Principles of Interstitial Brachytherapy - Seminar Dr Yamini Baviskar

1. PRINCIPLES OF INTERSTITIAL
BRACHYTHERAPY :
A CLINICAL DEMO
DR. YAMINI BAVISKAR JR1
DR. REVATHY KRISHNAMURTHY SR1

2. FLOW OF SEMINAR
INTRODUCTION
SOURCES
ISBT PHYSICS
BASIC CONCEPTS IN ISBT
SYSTEMS – PAST & PRESENT
CLINICAL CASE DEMO

3. BRACHYTHERAPY
• Placing Sealed Radioactive Sources Very Close To Or In
Contact With The Target Tissue
INTERSTITIAL INTRACAVITARY SURFACE TRANSLUMINAL
SHORT

4. Surgically Implanting Radioactive Sources
Directly Into The Tissues
In Or Around The Zone To Be Treated.
INTERSTITIAL BRACHYTHERAPY

5. RATIONALE
Highest dose in centre of tumour
Rapid Dose Fall off >> Max. sparing of adjacent
tissue
Ultimate form of conformal therapy
Prescribe High doses to CTV >> Better tumour
control
Volume Effect less for less normal tissue Irradiated
Short treatment time >> less repopulation
ALSO
1. Good cosmesis.
2. Tolerable side
effects
3. Day care procedure
4. Minimal radiation
morbidity
WHY??

6. CONCEPT OF INTERSTITIAL
BRACHYTHERAPY
• Alexander Graham Bell in 1903

7. TIME FRAMES
1896 – Becquerel discovered Radioactivity
1898 – Madam Curie / Pierre Curie – Radium
1901 - Danlos and Bloc - First radium implant in Paris
1920-50 – Various systems of dose prescription.
1957 – Ir-192 implants
1960 – Henschke / Fletcher - After-loading applicator
1962 – First Remote after-loading machine
1965 – Paris system for Interstitial BT
1970 – Co-60 HDR
1985 – HDR Ir- 192
2000 onwards - 3D BT planning ,CT/MR Compatible applicators,Inverse
planning
• Classical era 1940s to 1980s
• Maturation of classical BT
systems
• Transition from radium to
artificial radionuclides
• Modern era 1980 onwards
• Quantitative dosimetry
• Measurement of dose
distribution using computers.

8. IDEAL BRACHYTHERAPY SOURCE
1. Pure γ emitter, No charged particle emission
2. Medium γ energy
3. High specific activity
4. Non toxic
5. Stable SOLID daughter product
6. Long t ½ for temporary implant , Minimal decay correction
7. Short t ½ for permanent implants
8. Easily made into shapes and sizes -tubes, needles, spheres, wires

9. 1. Daughter product Radon – gaseous,
2. Rn Contamination risk, deposited permanently in bone
3. T ½ : 1626 yrs , high gamma ray constant- Radiation protection
4. Low specific activity : 1 Ci /gm - large diameter of needles
5. Higher gamma energy than is necessary for BT
6. High cost of source extraction and disposal
7. High beta energy: 3.26MeV - 1.4cm HVL shielding
Newer Sources With High Specific Activity Led To Replacement Of Radium
Until 1950s

10. 60COBALT
• Neutron activation of stable isotope Co 59
• Half-life 5.27 years
• Decay scheme
27Co
60
-----> 28Ni
60
+ -1β
0
+ γ
• Specific Activity - 1020 Ci/g
• Beta energies - 0.318MeV
• Photon energies - 1.17MeV ,
1.33MeV
• Half value layer in lead - 10 mm
• Neutron activation of stable isotope Ir 191
• Half-life - 73.83 days
• Decay scheme - 192 192 0
77Ir -----> 78Pt + -1β+ γ
• Specific Activity - 7760 Ci/g
• Beta energies - 0.079-0.672 MeV
• Photon energies - 0.2 - 1.06 MeV
Avg. 0.37 MeV
• Half value layer in lead - 4.5 mm
IRIDIUM 192

11. IRIDIUM 192
• Medium gamma energy
• Mono-energetic
• Flexible wire form
• Easy availability
• Low cost
Half – life
Single source max Activity : 10 Ci – 12 Ci
Active dimension: D - 0.6 mm, L-3.5 mm
With encapsulation of stainless steel : D- 0.9 mm,
L- 4.5 mm
High specific activity
High dose rate
Miniaturized

12. TEMPORARY
 Cs¹³⁷, Ir¹⁹²
 Implanted for Short period of time as compared to
their T1/2
 Removed when prescribed dose delivered.
 Rigid stainless steel needles/flexible Teflon / nylon
guides/plastic tubes
 Preloaded/After loaded
 Long T1/2
 PERMANENT
 I125 ,Au198,Pd103
 Indefinitely implanted
 Left in implanted tissue for gradually delivering
the dose until it decays.
 Photon energy used is low so that radiation
protection can be achieved
 Not suitable for remote afterloading
 Deliver a high total dose at a very low dose
rate.
 Short T1/2

13. CLASSIFICATION
PRELOADING
MANUAL
AFTERLOADING
REMOTE
AFTERLOADING
Pulsed dose rate:
Large no. of small fractions
To simulate LDR
requires less shielding
long treatment times
Time interval between 2 pulses < 4
hrs.
Report no.38

14. PHYSICS BEHIND
INTERSTITIAL
BRACHYTHERAPY
• Source Strength Specification
• Factors Influencing Single Source Dose
Distribution
• Basic Concepts And Definitions
• Systems of ISBT

15. SOURCE STRENGTH SPECIFICATION
1. MASS of Radium in milligram.
2. EXPOSURE RATE AT SPECIFIED DISTANCE : Wambersie et al
Exp. Rate = Specific gamma ray constant (k factor) / (Dist.)2 from a 1 mg point source encapsulated in 0.5 mm thick Pt sheath
K factor = 8.25 R/hr cm2 mg
3. MILIGRAM – HOUR
Exp. Rate α Amount of Ra in Source X Duration of exposure
4. ACTIVITY :
Number of spontaneous nuclear transformations/disintegrations in specified time interval.
SI unit – Bq. 1 miliCi = 3.7 X 10 7 Bq
5. EQUIVALENT MASS OF RADIUM :
That mass of Radium filtered by 0.5 mm Pt that has the same ref. air kerma rate as that of given source
1 mg.hr. Ra equivalent = 7.2 microGy at 1 m
MASS

16. KERMA : KINETIC ENERGY RELEASED IN MEDIUM
Radiation beam  absorbing medium = Two stage interaction
I : Energy of photons - the indirectly ionizing particles - transformed into kinetic
energy of high speed electrons
• II: Electrons - the directly ionizing particles - slowed down and deposit their
energy in the medium
QUANTIFIES the Transfer of Energy from Source to Matter
KERMA
ABSORBED DOSE

17. SOURCE STRENGTH SPECIFICATION
• EMISSION
REFERENCE AIR KERMA RATE: (AIR KERMA STRENGTH):
Air kerma rate in vacuum at reference distance of 1 m
from source centre , on its transverse axis
due to photons of energy greater than specified energy cut off (δ) to
exclude low energy contaminant photons.
mGy/h at 1 m
TOTAL REFERENCE AIR KERMA:
Sum of RAKR X Irradiation time for each source.
Analogous to Mg- hr
• Useful index for radiation protection of the personnel and nursing staff
• Cannot be used to estimate absorbed dose in immediate vicinity of
sources (i.e., in the tumour or target volume).

18. FACTORS INFLUENCING SINGLE SOURCE
DOSE DISTRIBUTION
• DISTANCE – INVERSE SQUARE LAW :
• Dominant factor irrespective of the energy of the emitted
radiation
• For a pure isotropic source, dose will decrease by a factor of
100 between distance of 0.5 and 5 cm
• ABSORPTION AND SCATTERING IN
• Source Core
• Encapsulation Medium
• PHOTON ATTENUATION AND SCATTERING IN
• Surrounding Medium

19. OBLIQUE FILTRATION
Photons from longitudinal source axis
traverse longer path lengths of capsule & core material
 more attenuation
than photons on the transverse source axis
To overcome –
1. Crossing in P-P
system
2. Longer active
lengths in Paris
System
3. Dwell time
optimization in
SSDS

20. BASIC CONCEPTS AND DEFINITIONS
PHYSICAL LENGTH:
Distance from proximal to distal end of source/ source assembly
ACTIVE LENGTH OF SOURCE:
Distance from most proximal to most distal end of radioactive
material contained in the source line.
EQUIVALENT ACTIVE LENGTH OF SOURCE LINE:
Active length of a uniform linear source that yields an isodose
distribution in region of interest equivalent to that from a uniform
source line made up of discrete spaced sources.

21. SYSTEMS OF INTERSTITIAL BRACHYTHERAPY
• Set of rules to obtain suitable dose distribution over volume to be treated by taking into account
• source types & strengths
• geometry
• method of application
• Consists of
1. Distribution rules
2. Dose specification & implant optimization criteria
3. Dose calculation aids
Implant should follow BOTH Source Distribution Rules
and Dose prescription & specification Method of a system

22. MANCHESTER
SYSTEM - 1934
• Developed to deliver uniform dose (+/- 10% ) throughout
implant with Preloaded Radium Needles.
• Distribution Rules for Moulds, Interstitial Single Plane & Volume
Implants
Dose Calculation Aids in the form of Tables.
Gives product of amount of radium (in mg) and
time (in hours) needed to give 1000 roentgens to
treated surface – mg-hrs per 1000R

23. DRAWBACKS OF PATERSON PARKER SYSTEM
1. Assumed k value of 8.4 R/hr cm2 mg
2. Ignored effect of Attenuation, Scattering,
3. Specified treatment in terms of Exposure
4. Fixed Spacing with Fixed Source Activity
5. Need of Crossed ends –difficult in deeper implants
6. Restricted to typical geometrical volumes

24. THE QUIMBY SYSTEM
UNIFORM SOURCE SPACING
UNIFORM SOURCE ACTIVITY
NON UNIFORM DOSE DISTRIBUTION
Higher dose in central region of the implant
THE MEMORIAL SYSTEM
• Developed by Laughlin et al
• Extension of the Quimby System
• Not used.

26. RULES OF PARIS SYSTEM
1. Sources straight and parallel to each other
2. Each source equidistant from each other
3. Centers of all sources contained on a single plane
perpendicularly bisecting sources ( >> Central
Plane)
4. Linear activity of all sources is uniform along
length and identical for all sources
5. For volume implants, intersection of source line
with central plane form either corners of a square
or apices of equilateral triangles

27. CENTRAL PLANE
Plane perpendicular to sources at right angles to long axis mid-way
along their length.

28. BASAL DOSE RATE
MINIMUM DOSE RATE BETWEEN A PAIR OR
GROUP OF SOURCES
• Total BDR - mean of elementary dose rates
• BD 1,2,3 are Elementary/ Local Basal Doses,
measured in Central Plane.
• Implant satisfactory if each elementary BDR
within ± 10% of the average BDR
MEAN CENTRAL DOSE:
Arithmetic mean of doses at mid- distance
between each pair of adjacent source lines,
taking into account dose contribution at that
point, from all sources in pattern

29. REFERENCE DOSE :
• 85 percent of the Basal Dose Rate , encompassing target
volume as closely as possible
• Treatment volume is enclosed by this reference isodose
• Used for calculating the total time of the implant
MINIMUM TARGET DOSE:
• The dose selected & specified by the rad. onc. as adequate to
treat PTV/CTV
• Min. dose at the periphery of CTV hence a.k.a MPD in USA

30. • LENGTH OF TREATMENT VOLUME
• Smallest distance b/w the invaginations of the treatment isodose at
either end of the volume.
• THICKNESS OF TREATMENT VOLUME
• Smallest distance b/w 2 parallel planes ,tangents to those
isodose invaginations which gives the target volume its least
thickness.
• WIDTH OF TREATMENT VOLUME
• Distance b/w most lateral sources + 37% of the source
separation, added on each side
• Sources implanted may be in more than one plane
depending on the thickness
• > 1 plane if t > 12mm

31. SAFETY /LATERAL MARGIN:
• Minimum distance measured between the reference
isodose and Outer catheters in the central transversal and
longitudinal plane.
SOURCE SEPARATION:
• Perpendicular distance between 2 adjacent sources
LOWEST ACCEPTABLE SEPARATION - 0.5 CM
MAXIMUM ACCEPTABLE SEPARATION - 2CM
OVERDOSE/ HYPERDOSE SLEEVE:
The volume of tissue immediately surrounding a
radioactive source which receives 170% BDR ( twice the
reference dose)

32. HIGH DOSE VOLUME:
Volume encompassed by the isodose corresponding to 150% of the MCD around
sources
LOW DOSE VOLUME:
Volume within the CTV encompassed by an isodose corresponding to 90% of
prescribed dose.

33. VOLUMES
• TREATED VOLUME:
• Volume of tissue according to the implant as actually achieved, receives a dose
at least equal to the dose selected and specified by the rad. oncologist as being
appropriate to achieve the treatment goal. (REFERENCE DOSE)
• IRRADIATED VOLUME:
• Volume larger than treatment volume ,which receives an absorbed dose
considered to be significant in relation to tissue tolerance.
• ORGANS AT RISK:
• These are those radiosensitive organs in or near target volume which would
influence treatment planning and/or prescribed dose.
• CTV based!

34. CASE SELECTION FOR BRACHYTHERAPY
 Easily accessible lesions, at least from one side
 Early stage disease
 T 1-T2 and sometimes early T3
 Ideally total size of implant ≤ 5 cm
 No local infection
 Proliferative and ulcerative lesions preferred

35. PRE- PLANNING
• CLINICAL:
• Accurate assessment of tumor dimensions and nodes
• Pre-treatment imaging
 TARGET VOLUME
 IMPLANT CONFIGURATION
 TOTAL DOSE
• INVESTIGATIONS:
• Fitness for anaesthesia
• CONSENT OF THE PATIENT
Hospital Admission 1 day prior to the
day of Implant

36. TECHNIQUES
• Hollow needle guide is pushed through
site
• Tubing inserted into hollow needle guide
and pushed through
• Needle is withdrawn leaving tubing
intact.
• Tubing secured on each end with buttons
• Breast
• Soft tissue sarcoma
• End of short needle guide is pulled
through with hemostats & inserted back
through in the opposite direction to exit
on the entry side
• Loops around/over the lesion
• Single plastic tube passes through both
needles
• Oral cavity
• BOT
BASIC/PUSHING TECHNIQUE LOOP/ PULLING TECHNIQUE

37. INTRA OPERATIVE SETTING POST OPERATIVE SETTING
PRE IMPLANT CAVITY ASSESSMENT

38. MARKING OF ENTRY EXIT POINTS – EQUAL
SPACING

39. INSERTION OF NEEDLES TO ACQUIRE
DESIRED GEOMETRY

41. REPLACING NEEDLES WITH TUBES

42. THE FINAL GEOMETRY
Insertion of Dummy plastic
wires

43. POST IMPLANT IMAGING AND MEASUREMENTS
• PLANNING CT 1.25 MM CUTS
• To display the tumour in relation to the applicator as accurately and
reproducibly as possible.
• Sectional images in three dimensions, parallel and perpendicular to the
(projected) axis of the needles or tubes.
• INDEXER LENGTH MEASUREMENT
• Catheter Length and Transporter length has to be measured
• Transporter length – 1001 mm (fixed)
• Indexer Length = Catheter Length + Transporter Length

44. THE COMPUTER SYSTEM
• Introduced In The Early 1960s
• Modern developments in BT planning and delivery technology :
1. Computer Isodose Calculations,
2. Dwell Weight Optimization Of Single-stepping Source Remotely
Afterloaded Implants
3. Utilization Of 3D Imaging To Define Target Volumes , Guide Applicator
Insertion.

45. STEPPING SOURCE DOSIMETRY SYSTEM
• System to optimize implants with needles or flexible catheters with an HDR source stepping through them.
• Use of increased DWELL TIME at the longitudinal ends
• Keep the active dwell positions inside the target volume.
• Reduce dwell time in central part of the implant to increase the dose homogeneity across target.
SSDS
PDS

46. STEPS IN PLANNING
1. Catheter Reconstruction
2. Activation
3. Normalization
4. Dose Prescription
5. Optimization
6. Plan Evaluation
IMAGE RECONSTRUCTION

47. CATHETER RECONSTRUCTION
Numbering the catheters
Left to Right ,
Deep to Superficial

48. DELINEATION OF TARGET VOLUME

49. ACTIVATION

50. CREATION OF BASAL POINTS

51. NORMALIZATION

52. DOSE
PRESCRIPTION

53. OPTIMIZATION
Optimization allows the active-
to-target length (AL/TL) ratio to
be reduced from 1.33–1.5 to 1.1–
1.25.
DWELL POSITION
DWELL TIME
MANUAL
GRAPHICAL – LOCAL
GLOBAL

54. PLAN EVALUATION
• Visual evaluation of the isodose distribution –
• Dose coverage
• Homogeneity
• Location of hot/ cold spots if any
• Target volume-oriented parameters:
• Dose that covers 90% of Target Volume (D90)
• Percentage of Target Volume receiving at least the prescribed dose (V100)
• Percentage of Target Volume receiving 1.5 times the prescribed dose (V150)

55. • Dose Homogeneity index (DHI)
• V(100%)-V(150%) / V(100%)
• DHI >0.75
• Conformality Index (COIN):
• Measure of the conformity of the plan, taking into account both the target
volume, critical organs and normal tissue.
• External volume index (EI):
• This is a fraction of the normal tissue receiving a dose at least equal to the
prescribed dose of the target volume.
• Sum index (SI):
• This index is equal to the weighted sum of V100, V200, DHI and EI.

56. LIMITATIONS OF ISBT
1. Cannot be performed in inaccessible areas
2. Require skill and expertise
3. Limited for well localized tumors
4. Limited for small lesions
5. Invasive procedure, require GA, may not be executed if multiple comorbidities
6. Labor intensive
7. Higher dose inhomogeneity
8. Greater conformation –small errors in placement of sources lead to extreme changes from
the intended dose distribution

57. TEMPLATE BASED IMPLANT
Boost Implant done after
completion of external
beam RT
MUPIT