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Table of Contents
Year : 2022  |  Volume : 5  |  Issue : 1  |  Page : 4-7

Dr. Kanhu’s COSID index: An acronym for plan evaluation in SRS & SBRT

1 Department of Radiation Oncology, Mahatma Gandhi Cancer Hospital and research Institute, Visakhapatnam, Andhra Pradesh, India
2 Department of Radiation Oncology, Acharya Harihar Post Graduate Institute of Cancer, Cuttack, India
3 Department of Medical Physics, Mahatma Gandhi Cancer Hospital and research Institute, Visakhapatnam, Andhra Pradesh, India

Date of Submission22-Oct-2021
Date of Decision14-Jan-2022
Date of Acceptance26-Mar-2022
Date of Web Publication02-Sep-2022

Correspondence Address:
Dr. Kanhu Charan Patro
Department of Radiation Oncology, Mahatma Gandhi Cancer Hospital and Research Institute, Visakhapatnam, Andhra Pradesh
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jco.jco_34_21

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Background: A major parameter in the workflow of radiation treatment is the plan evaluation. In order to achieve high dose to target, minimum dose to the critical structures and accurate delivery of treatment, various qualitative and quantitative parameters need to be assessed during plan evaluation. Material and Methods: Here we propose an acronym COSID to describe the five major indices that need to be evaluated during a stereotactic treatment plan. Results: The stereotactic radiation plan evaluation include good target coverage, minimum dose to the organs at risk (OAR), homogeneity and conformity of dose to the target. As very high dose is being delivered in stereotactic radiotherapy in one or small number of fractions, certain other parameters such as the dose fall of beyond the target and the complexity of plan must to be addressed. The proposed COSID index is an acronym for these parameters such as Coverage Index, OAR Index, Spillage Index, Imaging Index and Delivery Index. Conclusion: The paper highlights the five important parameters that need to be assessed while evaluating a Stereotactic Radiosurgery (SRS) or Stereotactic Radiotherapy (SRT) plan.

Keywords: Conformity index, gradient index, homogeneity index, stereotactic radiotherapy

How to cite this article:
Patro KC, Avinash A, Pradhan A, Kundu C, Bhattacharyya PS, Pilaka VK, Muvvala M, Chithambara A, Kumar AA, Aketi S, Prasad P, Damodara VN, Kumar Avidi VS, Atchaiyalingam M, Karthikeyan K. Dr. Kanhu’s COSID index: An acronym for plan evaluation in SRS & SBRT. J Curr Oncol 2022;5:4-7

How to cite this URL:
Patro KC, Avinash A, Pradhan A, Kundu C, Bhattacharyya PS, Pilaka VK, Muvvala M, Chithambara A, Kumar AA, Aketi S, Prasad P, Damodara VN, Kumar Avidi VS, Atchaiyalingam M, Karthikeyan K. Dr. Kanhu’s COSID index: An acronym for plan evaluation in SRS & SBRT. J Curr Oncol [serial online] 2022 [cited 2024 Feb 24];5:4-7. Available from: http://www.https://journalofcurrentoncology.org//text.asp?2022/5/1/4/355585

  Introduction Top

The technique of delivering the total prescribed dose of radiation to targets in the brain in a one to five fractions is called Stereotactic Radiosurgery (SRS). The development in the field of imaging and treatment planning software has allowed us to change over from 2-dimentional planning to 3-dimensional planning including the spatial geometry. This has allowed us to deliver treatment using non-coplanar beams in a more conformal way in order to give maximum dose to the target while being able to spare the critical structures. If the total dose of radiation is delivered in a few number of fractions to target outside brain is known as Stereotactic Body Radiotherapy (SBRT).

The modern 3-dimensional patient data has allowed us to calculate the dose of the target and the critical organs using the Dose-Volume Histogram (DVH). Various parameters such as maximum dose, minimum dose, mean dose of the target and the individual organs at risk (OAR) can be calculated simply using this DVH. It also helps us to find out the dose received by certain volume of the OARs. But the major drawback of DVH is that it doesn’t provide data regarding the spatial information.[1] Therefore, we require certain other objective parameters such as homogeneity, conformity, gradient index and slice by slice visual review of dose distribution for SRS/SBRT plan.

Material and Methods

Here, we describe five major indices for SRS/SBRT plan evaluation i.e. Coverage Index, Organ at Risk Index, Spillage Index, Imaging Index and Delivery Index that need to be evaluated for comparing rival plans and help in choosing the appropriate plan for treatment using an acronym COSID.


The COSID stands for Coverage index, Organ at risk index, Spillage index, Imaging index, Delivery index respectively. [Table 1]
Table 1: Showing the five major parameters of SRS/SBRT plan evaluation (COSID INDEX)

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Coverage index

As per the RTOG study, the plan is considered to have good coverage if >90% of prescription isodose covers 100% of the target volume. The plan is said to have a minor deviation if >80% but <90% of prescription isodose covers 100% of the target volume while a major deviation is considered if <80% of prescription isodose covers 100% of the target volume.[2] The quality of treatment plan coverage describe by RTOG (QRTOG) is given below.

QRTOG = Imin / PI,

Where QRTOG is the RTOG coverage index, Imin as minimum isodose within the target and PI represents prescription isodose. The value of QRTOG> 0.9 is acceptable. The QRTOG value between > 0.8 to <0.9 is considered as minor deviation while QRTOG value <0.8 is considered as a major deviation.[1]

As per ICRU Report 91, the following metrics need to be reported and considered in the SRS/SRT plan evaluation.[3]

Medianabsorbed dose of PTV, D50%: it is defined as the absorbed dose received by 50% of the volume.

SRT near-maximum dose, Dnear-max: Dnear-max is considered as D2% of PTV, if the volume of PTV > 2cm3 and Dnear-max is considered as absolute value of 35mm3, D35mm3, if the volume of PTV < 2cm3.

SRT near-minimum dose, Dnear-min: Dnear-min is considered as D98% of PTV, if the volume of PTV > 2cm3 and Dnear-min is considered as absolute value of 35mm3, DV-35mm3, if the volume of PTV < 2cm3.

OAR index

Organ at Risk (OAR) was first defined by ICRU Report 29 as any radiosensitive organ whose presence near the target has influence over the planned dose of radiation.[4] The OARs were divided as serial, parallel and serial-parallel organs by the ICRU Report 62. It also introduced the concept of planning organ at risk volume (PRV), where a margin is given around the OAR to compensate for organ motion similar to Planning Target Volume (PTV) margin around the Clinical Target Volume (CTV).[5]

For a serial organ we need to see the maximum dose (Dmax), for a parallel organ we look for the mean dose (Dmean) and volumetric constraints need to be seen for certain organs. The dose constraints for the OARs for plan evaluation of SRS/SBRT can be done by following the guidelines by Hanna GG et al.[6]

Spillage index

It is an acronym coined for the homogeneity, conformity and gradient index.

Homogeneity index

It is an objective assessment tool to evaluate the uniformity of dose distribution in the specified target volume. It was proposed by the RTOG for Stereotactic plan evaluation. It is calculated using the formula-

HIRTOG = Imax / RI,

Where HIRTOG represents RTOG Homogeneity Index, Imax as maximum isodose within the target and PI represents prescription isodose.

The plans with HI value of < 2 are desirable. The plans with HI value between 2 – 2.5 is viewed as a minor deviation while those with HI value >2.5 as major deviation.[2],[7]

Conformity index

Another important objective measure for plan evaluation of SRS/SRT is the conformity index that describes how tightly the radiation prescription dose encompasses the shape and size of the target volume. To note the conformity index of the SRS, here we describe the 2 most common types of conformity indices i.e. the RTOG conformity index and the Paddick conformity index.[8]

RTOG conformity index (CIRTOG)

In 1993, the RTOG group first proposed the conformity index that was also included in the ICRU report 62. CIRTOG is calculated as the ratio of the volume of prescription isodose (PIV) to the target volume (TV).


As per RTOG the CIRTOG value between 1–2 is considered to be desirable. The CIRTOG between 0.9 -1 and 2 -2.5 are considered as minor deviation as per RTOG and CIRTOG < 0.9 and >2.5 are considered as mojor deviation. [a] This method of calculation of conformity index was easy but was associated with certain criticism. If a treatment plan had CIRTOG of unity, it could not differentiate whether the PIV exactly covered the TV or not. To correct this dilemma Paddick I. proposed another conformity index that was popularly known by his name as Paddick conformity index.[9]

Paddick Conformity Index (CIPaddick) can be calculated using the following formula.

CIPaddick = (TVPIV)2 / TV x PIV,

Where TVPIV is the volume of target covered by prescription isodose.

The ideal CIPaddick = 1. If CIPaddick < 1 means under-treatment.

Gradient index

Apart from homogeneity and conformity index for objective SRS plan evaluation, Paddick I. et al., proposed the Dose Gradient Index (DGI) as an important measure to evaluate the steep dose fall off outside the target volume.[10] The DGI helps not only in comparison among plans with equal conformity indices but also helps in comparing various SRS treatment modalities such as Gamma Knife, CyberKnife and Linear Accelerator based treatment.

Dose fall off

The dose fall off observation acts as a much needed parameter in the plan evaluation under the heading of gradient index. For this we need to calculate the distance between various isodose lines. But none of the isodoses are spherical. In order to calculate the distance between the isodose lines we need to calculate the equivalent radius.

Equivalent radius calculation

The following points are used to calculate the equivalent radius.

Step 1: Note the specified isodose volume

Step 2: Calculate the radius of the isodose volume by using the formula:

V = 4/3 π r3

So, r = (3V/4 π)1/3

The formula for calculating gradient index is as given below.

Gradient Index = Equivalent radius of 50% isodose - Equivalent radius of prescription isodose. Ideally the gradient index should be between 0.3-0.9 mm.

Distance between various isodose lines

The gradient index tells us the spillage of dose between the 50% isodose and the prescription isodose. In addition, the distance between various isodose lines such as between 80% and 60% isodose lines and between 80% and 40% isodose lines also needs to be evaluated as it gives idea about the control of dose spillage across the lower and higher doses.

The ideal distance between 80% and 60% isodose lines should be <2 mm.[11]

The ideal distance between 80% and 40% isodose lines should be <8 mm.

Imaging index

Slice by slice evaluation

As DVH doesn’t give information regarding the spatial distribution of dose, the physician need to evaluate the dose distribution in a given plan slice by slice. It helps to view the regions of cold and hot spots and also to find out whether these cold and hot spots are clinically significant or not. Moreover, it also gives information regarding dose spillage outside the PTV.[12]

Delivery index

Complexity of plan

With the advent of intensity modulated radiotherapy and MLC based treatment; the treatment plans have become more complex as a result of beam modulation. A more complex plan has smaller beamlets and segments whose beam apertures are mostly irregular owing to higher modulation of machine parameters. This leads to overall treatment inaccuracy and lower quality of delivery of treatment. So, in order to achieve a better quality of treatment we need to choose an appropriate plan with less complexity.[13] Use of Flattening Filter Free technique can help in delivering treatment at an increased dose rate while reducing the overall treatment time thereby reducing the intra-fraction motion inaccuracy.

Monior units (MU) evaluation

A note should be made on the MU of all the rival plans for a single treatment. The MU depends on various planning parameters such as number of beams or arcs, arc increment, use of coplanar and non-coplanar beams, number of control points, number of beamlets and width of the segments. A plan with more number of beams/arcs/control points and beamlets, use of non-coplanar beams and smaller segment width leads to increase in the MU of the plan. Such parameters will improve the plan quality but will increase the planning time as well as delivery time. A plan with more MU can lead to increased toxicity and development of secondary malignancy in future due to increased integral dose. Therefore, optimum selections of these parameters need to be taken into consideration during plan evaluation in order to a get a deliverable plan.

Dose calculation parameters

The accuracy of the dose computation depends on the dose calculation grid and dose calculation algorithms. For a highly conformal plan such as SRS/SRT, we need to use a smaller dose calculation grid (eg. 2 mm grid) and a better algorithm for dose calculation (for eg. Monte Carlo Algorithm).

Pre-verification of treatment

Prior to start of SRS/SBRT treatment, Quality Assurance (QA) such as – point dose verification, fluence (2-Dimensional/Planar dose distribution) verification and mechanical isocentre check (Winston Lutz Check) need to be done. Also before commencing the treatment, we need to perform a trial run to check the collision among the gantry, collimator and couch.

  Conclusion Top

This article outlines the five plan evaluation parameters (COSID INDEX) in stereotactic radiotherapy that needs to be done prior to treatment delivery. This plan evaluation can also be applied to any Intensity Modulated Radiotherapy (IMRT) or Volumetric Modulated Arc Therapy (VMAT) plans. It gives an orientation to the beginners in the field of radiation oncology into one of the most important part of stereotactic radiation i.e. plan evaluation.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

  References Top

Menon SV, Paramu R, Bhasi S, Nair RK Evaluation of plan quality metrics in stereotactic radiosurgery/radiotherapy in the treatment plans of arteriovenous malformations. J Med Phys 2018;43:214-20.  Back to cited text no. 1
Shaw E, Kline R, Gillin M, Souhami L, Hirschfeld A, Dinapoli R, et al. Radiation therapy oncology group: Radiosurgery quality assurance guidelines. Int J Radiat Oncol Biol Phys 1993;27: 1231-9.  Back to cited text no. 2
Wilke L, Andratschke N, Guckenberger M, Blanck O, Brunner TB, Combs SE et al. ICRU report 91 on prescribing, recording, and reporting of stereotactic treatments with small photon beams. Strahlentherapie und Onkologie 2019;195:193-8.  Back to cited text no. 3
ICRU Report 29. Dose Specification for reporting external beam therapy with photons and electrons. Washington, DC: International Commission on Radiation Units and Measurements; 1978.  Back to cited text no. 4
ICRU Report 62. Prescribing, recording, and reporting photon beam therapy (Supplement to ICRU Report 50). Bethesda, MD: International Commission on Radiation Units and Measurements; 1999.  Back to cited text no. 5
Hanna GG, Murray L, Patel R, Jain S, Aitken KL, Franks KN, et al. Uk consensus on normal tissue dose constraints for stereotactic radiotherapy. Clin Oncol (R Coll Radiol) 2018;30:5-14.  Back to cited text no. 6
Kataria T, Sharma K, Subramani V, Karrthick KP, Bisht SS Homogeneity index: An objective tool for assessment of conformal radiation treatments. J Med Phys 2012;37:207-13.  Back to cited text no. 7
Petkovska S, Tolevska C, Kraleva S, Petreska E Conformity index for brain cancer patients. Proceedings of the second conference on medical physics and biomedical engineering of R Macedonia 2010;43:111.  Back to cited text no. 8
Paddick I A simple scoring ratio to index the conformity of radiosurgical treatment plans. Technical note. J Neurosurg 2000;93 (suppl 3):219-22.  Back to cited text no. 9
Paddick I, Lippitz B A simple dose gradient measurement tool to complement the conformity index. J Neurosurg 2006;105 (suppl):194-201.  Back to cited text no. 10
Kocher M, Soffietti R, Abacioglu U, Villà S, Fauchon F, Baumert BG, et al. Adjuvant whole-brain radiotherapy versus observation after radiosurgery or surgical resection of one to three cerebral metastases: Results of the Eortc 22952-26001 study. J Clin Oncol 2011;29:134-41.  Back to cited text no. 11
Prabhakar R, Rath GK Slice-based plan evaluation methods for three dimensional conformal radiotherapy treatment planning. Australas Phys Eng Sci Med 2009;32:233-9.  Back to cited text no. 12
Hernandez V, Hansen CR, Widesott L, Bäck A, Canters R, Fusella M, et al. What is plan quality in radiotherapy? The importance of evaluating dose metrics, complexity, and robustness of treatment plans. Radiother Oncol 2020;153:26-33.  Back to cited text no. 13


  [Table 1]


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