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ORIGINAL ARTICLE
Year : 2019  |  Volume : 15  |  Issue : 1  |  Page : 211-215

Search of an ideal location of isocenter in intensity-modulated radiotherapy treatment plans: A dosimetrical approach


Department of Radiotherapy and Radiation Medicine, Institute of Medical Sciences, Banaras Hindu University, Varanasi, Uttar Pradesh, India

Date of Web Publication13-Mar-2019

Correspondence Address:
Dr. Abhijit Mandal
Department of Radiotherapy and Radiation Medicine, Institute of Medical Sciences, Banaras Hindu University, Varanasi, Uttar Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jcrt.JCRT_985_16

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 > Abstract 


Aim: The aim of this study is to identify an ideal location of isocenter in intensity-modulated radiotherapy (IMRT) treatment plans.
Materials and Methods: A total of 28 clinical target volumes and 4 English capital letters (C, L, T, and H) target volumes were considered in this study. Two IMRT treatment plans were generated for each target volume in the ECLIPSETM treatment planning system (TPS), first one with isocenter automatically placed (ISOAUTO) by TPS and the second one with geometric center-based isocenter (ISOGEOM). The geometric center of a cuboid volume, which was formed encompassing around the target volume in sagittal, transverse, and frontal planes, is considered as the geometric center of the target volume as well as the isocenter (ISOGEOM) of the IMRT plans. While performing the IMRT treatment plans using the beam angle optimization and dose volume optimization, the normal tissue objectives and target volume objectives were kept similar in both the plans. The dosimetrical parameters between the two groups of plans were compared.
Results: The distance between ISOGEOM and ISOAUTO ranged from 0.16 cm to 3.04 cm with a mean and median of 0.85 cm and 0.69 cm, respectively. The ISOGEOM-based IMRT plans exhibited statistically significant advantages in total monitor units reduction (100% of cases, P ≤ 0.001), total number of field reduction (66% of cases, P ≤ 0.001), and reduction of patient mean dose (69% of cases, P ≤ 0.001) over ISOAUTO-based IMRT plans. The conformity index, homogeneity index and target mean dose were comparable between both group of plans.
Conclusion: Significant dosimetrical advantages may be observed, when the geometric centroid of target volume is considered as isocenter of IMRT treatment plan.

Keywords: Conformity index, homogeneity index, intensity-modulated radiotherapy, isocenter


How to cite this article:
Mandal A, Asthana AK, Pradhan S, Shahi UP, Choudhary S. Search of an ideal location of isocenter in intensity-modulated radiotherapy treatment plans: A dosimetrical approach. J Can Res Ther 2019;15:211-5

How to cite this URL:
Mandal A, Asthana AK, Pradhan S, Shahi UP, Choudhary S. Search of an ideal location of isocenter in intensity-modulated radiotherapy treatment plans: A dosimetrical approach. J Can Res Ther [serial online] 2019 [cited 2019 Nov 17];15:211-5. Available from: http://www.cancerjournal.net/text.asp?2019/15/1/211/243508




 > Introduction Top


Nowadays, intensity-modulated radiotherapy (IMRT) is a common technique used in radiotherapy management of malignant tumors, irrespective of variable size, shape, and location. To generate an IMRT plan, a number of editable machine parameters, patient information and treatment planning options are required. These editable parameters affect the quality of the generated plan.[1],[2] The location of isocenter is one of the most important editable parameters which may affect the quality of IMRT plans. The literature recommends that the isocenter should be placed at the center of the target volume or a generic location.[3] It is also difficult to identify the centroid of an irregular volumetric object, which requires a complicated mathematical formulation or algorithm which will fit to each volumetric object irrespective of size and shape. Most of the treatment planning system (TPS) automatically places the isocenter (ISOAUTO) of any treatment plan with respect to the target volume using an undisclosed algorithm.

In the present study, a method was formulated to identify the geometric centroid of any target volume. The target volume was encompassed by a cuboid formed by touching the proximal ends of the target volume in sagittal, transverse, and frontal planes [Figure 1]. The geometric centroid of the cuboid was considered as the geometric centroid of the irregular target volume as well as the geometric center-based isocenter (ISOGEOM) of the treatment plan.
Figure 1: Description about the algorithm to identify geometric centroid of an irregular volume. (a) Three-dimensional view of the cuboid, (b) Cross-sectional view of target center in transverse plane, (c) Cross-sectional of target center in frontal plane, (d) Cross-sectional view of target center in sagittal plane

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In this study, dosimetric comparisons were made between the IMRT plans generated using ISOAUTO and ISOGEOM.


 > Materials and Methods Top


A total of 28 clinical target volumes of various sites from the skull to the pelvis and 4 English capital letter phantom target volumes were included in this retrospective virtual planning study. Two IMRT treatment plans were generated for each target volume in ECLIPSE™ TPS (Version 11.0.47, supplied by Varian Medical System, Inc., Palo Alto, CA, USA) using beam angle optimization (BAO) (Plan Geometry Optimizer, version 11.0.31) and dose volume optimization (Dose Volume Optimizer, version 11.0.31). The first IMRT plan was generated using the ISOAUTO by TPS and the second plan was generated using geometric centroid of the target volume as isocenter. The target, organs at risk (OAR) and normal tissue objectives were kept similar in optimization process for both the IMRT plans. The comparisons were made between two IMRT plans of each target volume regarding total number of fields, total monitor units (MUs) to be delivered, conformity index (CI), homogeneity index (HI), target volume mean dose, and patient body mean dose. The percentage difference of any quantity was calculated as

Percentage Difference (%) = ([QuantityGEOM– QuantityAUTO]/QuantityAUTO) × 100%

Where QuantityGEOM is the quantity calculated in the treatment plan with ISOGEOM and QuantityAUTO is the quantity calculated in the treatment plan with ISOAUTO.

Two-tailed Fisher's exact test at 5% level of significance was used for statistical analysis.


 > Results Top


A total of 32 target volumes were included in this study, of which 28 were clinical patient target volumes and four were English capital letter (C, L, T, and H) phantom target volumes. Clinical target volumes were from the skull (12), face and neck (3), thorax (4), abdomen (7), and pelvic (2) region [Figure 2]. The target volume sizes ranged from 76.66 cc to 919.51 cc with a mean of 376.38 cc and median of 298.13 cc [Figure 3].
Figure 2: Site-wise distribution of target volumes

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Figure 3: Distribution of sizes of target volumes

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The distance between the isocenters (ISOAUTO and ISOGEOM) of two IMRT plans of any target volumes was calculated as

Distance = √([x1– x2]2+ [y1– y2]2+ [z1– z2]2)

Where (x1, y1, z1) and (x2, y2, z2) were the coordinates of the isocenters ISOAUTO and ISOGEOM.

The calculated distances ranged from 0.16 cm to 3.04 cm [Figure 4] with a mean of 0.85 cm and median of 0.69 cm.
Figure 4: Distances between the isocenters of geometric center-based isocenter and isocenter automatically placed-based intensity-modulated radiotherapy plans

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The number of fields used in any IMRT plan was decided using the BAO process. The total number of fields was significantly reduced in 66%, unchanged in 31%, and increased in 3% of cases (P ≤ 0.001) in ISOGEOM-based plans than ISOAUTO-based plans [Figure 5]. The maximum value percentage of reduction was 44.44%.
Figure 5: The percentage differences of total number fields used between geometric center-based isocenter and isocenter automatically placed-based intensity modulated radiotherapy plans

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The total MUs to be delivered were defined after the volumetric optimization and volumetric dose calculation (Anisotropic Analytical Algorithm, version 11.0.31). Highly significant reduction in total MUs was observed in all cases (P ≤ 0.001) in ISOGEOM-based plans than ISOAUTO-based plans [Figure 6]. The percentage of reductions ranged from 1.03% to 33.33% with a mean of 8.98% and median of 7.54%.
Figure 6: The percentage differences of total monitor units delivered between geometric center-based isocenter and isocenter automatically placed-based intensity modulated radiotherapy plans

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The CI and HI of both type plans were compared. ISOAUTO-based plans exhibit better conformity in 53% of cases, and ISOGEOM-based plans came out with better conformity in 47% of cases [Figure 7]. The advantages of CI in ISOAUTO-based plans were not statistically significant (P = 0.803). Whereas ISOAUTO-based plans showed little better homogeneity in 63% of cases, ISOGEOM-based plans came out with better homogeneity in 37% of cases [Figure 8]. Similarly, the advantages observed in ISOAUTO-based plans was not statistically significant (P = 0.079).
Figure 7: The percentage differences of conformity index between geometric center-based isocenter and isocenter automatically placed-based intensity modulated radiotherapy plans

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Figure 8: The percentage differences of homogeneity index between geometric center-based isocenter and isocenter automatically placed-based intensity modulated radiotherapy plans

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The percentage difference in target mean dose between two plans was seen to be very negligible in all cases, ranging from +1.21% to 0.89% with a mean of +0.04% and median of 0.0% [Figure 9]. While comparing the percentage difference in patient body mean dose between the two plans, there was a significant reduction in 69%, it was unchanged in 9%, and it was increased in 22% of cases (P ≤ 0.001) in ISOGEOM-based plans than ISOAUTO-based plans [Figure 10].
Figure 9: The percentage differences of target mean dose between geometric center-based isocenter and isocenter automatically placed-based intensity-modulated radiotherapy plans

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Figure 10: The percentage differences of patient body mean dose between geometric center-based isocenter and isocenter automatically placed-based intensity modulated radiotherapy plans

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 > Discussion Top


IMRT treatment is one of the most favorite techniques in modern-day precision radiotherapy. In IMRT treatment planning, many variable parameters are used. Of these, some parameters are TPS generated, some are planner/user defined and some are TPS generated but editable. The treatment plan quality is grossly dependent on the judicious use of these parameters. The coordinates of isocenter are automatically generated by TPS as well as an editable option of the planner. Articles recommend that the IMRT planning isocenter should be placed at the centroid of the target volume.[1],[2] The optimization of isocenter location was described by Salter et al.[4] for intensity-modulated stereotactic treatment and Tewatia and Tolakanahalli[5] in tomotherapy plans. Literature review recommends that IMRT plan isocenter should be located such as the multileaf collimator (MLC) can be used optimally.

In this study, a simple algorithm has been proposed to identify the centroid of an irregular volume as mentioned earlier. This centroid of the target volume is considered as isocenter (ISOGEOM) of the IMRT plan. In comparison with IMRT plan with TPS defined isocenter, ISOAUTO-based and ISOGEOM-based IMRT plans show a statistically significant advantage in reduction of the total number of fields used and total MUs to be delivered. These advantages effectively reduced the overall treatment time and machine load (reduced BEAM ON time), which leads to betterment in treatment execution, radiation safety, and less equipment maintenance cost. The effective and optimal use of MLCs caused the reduction in a total number of fields and total MUs.

While considering plan quality related to the target volume, the results of this study show comparable CI, HI, and target mean dose in both groups of IMRT plans. The plan quality related to the target volume is grossly dependent of the target, OAR, and normal tissue objectives and constraints during the optimization process. This study shows comparable values of plan quality indices as during generation of the IMRT plans, all the tissue objectives were kept same for both groups.

Again, the ISOGEOM-based IMRT plans exhibit higher merit in the reduction of patient body mean dose when compared to the ISOAUTO-based IMRT plans. The patient body mean dose is directly proportional to the integral dose. The reduction in integral dose may influence the reduction in associated complications such as radiation-induced malignancy, genetic changes, leukopenia, etc.


 > Conclusion Top


IMRT plans using geometric centroid as plan isocenter provide significant advantages in reduction of field number, total MUs, and patient body mean dose with comparable plan quality indices than ISOAUTO-based IMRT plans.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
 > References Top

1.
Wu VW. Effects of multileaf collimator parameters on treatment planning of intensity-modulated radiotherapy. Med Dosim 2007;32:38-43.  Back to cited text no. 1
    
2.
Mittauer K, Lu B, Yan G, Kahler D, Gopal A, Amdur R, et al. A study of IMRT planning parameters on planning efficiency, delivery efficiency, and plan quality. Med Phys 2013;40:061704.  Back to cited text no. 2
    
3.
Court LE, Balter P, Mohan R. Principles of IMRT. In: Nishimura Y, Komaki R, editors. Intensity-Modulated Radiation Therapy. Japan: Springer; 2015. p. 16-42.  Back to cited text no. 3
    
4.
Salter BJ, Fuss M, Sarkar V, Wang B, Rassiah-Szegedi P, Papanikolaou N, et al. Optimization of isocenter location for intensity modulated stereotactic treatment of small intracranial targets. Int J Radiat Oncol Biol Phys 2009;73:546-55.  Back to cited text no. 4
    
5.
Tewatia D, Tolakanahalli R. Does optimizing the placement of machine isocenter affect the overall optimized plan obtained using tomotherapy treatment planning system? A dosimetric and analysis study. Med Phys 2013;40:358.  Back to cited text no. 5
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10]



 

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