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 Table of Contents    
TECHNICAL NOTE
Year : 2019  |  Volume : 44  |  Issue : 4  |  Page : 283-286
 

4π radiotherapy using a linear accelerator: A misnomer in violation of the solid geometric boundary conditions in three-dimensional Euclidean space


1 Department of Radiation Oncology, Manipal Hospitals, Dwarka, New Delhi, India
2 Department of Medical Physics, Apollo Proton Cancer Center, Chennai, Tamil Nadu, India

Date of Submission08-Jan-2019
Date of Decision31-Aug-2019
Date of Acceptance31-Aug-2019
Date of Web Publication11-Dec-2019

Correspondence Address:
Biplab Sarkar
Manipal Hospitals, Dwarka, New Delhi - 110 075
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jmp.JMP_2_19

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   Abstract 

Purpose: The concept of 4πc radiotherapy is a radiotherapy planning technique receiving much attention in recent times. The aim of this article is to disprove the feasibility of the 4π radiotherapy using a cantilever-type linear accelerator or any other external-beam delivery machines. Materials and Methods: A surface integral-based mathematical derivation for the maximum achievable solid angle for a linear accelerator was carried out respecting the rotational boundary conditions for gantry and couch in three-dimensional Euclidean space. The allowed movements include a gantry rotation of 0–2πc and a table rotation of πc/2 - 0 - 3πc/2) . Results: Total achievable solid angle by cantilever-type linear accelerator (or any teletherapy machine employing a cantilever design) is 1/r2 π ∫ - θ=0 π ∫ φ=0 (rSinθdφ). (rdθ) = 2πc, which is applicable only for the foot and brain radiotherapy where the allowed table rotation is 90°–0°–270°. For other sites such as pelvis, thorax, or abdomen, achievable solid angle as the couch rotation comes down significantly. Practically, only suitable couch angle is 0° by avoiding gantry–couch–patient collision. Conclusions: Present cantilever design of linear accelerator prevents achieving a 4π radian solid angle at any point in the patient. Even the most modern therapy machines like CyberKnife which has a robotic arm also cannot achieve 4π geometry. Maximum achievable solid angle under the highest allowable boundary condition(s) cannot exceed 2πc, which is restricted for only extremities such as foot and brain radiotherapy. For other parts of the body such as pelvis, thorax, and abdomen, the solid angle is reduced to 1/5th (maximum value) of the 4πc. To obtain a 4πc solid angle in a three-dimensional Euclidean space, the patient has to be a zero-dimensional point and X-ray head of the linear accelerator has a freedom to rotate in every point of a hypothetical sphere of radius 1 m. This article establishes geometrically why it is not possible to achieve a 4πc solid angle.


Keywords: 4π, 4π radiotherapy, Euclidean space, linear accelerator, solid angle


How to cite this article:
Sarkar B, Ganesh T, Munshi A, Manikandan A, Mohanti BK. 4π radiotherapy using a linear accelerator: A misnomer in violation of the solid geometric boundary conditions in three-dimensional Euclidean space. J Med Phys 2019;44:283-6

How to cite this URL:
Sarkar B, Ganesh T, Munshi A, Manikandan A, Mohanti BK. 4π radiotherapy using a linear accelerator: A misnomer in violation of the solid geometric boundary conditions in three-dimensional Euclidean space. J Med Phys [serial online] 2019 [cited 2020 Jan 26];44:283-6. Available from: http://www.jmp.org.in/text.asp?2019/44/4/283/272669



   Introduction Top


This has reference to the concept called as “4π radiotherapy,” which in recent times has attracted much attention both scientifically and commercially.[1] The concept of 4π radiotherapy was developed in the University of California, Los Angeles (UCLA), during 2013.[2],[3] Subsequently, it was commercially adopted by Varian Medical Systems in their RapidArc planning approach.[3] Since the “4π radiotherapy” solution is commercially available, it is used by several researchers in their planning studies.[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15] The studies include sites such as thorax (lung stereotaxy),[2],[3],[7],[12],[15] brain,[12],[13],[14],[15],[16] head and neck,[5],[10] abdomen (liver stereotactic body radiotherapy),[7],[11],[15] pelvis (prostate),[9] and phantom [6] in knowledge-based planning [17] besides several other discussions in scientific forums.[8],[12] As claimed by Dong et al., in 2013, they have achieved a 4πc (steradian) solid angle at the tumor center for thorax cases and subsequently similar claims were made by rest of the authors until the last month (June 2019).[[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[17],[18] However, the question “whether it is possible to obtain a 4πc solid angle using a linear accelerator in three-dimensional Euclidean space?” is an interesting one to probe into against the backdrop of these mushrooming scientific literature and commercial products. The simple answer is “no”. It is not possible to obtain a 4πc solid angle using the current cantilever design of linear accelerator at any point in space with or without the patient on the couch. One may add some more variables such as vertical patient motion during therapy delivery or surface rendering using infrared to avoid gantry–patient collision, but these will not help in achieving 4πc solid angle at any point as explained below.

The aim of this article is to mathematically establish the infeasibility of 4πc radiotherapy with the present cantilever design of linear accelerator or any other teletherapy machines.


   Materials And Methods Top


The detailed mathematical derivation is provided elsewhere; however, for the completeness, we are presenting the summary of the result.[19],[20] Total solid angle (angle in three-dimension) in Euclidean space is defined by,

where ds is the surface area and r is the radius vector.

A 4πc solid angle can be achieved only at the center of a sphere.

And hence, solid angle as .

A linear accelerator with its accelerating arm attached to a vertical frame can geometrically be considered as a cantilever. The allowed movements include a gantry rotation of 0–2πc and a table rotation of . Therefore, the total solid angle obtained by a linear accelerator (or any teletherapy machine employing a cantilever design) is .


   Results Top


The total allowed solid angle is reduced to 2πc under maximum allowed boundary condition for a linear accelerator. Achievability of 2πc solid angle is only limited to the treatment of the extremities such as foot and brain radiotherapy under the condition that each point in the hemisphere created by gantry [−πc−0−+πc] and couch sees the target volume.

For example, total solid angle obtained by a 40 cm × 40 cm field size when gantry is rotated over a 0–2πc arc is only 2.51c. Total solid angle at the tumor center is reduced further with blocked or multileaf collimator-shaped fields as the total surface area becomes less due to shaping or blocking.

To elaborate the dimension of the solid angle encountered in radiotherapy, we provide a simple example of 4-field box technique or full-arc VMAT technique. With a 20 cm × 20 cm open field size, the solid angle calculated as:



= 0.0127πc solid angle

= 0.04c solid angle

For four beams = 0.16c solid angle.

Similarly, total solid angle for a single-arc VMAT technique with 20 cm × 20 cm open field is following. Surface area created by 20 cm × 20 cm open field over a path length created by single full gantry rotation of 360° is = 2π × 20 cm × 100 cm. Therefore, the solid angle calculated as = = 0.4πc.

A typical CyberKnife plan with 100 static segments having each field opening of 4 cm × 4 cm yields a total solid angle= =0.05πc, considering no overlap between beams, which is lesser than the typical VMAT/linear accelerator-based solid angle.


   Discussion Top


Several investigators have presented the pictorial representation of “4π radiotherapy” in their articles; however, none could achieve the complete 4πr 2 surface area in any one of those studies.[[2],[3],[4],[5],[6],[7],[8],[9],[10],[11],[12],[13],[14],[15],[17],[18]

Furthermore, patient–gantry–table collision is an additional potential risk when trying to achieve so-called “4πc solid angle.” Only the iPlan stereotactic planning system (BrainLab AG, Feldkirchen, Germany) offers a collision map, and other planning systems such as Eclipse (Varian Medical Systems, Palo Alto, CA) or Monaco (CMS Elekta, Sunnyvale, CA, USA) cannot generate a collision map. [Figure 1] presents a head and neck case planned with impractical combination of couch positions and gantry rotation; nevertheless, treatment planning system (TPS) did not warn about any impending collision. It was obvious that the plan was not deliverable as the gantry cannot cross 90° position even with a couch rotation as little as 20° [Figure 2]. Unless otherwise a surface-rendering technique is used, “4π radiotherapy” and planning with many noncoplanar beams is an inefficient and error-prone process.[6] All beams/arcs need to be verified for collision manually by therapists when the patient is on the couch. If it is found that the plan is nonexecutable due to collision issues, it would require a replanning, leading to delay in patient treatment.
Figure 1: Treatment planning using infeasible gantry and table angles, gantry rotation 0°–360° couch angle 0°, ±30° and 90° for a head and neck case

Click here to view
Figure 2: Gantry at 90° position colliding with couch at 20° position for the same head neck patient; no couch position ≥±12° practically possible as it does not allow the gantry to cross ±90° position for Elekta linear accelerator. For Varian accelerator, the accessible couch angle further reduces as the isocenter to gantry end clearance is lesser than Elekta accelerators

Click here to view


The term “4π radiotherapy” is a misnomer and does not represent the true geometry the technique is capable of achieving. It is impossible to deliver true “4π radiotherapy” using the present cantilever design of a medical linear accelerator or for that matter using other external beam therapy machines such as Tomotherapy (Accuray Inc., Madison, WI) or CyberKnife (Accuray Inc., Madison, WI). The maximum solid angle achievable in treating human individuals is limited to 2πc with cantilever-type medical accelerators and may increase, but never can reach 4π, even in advanced machines such as CyberKnife. Further, as the scope of gantry movement is very limited for the complex geometry beams from such advanced machines, only static intensity-modulated beams may be possible with a significant increase in the treatment time.

Among all the therapy delivery techniques, only brachytherapy comes closer to true “4π radiotherapy” if one considers the source as a point.

Therefore, solid angle encountered in radiotherapy is much less than 4πc; although it is theoretically possible to achieve maximum 2πc solid angle in brain or foot radiotherapy, it is not required clinically.

As a corollary of this study, while reviewing a selective list of six major journals in the field of radiation oncology and medical physics ([1] Physics in Medicine and Biology, [2] International Journal of Radiation Oncology-Biology-Physics, [3] Medical Physics, [4] Radiation Oncology, [5] Acta Oncologica, and [6] Journal of Applied Clinical Medical Physics), we found that a large number (30%; 5/15)[5],[6],[9],[13],[18] of these research activities were aided by the vendors, as stated in their financial disclosure statements. Further, the question of nonfeasibility of “4π radiotherapy” has been raised against two previous 4π articles, which was either not at all or answered unsatisfactorily.[19],[20],[21] For example, UCLA group ambiguously responded to the question regarding the feasibility of “4π radiotherapy;” however, it failed to establish mathematically (or geometrically) the achievability of 4πc solid angle using a linear accelerator.[17] Therefore, it is evident that efforts have been made to establish the superiority of “4π” technique over the standard noncoplanar treatment technique by identifying it with a fancy unscientific name and with commercial interests, which is not a healthy practice in terms of dignity of our profession.[22]


   Conclusion Top


We propose that the scientific and commercial use of the misnomer “4π radiotherapy” should stop forthwith because it is not possible to obtain 4πc solid angle at any point with an existing accelerator design. Further, peer reviewers should be also cautious in recommending articles on “4π radiotherapy” for publication.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
   References Top

1.
Smyth G, Evans PM, Bamber JC, Bedford JL. Recent developments in non-coplanar radiotherapy. Br J Radiol 2019;92:20180908.  Back to cited text no. 1
    
2.
Dong P, Lee P, Ruan D, Long T, Romeijn E, Yang Y, et al. 4π non-coplanar liver SBRT: A novel delivery technique. Int J Radiat Oncol Biol Phys 2013;85:1360-6.  Back to cited text no. 2
    
3.
Dong P, Lee P, Ruan D, Long T, Romeijn E, Low DA, et al. 4π noncoplanar stereotactic body radiation therapy for centrally located or larger lung tumors. Int J Radiat Oncol Biol Phys 2013;86:407-13.  Back to cited text no. 3
    
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Nguyen D, Rwigema JC, Yu VY, Kaprealian T, Kupelian P, Selch M, et al. Feasibility of extreme dose escalation for glioblastoma multiforme using 4π radiotherapy. Radiat Oncol 2014;9:239.  Back to cited text no. 4
    
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Rwigema JC, Nguyen D, Heron DE, Chen AM, Lee P, Wang PC, et al 4π noncoplanar stereotactic body radiation therapy for head-and-neck cancer: Potential to improve tumor control and late toxicity. Int J Radiat Oncol Biol Phys 2015;91:401-9.  Back to cited text no. 5
    
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Yu VY, Tran A, Nguyen D, Cao M, Ruan D, Low DA, et al. The development and verification of a highly accurate collision prediction model for automated noncoplanar plan delivery. Med Phys 2015;42:6457-67.  Back to cited text no. 6
    
7.
Nguyen D, Dong P, Long T, Ruan D, Low DA, Romeijn E, et al. Integral dose investigation of non-coplanar treatment beam geometries in radiotherapy. Med Phys 2014;41:011905.  Back to cited text no. 7
    
8.
Sheng K, Shepard DM, Orton CG. Non-coplanar beams improve dosimetry quality for extracranial intensity modulated radiotherapy and should be used more extensively. Med Phy 2015;42:531-3.  Back to cited text no. 8
    
9.
Tran A, Zhang J, Woods K, Yu V, Nguyen D, Gustafson G, et al. Treatment planning comparison of IMPT, VMAT and 4π radiotherapy for prostate cases. Radiat Oncol 2017;12:10.  Back to cited text no. 9
    
10.
Subramanian VS, Subramani V, Chilukuri S, Kathirvel M, Arun G, Swamy ST, et al. Multi-isocentric 4π volumetric-modulated arc therapy approach for head and neck cancer. J Appl Clin Med Phys 2017;18:293-300.  Back to cited text no. 10
    
11.
Tran A, Woods K, Nguyen D, Yu VY, Niu T, Cao M, et al. Predicting liver SBRT eligibility and plan quality for VMAT and 4π plans. Radiat Oncol 2017;12:70.  Back to cited text no. 11
    
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Wilson B, Otto K, Gete E. A simple and robust trajectory-based stereotactic radiosurgery treatment. Med Phys 2017;44:240-8.  Back to cited text no. 12
    
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Holm AIS, Petersen JBB, Muren LP, Seiersen K, Borghammer P, Lukacova S, et al. Functional image-guided dose escalation in gliomas using of state-of-the-art photon vs. proton therapy. Acta Oncol 2017;56:826-31.  Back to cited text no. 13
    
14.
Yu VY, Landers A, Woods K, Nguyen D, Cao M, Du D, et al. A prospective 4π radiotherapy clinical study in recurrent high grade glioma patients. Int J Radiat Oncol Biol Phys 2018;101:144-51.  Back to cited text no. 14
    
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Langhans M, Unkelbach J, Bortfeld T, Craft D. Optimizing highly noncoplanar VMAT trajectories: The noVo method. Phys Med Biol 2018;63:025023.  Back to cited text no. 15
    
16.
Available from: https://www.varian.com/news/radiation-oncology-researchers-glimpse-future-cancer-care [Last accessed on 2019 Sep 20].  Back to cited text no. 16
    
17.
Landers A, O'Connor D, Ruan D, Sheng K. Automated 4π radiotherapy treatment planning with evolving knowledge-base. Med Phys 2019;46:3833-43.  Back to cited text no. 17
    
18.
Ohira S, Ueda Y, Akino Y, Hashimoto M, Masaoka A, Hirata T, et al. HyperArc VMAT planning for single and multiple brain metastases stereotactic radiosurgery: A new treatment planning approach. Radiat Oncol 2018;13:13.  Back to cited text no. 18
    
19.
Tran A, Zhang J, Woods K, Yu V, Nguyen D, Gustafson G, et al. Treatment planning comparison of IMPT, VMAT and 4π radiotherapy for prostate cases. Radiat Oncol 2017;12:10.  Back to cited text no. 19
    
20.
Sarkar B. In regard to “Tran A, Zhang J, Woods K, Yu V, Nguyen D, Gustafson G, et al. Treatment planning comparison of IMPT, VMAT and 4π radiotherapy for prostate cases. Radiation Oncology 2017;12:10”. Radiation Oncology 2018;13:63.  Back to cited text no. 20
    
21.
Sarkar B. In regard to “Dong P, Lee P, Ruan D, Long T, Romeijn E, Yang Y, et al. 4π non-coplanar liver SBRT: A novel delivery technique. Int J Radiat Oncol Biol Phys 2013;85:1360-6.” Int J Radiat Oncol Biol Phys 2018;101:741-2.  Back to cited text no. 21
    
22.
Halperin EC. Restoring the honor of our specialty by minimizing financial ties of organized radiation oncology with industry. Int J Radiat Oncol Biol Phys 2018;101:257-8.  Back to cited text no. 22
    


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