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ORIGINAL ARTICLE
Year : 2016  |  Volume : 12  |  Issue : 2  |  Page : 1045-1049

Dosimetric study of beam angle optimization in intensity-modulated radiation therapy planning


1 Department of Radiotherapy, Post Graduate Institute of Medical Education and Research, Chandigarh; Department of Applied Sciences, Punjab Technical University (PTU), Jalandhar, India
2 Department of Physics, Goswami Ganesh Dutta Sanatan Dharma College, Chandigarh, India
3 Department of Applied Sciences, Punjab Technical University (PTU), Jalandhar; Department of Applied Sciences, Chitkasra University, Rajpura, India
4 Department of Radiotherapy, Post Graduate Institute of Medical Education and Research, Chandigarh, India

Date of Web Publication25-Jul-2016

Correspondence Address:
Arvind Kumar Shukla
Department of Radiotherapy, Post Graduate Institute of Medical Education and Research, Chandigarh-160 012
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0973-1482.157324

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 > Abstract 


Purpose: The quality of intensity-modulated radiotherapy (IMRT) highly depends on the choice of beam orientations and optimization algorithms used in the treatment planning. The present work reports dosimetric study of IMRT plans generated using preselected equiangular beam orientations (PSBO) and beam angle optimization (BAO) for the patients of prostate, head and neck, and esophagus carcinoma.
Materials and Methods: The computed tomography (CT) datasets of 30 patients treated with IMRT for prostate, head and neck, and esophagus carcinoma were selected. Two IMRT plans were made for each patient. First plan was prepared for seven to nine fields PSBO in coplanar arrangement. The second plan was made using the BAO by a computerized algorithm compatible treatment planning system (TPS). The dose–volume histograms (DVHs) of PSBO and BAO plans were compared for all the patients. The treatment plans were compared using the parameters delivered monitor units (MUs), doses delivered to organs at risk (OARs), target coverage (conformity index (CI)), homogeneity index (HI), and quality index (QI).
Results: DVHs generated showed that OARs receive almost identical or slightly better doses in case of BAO as compared to PSBO. CI, HI, and QI values were almost same for two plans. However, we have noticed significant reduction in MUs for all the plans generated using BAO.
Conclusions: It is concluded that BAO provides superior plan with respect to MUs and should be used whenever possible in IMRT planning.

Keywords: Beam angle optimization, conformity index, intensity-modulated radiotherapy, monitor units


How to cite this article:
Shukla AK, Kumar S, Sandhu I S, Oinam AS, Singh R, Kapoor R. Dosimetric study of beam angle optimization in intensity-modulated radiation therapy planning. J Can Res Ther 2016;12:1045-9

How to cite this URL:
Shukla AK, Kumar S, Sandhu I S, Oinam AS, Singh R, Kapoor R. Dosimetric study of beam angle optimization in intensity-modulated radiation therapy planning. J Can Res Ther [serial online] 2016 [cited 2019 Sep 21];12:1045-9. Available from: http://www.cancerjournal.net/text.asp?2016/12/2/1045/157324




 > Introduction Top


External beam radiotherapy (EBRT) has become universally accepted as an appropriate treatment modality with the introduction of megavolt equipment. It has an advantage of low skin dose and greater dose homogeneity at given depth over orthovoltage X-rays. Most of the advance techniques in EBRT, that is, intensity-modulated radiation therapy (IMRT), image-guided radiation therapy (IGRT), stereotactic radio surgery, and stereotactic radiotherapy provides tool to achieve better therapeutic gain. IMRT is a powerful clinical technique designed to deliver precisely radiation dose to the target. IMRT has become standard technique to treat patients with prostate, esophagus, and head and neck cancers.[1],[2],[3] In IMRT, highly conformal radiation doses are delivered from different directions to target volume while minimizing radiation dose to adjacent organs at risk (OARs) and other normal tissues. The quality of plan does not depend only on the treatment planning system (TPS) and beam parameters used for optimization. IMRT optimization is a complex process and iteratively performed to achieve the set desired goal. Considerable efforts have been put to make IMRT easily implementable through increasing the automatization of beam setup, shortening the optimization time, and enhancing ability of the dose verification.[4],[5],[6],[7],[8]

It is well established that selection and optimization of beam angles in IMRT play crucial role to meet the goal of radiotherapy, that is, maximize the dose to tumor target and minimize the dose to critical structures and normal tissue. Usually, the dosimetrist first selects the number of beams with the beam directions to be equally spaced angles. The beam angles are so adjusted that no particular beam affects the critical organ directly. Normally several trial-and-error attempts are needed to find a group of adequate beam angles. Although this procedure leads to good treatment plan, it may not be optimal.[9] The optimal IMRT plans for each individual patient can be generated using computer optimization technique. Optimization routines for IMRT are exceedingly complex and computationally challenging to achieve the goal in a relatively short time. Many optimization algorithms have been introduced for beam angle optimization (BAO) in IMRT. Number of computerized algorithms [4], 6, [10],[11],[12] gives case specific solutions to the beam angle problem where various OARs and target volumes are in close vicinity of tumor target. Some investigators have attempted to provide optimization for the gantry angle and the beam collimator angles.[4]


 > Materials and Methods Top


Computed tomography (CT) datasets of 30 patients of prostate, head and neck, and esophagus carcinoma were used in sliding window IMRT planning. Each of 30 patients was scanned with the slice thickness of 2.5 mm using CT simulator (GE Healthcare, Milwaukee, WI). CT images were sent to the Eclipse TPS (Varian Medical Systems, version 8.6) for standard volume delineation and treatment planning. Planning target volume (PTV) and clinical target volume (CTV) are drawn according to the definition given by international commission on radiation units and measurements.[13] The tolerance of normal tissues to the late effects of radiation limits the dose that can safely be prescribed to the tumor. The tolerance dose varies between tissues and is influenced by the proportion of the organ treated, the length of follow-up, and the end point assessed. Therefore, it would be necessary to identify the various OARs for the patients suffered with prostate, head and neck, and esophagus carcinoma. In case of prostate carcinoma; the OARs are rectum, bladder, and sigmoid. For head and neck carcinoma, the OARs are eyes, eye lens, optic nerve, optic chiasma, brainstem, and temporal lobes. The critical structures for a site with carcinoma esophagus are lungs, liver, heart, and spinal cord. In present measurements, two plans are made for each patient treated with sliding window IMRT technique. First plan for each patient was prepared using seven to nine fields preselected equiangular beam orientations (PSBO) in coplanar arrangement. In the second plan, the selection of optimal gantry angle was chosen by the algorithm used in the BAO. BAO is performed with plan geometry optimization (PGO) algorithm [14],[15] compatible with eclipse TPS.

All the constraints of plan PSBO were used for the optimization of plan BAO and same total dose was prescribed for in each patient. The treatment planning was performed using Eclipse TPS using beam data generated for 6 MV photon beam of Clinac DBX linear accelerator equipped with 80-leaf Millennium Multileaf Collimator (MLC; Varian Medical System, Palo Alto, USA). The constraints applied to these dose–volume histograms (DVHs) consist of minimum, maximum, and specific dose/volume for each structure. For consistency of comparison, all IMRT plans were normalized with delivered dose to percentage volume of the PTV.

Dose conformity, homogeneity, and quality of a plan were represented by conformity index (CI), homogeneity index (HI), and quality Index (QI),[16],[17] respectively. The doses to the OARs like bladder and rectum for carcinoma prostate; right and left eye, right and left lens, and right and left optic nerve were for carcinoma head and neck; and right and left lung, heart, and spinal cord for carcinoma esophagus were also compared.

The sliding window technique was used for IMRT delivery. The optimal fluence was converted into actual fluence by using leaf motion calculator (LMC) that designs the leaf motion patterns. The LMC takes into account several parameters of MLC such as leaf speed, leaf span, transmission, minimum leaf gap, and rounded end effects. The final dose was calculated both for the plan PSBO and plan BAO with a grid size of 2.5 × 2.5 mm 2 including inhomogeneity correction. The dose distribution for the actual fluence was calculated using pencil beam convolution (PBC) algorithm. A paired two-tailed Student's t-test was used for data analysis. The threshold for statistical significance was P < 0.05.


 > Results Top


Dose distributions and DVHs generated using plan PSBO and plan BAO were compared for each patient. It is clear from [Figure 1]a and [Figure 1]b that BAO IMRT plan exhibits more confirmed dose distribution in axial CT slice over PSBO IMRT plan with the dose shades covered by 95% of the prescribed dose. DVH is a commonly used clinical tool for the treatment plan evaluation. Typical DVHs curves from three patients each suffered with prostate, head and neck, and esophagus cancer are illustrated in [Figure 2]a,[Figure 2]b,[Figure 2]c, respectively. It is clear from these figures; the OARs receive almost identical or slightly better doses in optimized plan BAO, compared with that of manual plan PSBO. Furthermore, the DVH for OARs are similar for both techniques, which implies that both of the plans provide nearly identical or slightly better coverage. Similar observations on BAO has been reported by Liu et al.,[18] for IMRT of nonsmall cell lung cancer and Srivastava et al.,[12] for limited patients suffered with head and neck and prostate cancer. These measurements clearly recommended the development of a multiresolution search scheme for BAO using fewer and non-modulated beams.
Figure 1: Typical axial CT slice of (a) BAO and (b) PSBO IMRT plans for esophageal cancer patient. CT = Computed tomography, PSBO = preselected equiangular beam orientations, BAO = beam angle optimization, IMRT = intensity-modulated radiotherapy

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Figure 2: Typical dose volume histograms of (a) prostate, (b) esophagus, and (c) head and neck cancer patients

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Major advantages and superiority of BAO over PSBO is confirmed by analyzing the monitor units (MUs) calculated using PBC model. Dose MUs for prostate, head and neck, and esophagus carcinoma patients calculated from IMRT plans are compared in [Table 1]. In case of for the prostate, mean values of MUs for 10 patients in PSBO and BAO was 888 ± 304 and 830 ± 309, respectively. The data was statistically significant with a difference of 7.75 ± 4.17% (P = 1.8 × 10-5). Likewise, mean values of MUs for head and neck and esophagus cases were 909 ± 263 (in PSBO) and 854 ± 256 (in BAO) and 493 ± 68 (in PSBO) and 461 ± 60 (in BAO), respectively. Also, the differences in MUs for the head and neck and esophagus cases in PSBO and BAO were 6.59 ± 3.6% (P = 3.53 × 10-4) and 6.93 ± 3% (P = 1.2 × 10-4), respectively. The reduction in MUs in case of BAO is of clinical relevance. The number of MUs has been correlated with the whole-body radiation dose and should be minimized to reduce radiation-related long-term complications.[19],[20],[21]
Table 1: Comparison of MUs for BAO and PSBO in prostate, head and neck, and esophagus cases

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The CI, HI, and QI were used to evaluate the dose coverage of the PTV by each plan. CI is used to evaluate tightness of fit between PTVs and isodose volumes in treatment plans. CI, HI, QI, and mean dose were compared using paired t-test and their numerical values are listed in [Table 2]. It is clear from [Table 2], the values of CI, HI, QI, and mean dose in both the techniques used in IMRT planning of prostate, head and neck, and esophagus cases are not significantly different (P > 0.05). [Table 3] shows the doses of OARs for all three sites and shows that these are not significantly different (P > 0.05).
Table 2: CI, HI, and QI and mean dose for the planning target volume of prostate, head and neck, and esophagus cases

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Table 3: Comparison of average doses of maximum, mean, and D20 and D50 to the OAR

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 > Discussion Top


In fact, almost all of the published works, including the proposed one in this paper, focused on the clinically meaningful improvement of IMRT by trying to provide the most preferable beam angles within a clinically acceptable computation time. Our study shows that the IMRT plans based on optimal selection of beam angles has sufficient target coverage and superior OARs sparing to the manual beam selection. The BAO plans reveal almost identical or even better DVH for PTV and much better sparing to OARs compared with PSBO. The MUs are significantly lower in BAO than the PSBO in each site investigated. The lower MUs will be a potential advantage for breath-control radiotherapy and adaptive therapy. In spite of this, reduction of MUs will reduce the overall treatment time for each patient treated with IMRT technique, and hence patient throughput will be increased. It is suggested that optimized beam angles maybe suboptimal, but are better than those manually selected. Besides the improvements of optimization algorithms themselves, another important way to improve the efficiency of BAO should come from the external intervention or guidance to the optimization process, such as the usage of the expert knowledge accumulated by the medical physicists and oncologists over time.


 > Conclusions Top


The present dosimetric study clearly reveals the advantages of BAO IMRT over PSBO IMRT plans for prostate, head and neck, and esophagus carcinoma using sliding window technique. The BAO plans exhibit almost identical or better sparing to OARs and reduction in radiation MU as compared to PSBO plans. The significant reduction in MUs was directly related to the improvement of delivery efficiency and treatment delivery time. The BAO heavily relies on the original beam orientations, which implies that the final results may not be most optimal one. But present measurements show that the BAO strategy can still achieve plans with sufficient plan quality and delivery efficacy. Besides, prostate, head and neck, and esophagus cancer; the BAO can be used for other site tumors such as liver, lung, etc., and will be the subject of future research.


 > Acknowledgement Top


The authors would also like to thank the Punjab Technical University, Jalandhar for the Ph. D. registration.

 
 > References Top

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Webb S. Intensity modulated radiation therapy. Bristol and Philadelphia: Institute of Physics Publishing; 2001. p. 1-34.  Back to cited text no. 1
    
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Bortfeld T. The number of beams in IMRT--theoretical investigations and implications for single-arc IMRT. Phys Med Biol 2010;55:83-97.  Back to cited text no. 10
    
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Fenwick JD, Pardo-Montero J. Numbers of beam angles required for near-optimal IMRT: Theoretical limits and numerical studies. Med Phys 2011;38:4518-30.  Back to cited text no. 11
    
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Srivastava SP, Das IJ, Kumar A, Johnstone PA. Dosimetric comparison of manual and beam angle optimization of gantry angles in IMRT. Med Dosim 2011;36:313-6.  Back to cited text no. 12
    
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International Commission on Radiation Units and Measurements (ICRU). Prescribing, recording and reporting photon beam therapy. ICRU Report 50. Bethesda (MD): ICRU, 1993.  Back to cited text no. 13
    
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Das S, Cullip T, Tracton G, Chang S, Marks L, Anscher M, et al. Beam orientation selection for intensity-modulated radiation therapy based on target equivalent uniform dose maximization. Int J Radiat Oncol Biol Phys 2003;55:215-24.  Back to cited text no. 14
    
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Liu HH, Jauregui M, Zhang X, Wang X, Dong L, Mohan R, et al. Beam angle optimization and reduction for intensity-modulated radiation therapy of non small-cell lung cancers. Int J Radiat Oncol Biol Phys 2006;65:561-72.  Back to cited text no. 18
    
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Hall EJ, Wuu CS. Radiation-induced second cancers: The impact of 3D-CRT and IMRT. Int J Radiat Oncol Biol Phys 2003;56:83-8.  Back to cited text no. 19
    
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Kry SF, Salehpour M, Followill DS, Stovall M, Kuban DA, White RA, et al. The calculated risk of fatal secondary malignancies from intensity-modulated radiation therapy. Int J Radiat Oncol Biol Phys 2005;62:1195-203.  Back to cited text no. 20
    
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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

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