Dosimetric errors of radiotherapy techniques involving small fields

Biplab Sarkar Ph.D.
Senior Medical Physicist
Radiation Oncology,
Manipal Hospital, New Delhi
biplabphy@gmail.com
Dosimetric errors of radiotherapy
techniques involving small fields
XVII Annual TMH Radiotherapy Practicum 20-21 Sep 2019

1.0 Physical or Physics principle
2.0 Clinical Scenarios
3.0 Dosimetric Measurements

Treatment machines and technique
Gamma Knife
CyberKnife
Linear accelerator based
Steriotaxy (Winston and
Lutz, 1988)
Tumour Dimension
(diameter) treated by SRS
AVM & Brain Metastasis≤ 3
cm
Gliomas and others ≤ 4 cm
Required overall spatial accuracy ≤ 1mm
IPEM Report ≤ 4 cm “Small field”

The Dimension Analysis: Small-Field
Radiation Therapy Equipment
Gamma Knife – Filed size (diameter)
4, 8 ,14 and 18 mm. – mechanical
precision of 0.5 mm or less
Linac (2000) : Novalis Varian (3mm
MLC cones) ; Elekta 3-Dline (2.5 mm
MLC & cones)
Inbuild MLC 4-5 mm
Beam Positing accuracy 0.4 mm
CK: Cone (5-50mm dia) and 2.5 mm
MLC, mechanical accuracy : (??)

Linear Accelerator based stereotaxy
Isocentric
Varian Novalis® TX (with
ExacTrac®, Brainlab),
Varian TrueBeam STX® with
Novalis, Varian EDGE®
Elekta Axesse® and VersaHD®
Non Isocentric
CyberKnife

Choice of beam Energy: Cranial and Extra-cranial
Recommended energy ≤ 10 MV
2ndary e- path 15x ≈3cm significantly affecting
penumbra width.
Accuracy of dose calculation better for low energy
Low energy Photon better blocked by MLC
Neutron production (for very high energy 18X)
For Lung : Definitely 6 MV accuracy in the low
density region.

The conflict in Small field
Is the problem exclusive for radiotherapy?
Or
we see some more example in nature (Physics,
Chemistry or Biology)?
Understanding why there is a problem with small
field which is not with the large fields
Quantum mechanical approach to the small field
problem

1 cm4 mm

Cross over
without
notice OR

1 cm
End up with (1)Crossing or (2) A dead
Ant and a flat surface:
But Why two scenarios are different for
a small and objects/ animals?

Do we have similar incident in Physic?
The associated de-Broglie
wavelength of x-ray (≈ 1 Angstrom)
is nearly equal to the band gap or
lattice gap.
Similarly Ant will climb the hill if
the hill dimension more or equal to
its own dimension. Reaction is
possible
(an ant may not able to climb Mt. Everest in its entire
life span: crashed: as dimensions are too
mismatched
What is dimension : de-Borglie
wavelength (probability density of finding
the object at a given point of the
configuration space)
X-Ray Diffraction pattern.
Quantum mechanical Equivalence

Tumour dimension equivalent to
to
(1) MLC dimension
(2) Dose calculation grid resolution.
(3) Chamber Dimension
0.6cc
Problem of matched dimension
0.125cc 0.01cc

Small and very small Detectors
0.5 mm to 15 mm

2.0 Clinical
We encounter different clinical situation where “small
field” is involved (1) radiosurgery or (2) conventional
fractionations

Clinical Case-1: with follow-up
14Gy in 1#; Tumour invading Brainstem : This is not actual
clinical delivered plan Just to show tumour position
6 Month Follow-
up MRI
Perfect Necrosis
at the tumour
location:
Exact matching
in the
stereotactic co-
ordinate.
Conclusion: dose Sufficient and no spatial error

Clinical Case : 2
SRS 24 and 20 Gy in 1# in two spatially isolated lesion (22-08-
2017)
Follow-up MRI (27-11-2017); in GTV1 well defined necrotic
tissue ;GTV-2 is unremarkable.
Follow-up MR-3.5 monthsPlanning MR
Planning MR
Planning MR

Case-3: SRS 12Gy in 1#
Pre treatment -MR6 month follow up MR
Edema

3.0 Dosimetry
ERGO++ and 3DLine µMLC
Elekta-iPlan1st Installation
Installation

Beam data Measurement error:
Absorb dose : Required multiple level of redundancy ; use of
detector dependent correction factor.
Just relative reading data from different detector types is not the
same as the relative dose in small fields;
Accurate depth Field-size dependent correction factors are required
to ensure that the detector signal is faithfully converted into absorbed
dose
Measurement: Challenges
1. Lateral Charge particle disequilibrium
2. Partial Source Visibility
3. Detector Size relative to field size.

Measurement Challenges: IPEM report- 103
Measurement GuidelinesBest Practice Guidelines

According to the WHO Radiotherapy Risk
Profile, most common sources
of actual adverse events in radiation therapy?-
Commissioning and treatment planning

Two Major Categories small field
 Commissioning
Small field measurements
Absolute calibration
Treatment parameter transfer –
SRS Cones
Wrong side/site treatments

Background
Different measuring devices were used by the
physicist
– A measuring device not suitable for calibrating the
smallest microbeams was used
Wrong data Entry to TPS
All patients treated with micro MLC were planned based on
this incorrect data
Patients treated with conical collimator were
not affected
Undetected for 5 years

Discovery of the accident
The BrainLAB company discovered that the
measurement files did not match up with those at
other comparable centres, during a worldwide inter-
comparison study
We were in the same risk group being the first
worldwide user (2012) of the Elekta-Iplan system
Complete iPlan data was measured 3 times (yes)
over 4 months time because Brainlab didn’t have
any Elekta library or Golden Beam data to
compare and choice of the appropriate chamber
was not known.

Practical Problem: Brainlab Did Not
rectify until now

A 1x1 cm2 6 MV output factor measured with a
0.6cc Farmer chamber
Would be 40% smaller than the true output factor
output measurement with 0.6 cc chamber for 1x1 FS.

Normalised TSCF : 1x1 to 40x40
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1x1 2x2 3.x3. 4x4 5x5 7x7 10x10 12x12 15x15 20x20 25x25 30x30 35x35 40x40
TSCF

How do you know if your data is good?
Compare with Other Institutions / Machines
Better That Vendor take responsibility of data
verification; instead inter-comparision by the users

Patient Specific QA: Small field: Our experience
Steriotaxy -310 – Dose to MU verification for all Plans
Small field 206 ≤33.5 cc(≈ radius=2cm)
0.01
0.1
1
10
100
1
4 7 10 13
16
19
22
25
28
31
34
37
40
43
46
49
52
55
58
61
64
67
70
73
76
79
82
85
88
919497100103106109112115
118
121
124
127
130
133
136
139
142
145
148
151
154
157
160
163
166
169
172
175
178
181
184
187
190
193
196199202205
GTV and PTV Volume
GTV 1
PTV1 Minimum
PTV Volume
Encountere
d is 0.2 cc ≈
4mm radius
(2 MLC)
XVIIAnnualTMHRadiotherapyPracticum20-21Sep2019

Measurement Result Dose to MU
-10.0
-8.0
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
10.0
1
5 9 13 17
21
25
29
33
37
41
45
49
53
57
61
65
69
73
77
81
85
89
93
97
101
105
109
113
117
121
125
129
133
137
141
145
149
153
157
161
165
169173177181185189193197201205
209
213
217
221
225
229
233
237
241
245
249
253
257
261
265
269
273
277
281
285
289
293
297
301
305
309
313
317
321
325
329
333
337
341
345
349
353357361365369
Small Fields measurement : Pin Point Chamber
% Point VARIATION % Volumetric Variation
369patients:SmallFieldsmeasurement(steriotaxy+
conventionalfractionations):Verifiedforpointdoseand
volumetricdoseusing0.015ccionizationchamberand
plasticwaterphantom.
All True gantry
Nominal couch
measurement
We have to postponed the treatment re-do
the planning change parameters, re-do QA to
ensure the correctness of dose delivery 4
times (≈1%) out of 369 patients.
XVIIAnnualTMHRadiotherapyPracticum20-21Sep2019

Ionization chamber array ??
2%-2mm

IC Array : Allowed or not allowed
Radiotherapy Oncology
Medical Physics
Medical Physics
Medical Physics

IC Array : Allowed or not?

Brachy Dose Gradient >>>than Steriotaxy DG

Arc Check: Results are average and sometime unpredictable

Additional Problem: small tumour; small field

Dose delivery: Challenges: positional error
Planned
Without
positional
Correction After positional
Correction
Dose
loss
No
Dose
loss

Planned
Without 6D
table
Correction After 6D table
Correction
Planned
With positional
correction
Without positional
correction
Dose delivery: Challenges: positional error

No problem with rotational error (<5%) If Radius ≥ 2 cm
0
5
10
15
20
0.30 0.80 1.30 1.80
Volumeinccand%
GTV radius
Post correction variation of % volume
between PTV_R and PTV_NR
Pre correction variation of absolute volume
between PTV_R and PTV_NR (cc)

So can we solve the Measurement problem?
With Present ion chamber system –No its not possible to solve the
“uncertainty” associated with “small fields”-Prohibited by
fundamental physics rules.
Best Detector is film=80 µm ; Resolution (≡10
-5m) different by
102with the resolution we deal with 1 mm (10
-3).
Ion chamber  need an quantum IC of similar order (10
-5 m).
Problem with small or very small detector they are not “volume”
represented.
Probably GEL will work in this dimension (at least theoretically)
Conclusion

Degree of correctness.
Small field : Uncertainties we have to live with.
 Base line dosimetric data TPS data should be taken very
cautiously most accurately as per the international and
vendor recommendation – Needs comparison.
 Point dose Measurement before delivery is Must
 IC/diode Array: Need to evaluate case by case basis.
 Many uncertainties involve , specially in contouring and
dose calculation and we have to accept these
approximation.
 Small Field Clinical Results are good (since Lakshell time)
With all errors and uncertainties.
Conclusion

Dosimetric errors of radiotherapy techniques involving small fields

Dosimetric errors of radiotherapy techniques involving small fields

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