0.003) and conformity (P = 0.001) as compared to the IMRT plans. There were no significant differences in D98%, D2%, V107%, and DHI between IM_ENT+EXT and VM_ENT+EXT. There was no significant variation for V30Gy, V40Gy for bladder and bowel. However, the variations are significant in V40Gy (P = 0.018) for rectum and bowel Dmax (P = 0.030) for both plans. There was no significant difference in 30%, 50%, 60%, and 70% spill outside target volume but there was significant decrease in 40% (P = 0.030) spill outside target volume between IM_ENT+EXT to VM_ENT+EXT plans. [Figure 3] shows the dose distribution given by different optimization methods and techniques.
Figure 3: (a-c). The dose distribution of intensity-modulated radiotherapy plans with different optimization methods and (d-f). Were the dose colorwash for volumetric-modulated arc therapy plans

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To validate the optimization methods, we performed phantom measurements for all techniques used in this study. The point doses were well within ± 3% for target, and the variations in avoidance were <±10%.[27],[28] [Table 4] shows all the phantom measurements and doses from TPS.
Table 4: Phantom measurements

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   Discussion Top


We compared the plan qualities for IM_Base and IM_ENT. We found that the plans were clinically acceptable with no change in target coverage, minimum dose, and conformity. Although with restrictions of beam entry from particular angles, the plans had increased Dmean, maximum dose, and heterogeneity for the target without affecting the OARs and spillage outside the target. When we compared IM_Base with IM_ENT+EXT, the plan's quality deteriorates in all the aspects of the target's coverage, homogeneity, and other parameters. The drastic change in the plan quality was because of the decreased degree of freedom for optimization for the IM_ENT+EXT plans. Comparing the constrained plans IM_ENT and IM_ENT+EXT, the IM_ENT plans were found superior in target dose parameters, but there was little change in OAR doses and spill doses.[29]

Evaluation of the VMAT plans also showed similar observations VM_Base and VM_ENT plans were similar in all aspects. VM_Base plans had better plan quality and OAR sparing than VM_ENT+EXT. In VM_ENT, the target volume parameters were comparatively better than VM_ENT+EXT, without appreciable change in OAR and spillage dose.

While comparing the two techniques, IMRT and VMAT keeping the optimization method the same, we found the VMAT plans were better in target coverage and OAR sparing. However, there was not much variation in spillage outside target volume. The dominance of VMAT plans was due to a higher degree of freedom in optimization for VMAT plans than IMRT.

In our study, all comparisons suggested that the optimization method, where only the beam's entry was blocked, was the best approach to dealing with the patients having prosthetic implants. This approach helped the optimizer avoid the perturbed photon dose profile along the beam path. It provided adequate degrees of freedom to the optimizer to give the feasible solutions.[19],[30] The optimization method three, which completely avoids the implants, results in heterogeneous dose distribution because the optimizer had a minimal scope of beam modulation to provide the optimal solution for the target and OARs. Method two avoided the backscatter, but the study shows less significant spillage outside the target than method three. Method one did not consider any constraints on the entry and exit of the beam. This method included photon profiles for dose calculations, perturbed and unperturbed (without being impacted by the implant), which may lead to higher variation in dose delivery parameters. Therefore, we must avoid using this method for metallic implant patients.

The IM_ENT plans had provided similar dose distribution described in the study conducted by Kung et al. where they used nine equally spaced fields avoiding the implant to plan the prostate cases.[16] R. Prabhakar et al. conclude in their study that the two arcs VMAT plans with avoidance structure are the best approach for hip prosthesis patients.[15] Our results also show consistency with them. The study of Koutsouvelis et al. investigated the need for the avoidance sector in hip prosthesis patients, and they concluded that the avoidance sector was not necessary for VMAT planning in these patients.[31] We also observed the same when we evaluated the parameters for VM_Base and VM_ENT. We only blocked the implant in method two without creating any extra structure around it therefore, method two was the midway approach between the studies conducted by Prabhakar et al. and Koutsouvelis et al. We can quickly implement this method for planning prosthesis cases.

There were many publications on the planning of hip implant patients.[4],[16],[32],[33] For the hip prosthesis patients, the optimization strategies used in our study were less explored. We used seven fields for IMRT plans and two arcs for the VMAT plans. The two options used in this study are part of modern TPS. The options help the planner create templates and write scripts to automate the planning of the patients having metallic implants. The planners can efficiently utilize the characteristics of metallic implant the ED or HU number to discriminate from the human body. Different researchers used metal artifact reduction algorithm, maximum transmission algorithm, and other simulation studies to reduce artifacts in hip prosthesis patients.[34],[35] The absence of actual metallic implant in phantom, artifact reduction software and the effect of positional variation in implant are a few limitations of our study. As future prospects the other sites and effects of Monte Carlo simulation studies should be carried out to validate these methods.


   Conclusion Top


The best approach is to plan hip prosthesis cases with blocked entry of radiation beam for IMRT and VMAT. However, VMAT omits these requirements. Template-based IMRT and VMAT plans can be utilized to plan the patients having prosthesis in the femur and physical characteristics of implants may help implement automated methods.

Acknowledgments

The authors thank the management of Rajiv Gandhi Cancer Institute and Research Centre in New Delhi, India, for their continued support and encouragement to complete this research work.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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