Step-by-step guide to SRS planning for brain metastasis cavity

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Address for correspondence: Dr. Kanhu C. Patro,
Department of Radiation Oncology, Mahatma Gandhi Cancer Hospital and
Research Institute, Visakhapatnam, India.
E-mail: drkcpatro@gmail.com
Received: 18 December 2021; Revised: 26 March 2022;
Accepted: 10 May 2022; Published: 02 September 2022
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How to cite this article: Patro KC, Avinash A, Pradhan A, Tatineni
S, Kundu C, Bhattacharyya PS, et al. Step-by-step stereotactic
radiotherapy planning of brain metastasis in a surgically resected
setting: A guide to radiation oncologists: Dr Kanhu’s ROSE case
[Radiation Oncology from Simulation to Execution]. J Curr Oncol
2022;5:13-20.
Original Article
Step-by-step Stereotactic Radiotherapy Planning of Brain
Metastasis in a Surgically Resected Setting: A Guide to
Radiation Oncologists: Dr Kanhu’s ROSE Case [Radiation
Oncology from Simulation to Execution]
Kanhu C. Patro, Ajitesh Avinash1
, Arya Pradhan1
, Suresh Tatineni2
, Chittaranjan Kundu, Partha S. Bhattacharyya, Venkata K. R. Pilaka,
Mrityunjaya M. Rao, Arunachalam C. Prabu3
, Ayyalasomayajula A. Kumar3
, Srinu Aketi3
, Parasa Prasad3
, Venkata N. P
. Damodara, Veera S. P
. K. 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, Odisha, 2
Department of Neurosurgery, Medicover Hospital, Visakhapatnam, Andhra Pradesh, 3
Department
of Medical Physics, Mahatma Gandhi Cancer Hospital and Research Institute, Visakhapatnam, Andhra Pradesh, India
Abstract
Background: Surgical resection of brain metastasis is followed by adjuvant radiation in order to reduce the risk of local
recurrence. Traditionally, adjuvant radiation was practiced in the form of whole brain radiation therapy that was associated
with adverse neurocognitive outcomes and poor quality of life of the patients. In the recent times, stereotactic radiosurgery
(SRS) is being practiced as the standard of care for treating brain metastasis cavity with good local control and improved
the patient’s quality of life by sparing the normal tissues of adverse effects of radiation. Here, we describe procedure details
for stereotactic planning of surgically resected brain metastasis. Materials and Methods: The step-by-step procedure for
stereotactic planning of brain metastasis cavity has been described using a clinical scenario of brain metastasis. Results: The
stereotactic radiation planning of brain metastasis cavity starts with the basic history and relevant evaluation of symptoms.
Magnetic resonance imaging (MRI) of the brain is the imaging modality of choice. The radiation planning of brain metastasis
cavity starts with computed tomography (CT) simulation and MRI of brain that should be done in a prescribed format to
achieve uniformity in radiation planning. After CT and MRI image fusion, contouring of target, organs at risk (OAR), and
radiation planning should be done. The plan evaluation includes target and OAR coverage index, conformity, homogeneity and
gradient index, and beam arrangement. After radiation plan evaluation, treatment is delivered after quality assurance and dry
run. Conclusion: The paper highlights the sequential process of radiation planning for SRT of brain metastasis cavity, starting
from simulation, planning, evaluation of plan, and treatment.
Keywords: Brain metastasis, cavity SRS, plan evaluation, radiotherapy planning
Introduction
The management of brain metastasis has evolved from
the traditional whole brain radiotherapy (WBRT) to
stereotactic radiosurgery (SRS) with time. Following
surgical resection of brain metastasis, adjuvant treatment in
the form of WBRT or SRS is advocated in order to decrease
the risk of local recurrence. Various randomized trials have
proved that WBRT is effective in higher local control at
[Downloaded free from http://www.journalofcurrentoncology.org on Friday, September 2, 2022, IP: 117.239.144.217]

Patro, et al.: SRS planning of surgically resected brain metastasis: The ROSE case

14 14 Journal of Current Oncology ¦ Volume 5 ¦ Issue 1 ¦ January-June 2022
post-operative tumor bed as well as distant recurrence in the
brain, but it was associated with increased deterioration of
patients’ cognition and quality of life. Recent publications
and guidelines advocate SRS as the standard of care for
limited brain metastasis when compared with WBRT
because of similar local control with advantage of higher
patients’ cognition and quality of life.[1]
Material and Methods
Here, the various steps of radiation planning for SRS have
been illustrated in an easy way for the beginners using a case
of post-operative cavity in a patient with brain metastasis.
Results
Case History
A 40-year non-smoker male with ECOG 1 had an episode
of seizure in the month of March 2020 for a duration of
3–4 min, followed by aura. The seizure was not associated
with headache and vomiting. There was no history of
involuntary urination or defecation. It was followed by
another episode in the month of June 2020 for 2–3 min
duration, followed by aura. Post-ictal confusion lasted for
25–30 min. This was also not associated with headache,
vomiting, involuntary urination, or defecation.
Imaging
Contrast-enhanced magnetic resonance imaging
(CE-MRI) of the brain showed a well-defined lesion
of size 3.2 cm × 3.2 cm in the left occipital and inferior
temporal lobe that was hypointense on T1 and brilliantly
heterogeneous on T2-weighted image associated with
perilesional edema [Figure 1(A)]. Magnetic resonance
spectroscopy showed increased choline and decreased
N-acetyl aspartate. With these imaging findings, a
preliminary diagnosis of ganglioglioma was made.
Surgery
The patient underwent left parieto-occipital craniotomy
with gross total excision. The excised tumor was vascular
and there was a clear plane of cleavage.
Histopathology
On post-operative histopathological examination, the tumor
was confirmed to be of size 4 cm × 3.5 cm × 1.5 cm and
was suggestive of metastatic papillary adenocarcinoma. On
immunohistochemistry study, CK 7 and TTF 1 were positive.
Post-operative Imaging
CE-MRI of the brain revealed a post-surgical defect of
size 3.2 cm × 3 cm in the left temporo-occipital region. The
cavity was thick-walled having minimal irregular outline.
The lesion was hypointense on T1 and hyperintense on
T2-weighted image [Figure 1(B)].
Whole body positron emission tomography-computed
tomography (PET-CT) scan showed a surgical defect in
the left parieto-occipital region of the brain measuring
size 3.2 cm × 2.4 cm. There was a spiculated lesion in the
upper lobe of the right lung of size 2.6 cm × 2.1 cm (SUV
max. 3.5). There was also a right paratracheal lymph
node of size 1.1
×
1.6 cm (SUV max. 3), and multiple
hypermetabolic lymph nodes were seen in the right
paratracheal and subcarinal regions [Figure 1(C)]. With
these above findings, a final diagnosis of the carcinoma
right lung with brain metastasis (c T4N2M1b) was made.
Tumor Board Decision on Further Line of Treatment
The patient details were discussed among neurosurgeon,
radiation oncologist, and medical oncologist in the
tumor board, and stereotactic radiotherapy followed by
chemotherapy was decided by the board as the treatment
plan.
Discussion With the Patient
The patient was explained about the procedure, regarding
imaging and follow-up, tumor response, the need of
radiotherapy (in the form of whole brain radiotherapy/
SRS) in the future, and post radiotherapy-raised
Figure 1: Pre-operative magnetic resonance imaging (MRI) of brain
showing lesion of size 3.2 cm × 3.2 cm in the left temporo-occipital
region (A), post-operative MRI of brain showing post-surgical defect of
size 3.2 cm × 3 cm in the left temporo-occipital region (B), and positron
emission tomography-computed tomography (PET-CT) scan showing
lesion in the upper lobe of right lung of size 2.6 cm × 2.1 cm (C)

Journal of Current Oncology ¦ Volume 5 ¦ Issue 1 ¦ January-June 2022 15
intracranial tension. He was also explained regarding the
tumor control, appearance of new lesions in the future,
and the possibility of radionecrosis.
Pattern of Recurrence
Followingsurgicalresectionof brainmetastasis,recurrence
of leptomeningeal disease (LMD) is seen in about 30%
of the cases. There are basically two types of recurrence,
i.e., classic LMD and nodular LMD. LMD develops due
to hematogenous spread, direct spread of the disease, or
tumor spillage to cerebrospinal fluid during surgery.[2]
Dose Selection
Currently, single fraction SRS (12–20 Gy in 1 fraction) is
advocated as the standard of care for adjuvant therapy
for post-operative cavity, but it requires dose reduction
for tumors larger than 3 cm for radiation-induced
safety. The tumor with size 3 cm requires coverage of
the areas of affected meninges and part of craniotomy,
thereby increasing the volume to be encompassed within
the target volume, thus increasing more tissue at risk of
radiation necrosis. The use of fractionated SRS (27 Gy in
3 fractions or 30 Gy in 5 fractions) gives the liberty of
coverage of such larger volumes and delivery of dose with
higher biologically effective dose than single fraction SRS
while maintaining safety. In multiple studies, it was seen
that local tumor bed control was better with fractionated
SRS when compared with single fraction SRS.[1]
Decision by the Radiation Tumor Board
As the tumor size was 3 cm and in order to include the
larger volume of adjacent dura and craniotomy tract,
fractionated SRS was planned by the Radiation Tumor
Board to a marginal dose of 30 Gy in 5 fractions at 5 Gy/
fraction.
Radiation Planning
Here we describe the steps of treatment of brain metastasis
post-operative cavity by stereotactic radiotherapy from
simulation to plan execution.
Time Interval Between Surgery and Start of
Radiation
Alghamdi et al.[3]
in their paper have reported that there
was greater shrinkage of post-operative cavity in larger
tumors, i.e., 3 cm and in early post-operative period
(21 days) as this may lead to irradiating more normal
tissues. Thus, radiation should begin not less than
21–28 days following surgery.
Step 1: CT simulation
During simulation, the patient was set up in the supine
position with neutral neck position and immobilization
was done using the FRAXION thermoplastic mask and
stereotactic frame [Figure 2(A)]. Fiducials were placed on the
thermoplastic mask after proper alignment with the lasers.
Intravenous contrast was given at a dose of 1 mL per kg body
weight. Then, CT scan was taken from the vertex to neck with
CT slice thickness of 1 mm, as depicted in Table 1 and Figure
2(B). After simulation, the DICOM CT images were sent to
our Oncentra server which was then imported for delineation
of targetandorganatrisk(OAR).TheCTscanof thispatient
was done on the post-operative 28th day of surgery, keeping
in mind the cavity remodeling.
Step 2: MRI protocol
MRI brain of the patient was done using 512 × 512 matrix
in the neutral neck position similar to that of CT scan
during simulation with no gap, no tilt, and 1 mm slice
thickness, as depicted in Table 1. The field of view included
the body contour along with nose, eyes, and skull. The
MRI included the usual T1, T2, FLAIR sequences. In
addition, the 3D FSPGR was used to view the normal
anatomy.
Step 3: Image fusion
These acquired MRI sequences were fused with the
planning CT scan by contouring the eyes, lens, basilar
artery, sinuses, and calcification, and matching was done
using the auto-fusion technique to help in target and OAR
delineation [Figure 2(C)].
Step 4: Target delineation
The target delineation was performed as per the consensus
contouring guidelines for post-operative completely
resected cavity SRS for brain metastasis.[4]
As this was a
post-operative setting, there was no gross tumor volume
(GTV). The clinical target volume (CTV) included the
entire contrast-enhancing surgical cavity seen in the
gadolinium-enhanced T1-weighted MRI scan excluding
the edema. The CTV included a 5 mm margin along the
bone flap beyond the initial region of pre-operative tumor
contact as the tumor was in contact with the dura in the
pre-operative MRI scan. If the tumor was not in contact
with the dura, then CTV should include a margin of
1–5
mm along the bone flap. CTV included a margin of
1–5 mm along the sinus as the tumor was in contact with a
venous sinus in the pre-operative MRI scan.
Here, we propose a simple mnemonic “ABCDE” that
can be used while target delineation was performed
for the post-operative cavity SRS, where A = adjacent
dura to lesion and surgical tract, B = bone flap inner
part, C = cavity proper, D = dural sinus, E = enhancing
component on CE-MRI.
Thus, the total CTV: A + B + C + D + E [Figure 3(A)-(F)].
The planning target volume (PTV) was drawn taking
1 mm around the CTV [Figure 4(A)]. Smoothing of the


contour was done from the adjacent bone. Multi-planar
evaluation, i.e., evaluation of both the GTV and PTV, was
done in all the three planes: axial, coronal, and sagittal
[Figure 4(B)-(D)].
In the present case, the CTV volume was 4.65 cc and the
PTV volume was 37.491 cc.
Step 5: OAR delineation
The OAR delineation included the cochlea, brainstem,
optic chiasma, and optic apparatus. The cochlea was
contoured in the bone window setting, whereas other
OARs, i.e., brainstem, optic chiasma, and optic apparatus,
were contoured using the MRI that was fused with the
planning CT. Also brain CTV was also drawn as an OAR
[Figure 5(A)].
Step 6: Radiation technique
Radiation planning can be done using any of the RT
techniques such as intensity-modulated radiotherapy
(IMRT), volumetric-modulated arc therapy (VMAT),
dynamic conformal arc therapy (DCARC), or three-
dimensional conformal radiotherapy (3DCRT),
according to the convenience of the radiation physicist
and physician.
In the present case, planning was done using the VMAT
technique.
Step 7: Plan evaluation
After the completion of planning by the physicist, the
evaluation for treatment plan was done using the following
indices as noted below:
PTV coverage index
Following planning, the coverage of the PTV needs to be
seen. The prescription isodose level was such that not 100%
of the prescribed dose covered 100% of the PTV. Often, 95%
of the prescription dose covered 95% or higher percentage
of the PTV, and otherwise 100% of the prescription dose
covered 95% or higher percentage of the PTV.[5]
In the present case, 95% of the prescription dose
covered 99.9% of the PTV and 100% of the prescription
dose covered 99.15% of the PTV, which satisfied the
aforementioned parameter for the PTV coverage as
depicted in Table 2.
Intracranial organ at risk index
Keeping in mind the desirable dose constraints to the
OAR, we need to check the dose to individual OARs.[6]
The dose desirable and dose achieved for all the OARs in
the present case are depicted in Table 3.
Whole brain minus CTV dose
As per Faruqi et al.,[7]
in order to reduce the risk of
adverse effects following fractionated SRS, the volume of
brain minus CTV receiving 30 Gy dose should be limited
to 10.5 cc.
Table 1: CT simulation and MRI protocol to be followed for
brain metastasis cavity SRS
CT simulation protocols for simulation
Supine position
Immobilization using stereotactic thermoplastic mask
Intra venous contrast at a rate of 1 mg/kg
CT scan taken from vertex to neck 15 min after contrast
administration
1 mm slice thickness
MRI protocol Utility
T1/T2/FLAIR sequence Usual sequence
3D FSPGR sequence Normal
anatomy
512 × 512 matrix
1 mm slice thickness
No gap
No tilt
Neutral neck
Field of view should include body contour, nose, eye, and skull
Figure 2: Immobilization of the patient using the stereotactic thermoplastic mask and frame during CT simulation (A), CT scan taken during CT
simulation (B), and fusion of MRI of the patient with planning CT scan (C)

In the current case, the 30 Gy volume of brain–CTV
was 10.30 cc.
Conformity index
To note the conformity index of the SRS, here we used two
types of conformity indices, i.e., the RTOG conformity
index and the Paddick conformity index.[5,8]
The RTOG conformity index (CIRTOG
) was calculated
using the following formula:
CIRTOG
= Volume of prescription isodose/PTV volume.
In this case, the RTOG conformity index was 1.17
[Table 2].
Paddick conformity index (CIPaddick
) was calculated using
the following formula:
CIPaddick
=
(Volume of prescription isodose in the area of
interest, i.e., PTV)2
/PTV volume × Volume of
prescription isodose.
Here in the current case, the Paddick conformity index
was 0.96 [Table 2].
Homogeneity index
It is calculated using the following formula:
Figure 3: Clinical target volume (CTV) delineation showing delineation of adjacent dura (A), bony flap (B), surgical cavity (C), dural sinus (D),
contrast-enhanced region (E), and total CTV (F)
Figure 4: Delineation of planning target volume (PTV) (A), evaluation of
CTV and PTV in axial view (B), coronal view (C), and sagittal view (D)


Homogeneity index = Maximum dose/Prescription dose.
In this case, the homogeneity index was 1.21 [Table 2].
Dose fall off
The dose fall off observation is very much needed in the
plan evaluation under the heading of gradient index. For
this we need to calculate the difference between various
isodose lines. In order to calculate the difference between
the isodose lines, we need to calculate the equivalent
radius.
Equivalent radius calculation
To evaluate the dose gradient, we have to find out the
difference between the radius of various isodose lines. But
none of the isodoses is spherical. So, we use the following
formula to calculate the equivalent radius.
1st: Find out the specified isodose volume.
2nd: Calculate the radius of the isodose volume by using
the formula:
V = 4/3 π r3
,
r = (3V/4 π)1/3
.
The calculation of volume and radius of various isodose
lines in the present case is shown in Table 2.
Gradient Index
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 and 0.9 mm.
Figure 5: Delineation of organs at risk (A), isodose lines: 100% (dark
green), 80% (light green), 60% (light blue), 50% (pink), 40% (dark blue)
(B), and cone beam computed tomography correction of patient during
treatment (C)
Table 2: Various indices of plan evaluation of brain metastasis cavity SRS in the current case
Parameter Value Desirable
Dmax 36.43 Gy —
D95% 31.01 Gy —
D100% 28.23 Gy —
V95% 99.99% —
V30Gy (V100%) 99.56% —
V110% 44.45% —
V120% 0.03% —
V130% 0 —
PTV volume 37.491 cc —
Volume of prescription isodose 43.798 cc —
Volume of prescription isodose within the PTV 39.764 cc —
Maximum dose 36.43 Gy —
Prescription dose 30 Gy —
RTOG conformity index 1.17 1–2
Paddick conformity index 0.96 0.85–1
Homogeneity index 1.21 1.1–1.3
Parameter Volume Radius
100% isodose line 43.79 cc 2.19 mm

In the current case, the gradient index was
2.19−3.15 mm = 0.96 mm, which is close to the ideal
gradient index [Table 2].
Distance between various isodose lines
The various isodose lines are depicted in Figure 5(B).
The ideal difference between 80% and 60% isodose lines
should be 2 mm.[9]
In the current case, it is 0.4 mm.
The ideal difference between 80% and 40% isodose lines
should be 8 mm.
In the present case, it is 1 mm.
Beam arrangement
The arrangement of the beams was done such that there
is adequate coverage of the target while giving less dose to
the OARs. It should be noted that the beams should not
pass through the ipsilateral eye.
Choosing the appropriate plan
During radiation planning, out of all the plans, three plans
were chosen for comparison, as shown in Table 4. Plan 3
had the highest V30Gy, D100%, and monitor units (MUs)
with lowest (brain—PTV) dose and 50% volume. Thus
plan 3 was chosen for treatment which has less integral
dose, even though MU is more.
Step 8: Quality assurance (QA)
Mechanical isocenter check was done using the Winston–
Lutz test, and the point dose verification was done keeping
the tolerance as 1 mm.[10]
Step 9: Dry run
Treatment verification consists of setup reproduction,
isocenter verification, and clinically verifying each
treatment field—check beam clearance, check any
interlock—MLC interlock, and potential MU problems.
Then the immobilization devices are clearly marked after
successful dry run.
Step 10: Pre-medication protocol
Prior to the start of the treatment, pre-medication was
delivered in the form of tablets as described below: all
starting the day before start of RT treatment.
Tablet Dexamethasone 8 mg thrice daily
Tablet Ondansetron 8 mg thrice daily
Tablet Pantoprazole 40 mg once daily
If the patient is diabetic, proper diabetic care needs to be
taken.
Step 11: Setup verification and treatment delivery
It includes cone beam CT correction [Figure 5(C)]. After
all the corrections had been done treatment was delivered.
Step 12: Post-medication
It is an optional protocol that usually includes anti-
emetics, proton pump inhibitors, and tapering the dose of
steroids over a week. Steroids and anticonvulsants were
given to the patient to decrease the chance of edema and
seizures during and after treatment.
Step 13: Advice and follow-up
After the completion of the treatment, the patient was
usually advised to follow after 6 months for imaging.
Supplementary file
Here, we also provide the Brain Metastasis SRS Plan
Evaluation sheet as a supplementary file that will help
in proper and accurate plan evaluation for every post-
operative cavity SRS case of brain metastasis.
Conclusion
This paper conceptualizes and acts as an easy guide for
the stereotactic radiotherapy treatment of post-operative
cavity brain metastasis.
Table 3: Organs at risk with their desirable dose and dose
achieved in the current case
Organ Desirable dose (Gy) Achieved dose (Gy)
Right eye DMax
22.5 1.97
Left eye DMax
22.5 4.4
Right optic nerve DMax
22.5 2.3
Left optic nerve DMax
22.5 5.5
Optic chiasma DMax
22.5 7.5
Brain stem DMax
23–31 10.01
Right cochlea DMean
25 1
Left cochlea DMean
25 1
Table 4: Comparative assessment of three SRS plans of cavity SRS
Plan V30 Gy D100% Brain—PTV
(12 Gy volume)
50% volume (cc) Monitor units Integral dose (volume of
patient × mean dose)
1 98.35 27.76 7.1% 148 1885 4868.586 (3321 × 1.46 Gy)
2 97.96 26.84 6.23% 132 2247 4549.77 (3321 × 1.37 Gy)
3 99.56 28.23 6.14% 130 3679 4649.4 (3321 × 1.4 Gy)


Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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