COSID INDEX

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Website:
www.journalofcurrentoncology.org
DOI:
10.4103/jco.jco_34_21
Address for correspondence: Dr. Kanhu Charan Patro,
Department of Radiation Oncology, Mahatma Gandhi Cancer Hospital and
Research Institute, Visakhapatnam, Andhra Pradesh, India.
E-mail: drkcpatro@gmail.com
Received: 22 October 2021; Revised: 14 January 2022; Accepted: 26 March 2022;
Published: 02 September 2022
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© 2022 Journal of Current Oncology | Published by Wolters Kluwer - Medknow
How to cite this article: Patro KC, Avinash A, Pradhan A, Kundu C,
Bhattacharyya PS, Pilaka VKR, et al. Dr. Kanhu’s COSID index:
An acronym for plan evaluation in SRS SBRT. J Curr Oncol
2022;5:4-7.
Original Article
Dr. Kanhu’s COSID Index: An Acronym for Plan Evaluation in
SRS SBRT
Kanhu Charan Patro, Ajitesh Avinash1
, Arya Pradhan1
, Chittaranjan Kundu, Partha Sarathi Bhattacharyya, Venkata Krishna Reddy Pilaka,
Mrityunjaya Muvvala, Arunachalam Chithambara2
, Ayyalasomayajula Anil Kumar2
, Srinu Aketi2
,Parasa Prasad2
, Venkata Naga Priyasha Damodara,
Veera Surya Premchand Kumar Avidi, Mohanapriya Atchaiyalingam, Keerthiga Karthikeyan
Department of Radiation Oncology, Mahatma Gandhi Cancer Hospital and research Institute, Visakhapatnam, Andhra Pradesh, 1
Department of Radiation Oncology,
Acharya Harihar Post Graduate Institute of Cancer, Cuttack, 2
Department of Medical Physics, Mahatma Gandhi Cancer Hospital and research Institute, Visakhapatnam,
Andhra Pradesh, India
Abstract
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
Introduction
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,
[Downloaded free from http://www.journalofcurrentoncology.org on Friday, September 2, 2022, IP: 117.239.144.217]

Patro, et al.: COSID index: An acronym for plan evaluation in SRS SBRT
Journal of Current Oncology ¦ Volume 5 ¦ Issue 1 ¦ January-June 2022 5
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.
Results
The COSID stands for Coverage index, Organ at risk
index, Spillage index, Imaging index, Delivery index
respectively. [Table 1]
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
prescriptionisodose.Thevalueof QRTOG
0.9isacceptable.
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]
Median absorbed 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
,
D35mm
3
, 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-35mm
3
, 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
Table 1: Showing the five major parameters of SRS/SBRT
plan evaluation (COSID INDEX)
COSID INDEX
Index Name
C Coverage Index
Median absorbed dose of PTV, D50%
Near-maximum dose, Dnear-max
Near-minimum dose, Dnear-min
O Organ at risk Index
S Spillage Index
Conformity Index
Homogeneity Index
Gradient Index
I Imaging Index
Slice by slice evaluation
D Delivery Index
Complexity of Plan
Monior Units (MU) Evaluation
Dose Calculation Parameters
Pre-verification of Treatment


6 6 Journal of Current Oncology ¦ Volume 5 ¦ Issue 1 ¦ January-June 2022
is calculated as the ratio of the volume of prescription
isodose (PIV) to the target volume (TV).
CIRTOG
= PIV / 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,
WhereTVPIV
isthevolumeof targetcoveredbyprescription
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

Journal of Current Oncology ¦ Volume 5 ¦ Issue 1 ¦ January-June 2022 7
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
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
Nil.
Conflicts of interest
There are no conflicts of interest.
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