Brachytherapy Notes

Figure below taken from the book chapter “Isotopes and delivery systems for brachytherapy” by Anthony Flynn, additional tables from Podgorsak (IAEA) and Cember
Ruthenium-106 is a beta emitter (hence it is packaged in a small plastic container inside a lead pig). It nominally has a half-life of 369 days and emits beta particles with maximum energy 3.53 MeV.

In reality it has a half-life of 369 days but decays into Rhodium-106 by emission of a beta particle with maximum energy of just 39 keV. Rhodium-106, in turn, has a half life of just 30.4 seconds and decays into Paladium-106 by emission of 3.53 MeV beta particles. Given the brief half-life of Rh-106,, Ru-106 can be considered, overall, as emitting betas up to 3.53 MeV and having a half life of 369 days.

Radial Dose Function for Ir-192

- See spreadsheet and TG-43

HDR vs LDR

LDR if for doses in the range 0.4 Gy/hr to 2 Gy/hr
MDR is for doses in the range 2 G/hr to 12 Gy/hr
HDR is for > 12 Gy/hr (ie 1 Gy per 5 mins)

HDR at the MGH uses Ir-192 with a starting activity on the order of 10 Ci for a new source
The Air KERMA rate constant for Ir-192 is about 108
A 10 Ci source is
Thus the Air Kerma rate at 1 cm would be

This is significantly higher than the definition of HDR, thus the MGH is definitely HDR when using Ir-192 wires

HDR offers the advantage of not having permanent implants

Permanent implants are not good for ALARA
Patient should not to be near pregnant women for a long time
Patient should wear condom during sex

Probably HDR is less expensive as source can be reused multiple times
HDR may have worse toxicity to OARs, since Ir-192 is more penetrating than I-125 or Pd-103
LDR may be superior radiobiologically

Longer protracted treatments may be better in terms of toxicity for OARs
Fractionation is the solution

Note:

The only difference between LDR and HDR is the dose rate, otherwise dose rate distributions are exactly the same for the same geometry and source type

Brachytherapy Dummy Wires

Used to see distances in radiographs
At the MGH, the following are used

GYN - see photo below

Colpostats: 1 cm apart dummies
Tandem: 2 cm apart dummies

Esophagus: 1 cm apart dummies
Multiple catheter lung: various different bead distances used to distinguish between the different catheters

Radiation Safety in Brachytherapy

Emergency stop button locations

At and on console, inside door and on walls of suite

Radiation survey meters and alerts

On walls and hand-held

Emergency equipment

Suture kit, wire cutters, pig

In the event of an emergency

Be sure it is an emergency
Press interrupt button
Press emergency stop buttons one at a time from outside room to inside
Enter room with pocket dosimeter
Go for gold - crank the gold hand crank on the afterloader
If doesn’t come out, pull out catheter manually and place in pig
Remove patient
Survey the room
Notify RSO
Notify Nucleotron
Close off room

A NEW can be 1 m from the source for 1 hour before reaching 50 mSv annual limit
Risk to patient is much higher since the source is inside them - remove it as fast as possible

Brachytherapy Treatments

Site	Prescription	Notes
Cervix	3 fractions of 8 Gy each with EBRT (typically 45 Gy with EBRT)	See paper by Souhami et al 2005
Prostate	10 Gy in 1 fraction + 50 Gy in 20 fractions EBRT
Lung
Esophagus
Eye (Choroidal melenoma, squamous cell carcinoma of the conjunctiva)	- Choroidal melenoma - 85 Gy - SCC of conjunctiva - 100 Gy	Eye plaque - prescription doses higher than for EBRT since better OAR sparing (eg 60 Gy in 10 fractions for SRS)

GYN Cases

See Guidelines by the ABS
Ir-192 HDR intracavitary treatment using tandem and colpostats
Prescription is to point A

Point A is situated 2 cm laterally from the uterine canal and 2 cm above the lateral fornix, as shown in figure below (from Johns and Cunningham).
From talking to William, point A is a point in reference to the applicator - 2 cm up from the cervical oss, which is represented by the flange, and 2 cm out laterally.
There are technically two point As but they can be considered as just one since they should both receive the same dose. They may not receive the same dose if the setup for some reason is asymmetric

OARs are on parametrial wall, represented by point B

Point B is with reference to the anatomy, (in contrast to point A, which is in reference to the applicator)

ICRU points are on the rectum and bladder walls

Rectum point - 5 mm posterior to the vagina packing on a transverse slice
Bladder point - at the most posterior part of the foley, which is inside the bladder
If there are areas near the bladder and rectum points that are hotter than the points themselves, then move the points there - to error on the side of caution
Typically the rectum and bladder get around 4 Gy per 8 Gy to point A

In cases where colpostats are used without a tandem, the prescription may be to point X, which is a point 2 cm superior along an imaginary tandem from the point of intersection of the colpostats and an imaginary tandem placed centrally between them. See the figure below

From this figure it is clear that the bladder is anterior to the tandem and colpostats (which are inserted into the vagina) and the rectum is posterior to them.

Tandem and colpostats along with packing (to move bladder and rectum out of the way) are inserted by physician and act as applicators for the HDR brachy source

The tandem and colpostats are designed to bring the brachy source to the correct point for delivery of the pear-shaped radiation dose - see picture below

After their insertion, the tandem and colpostats are clamped to treatment couch for immobilization.

The ovoids of the colpostats provide the correct geometry for the pear-shaped dose distribution that is to be delivered.

Patient gets AP and Lat kV radiographs and a CBCT

The right side of the patient is marked with a CT marker

Generally right and left are not an issue unless for some reason the applicator didn’t get inserted symmetrically - when the setup is assymetric computerised planning, rather than manual planning, should be done.

Radiographs are used only for manual calculations
CBCT is used for post-treatment (or pre-treatment) computerised planning with Oncentra

Manual (tables) versus Computerized Planning

Manually planning uses tables that trace their origins to source trains of Co-60 pellets that were used when HDR was first implemented at the MUHC. Then, the source trains were prepared in advance and inserted into the patient as a single piece - thus all dwell positions had the same dwell time. The dose distributions were nice and so today manual planning uses similar distributions even though it involves a single source being stepped through each of the dwell positions.
Manual planning is good in that it is fast and the patient can be treated quickly
Manual planning should not be used when the setup is asymmetric or when the physician wants to deviate from the setup that the tables were designed for - see figure below for good and bad setups

Tandem and Colpostat Cases

Picture of tandem and colpostats shown below
The tandem goes through the flange and the colpostats go out to the level of the flange
The flange sits against the cervical os, which is the circular opening from the vagina into the cervix.
The flange helps steady the setup and it is also useful for visually determining the position of the cervical os (and hence points A and B) on the radiographs and CBCT

Photo of tandem and colpostats - tandem goes into the cervix, colpostats with their ovoids stay in the vagina. Notched positions at 1 cm apart are visible on the tandem. The dummy markers inserted into the tandem, however, are 2 cm apart.

Need to be careful when going from dummy positions (2 cm apart in tandem, 1 cm apart in colpostats) to source dwell positions, which are 2.5 mm apart. Note: the tandem used here is just less than 6 cm long, being the distance from the tip to the flange as evidenced by the dummies.

This figure shows the loading as per the table. The dummy and actual source positions are shown. Dummies are 2 cm apart for the tandem and 1 cm apart for the colpostats. The source positions are 2.5 mm apart for both tandem and colpostats.

The loading should be done to the flange as shown here - this means that the length of tandem to choose in the table corresponds to the distance from the tip to the flange, in this case 6 cm. If unsure of the flange position in the lateral view, check it out on the AP view

AP radiograph of the tandem and colpostats inserted into the vagina and uterus. The packing is clearly visible in this photograph - packing is used to increase the separation of the source from the posterior bladder and anterior rectal walls.

Manual Calculation

Use standard gynecological loadings to obtain a dose rate at the prescription points (A) as previously calculated by Horacio

Need to confirm which tandem is being used (physician will say which he/she inserted but physicist should double-check by examining the radiographs).
The distance to measure is from the tip to the flange - each dummy position is 2 cm and each source position is 2.5 mm (or 0.25 cm)

Distance (cm)	Dwell position	Dummy Number
0	1	1
0.25	2
0.5	3
0.75	4
1	5
1.25	6
1.5	7
1.75	8
2	9	2
2.25	10
2.5	11
2.75	12
3	13
3.25	14
3.5	15
3.75	16
4	17	3
4.25	18
4.5	19
4.75	20
5	21
5.25	22
5.5	23
5.75	24
6	25	4
6.25	26
6.5	27
6.75	28
7	29
7.25	30
7.5	31
7.75	32

Three colpostat dwell positions are used, corresponding to the three locations nearest the intersection of the colpostats and the tandem, as seen on the lateral kV radiograph
The calculation boils down to taking the pre-determined dose rate, from the appropriate row in the tandem and colpostats table (in cGy/secCi) and using the prescription dose (in cGy) and the source activity (in Ci) to determine the dwell times (in sec) for input to the treatment delivery computer

The tandem and colpostats table lists tandem lengths along with the number of dwell positions and their locations and the expected resultant dose rate at the prescription point as a function of the source activity and equal dwell times at the dwell position

Example:

Give 8 Gy = 800 cGy to point A, for large colpostats and tandem of length 6 cm
From the table below, the dose rate at point A is 1.95 cGy/secCi
Activity at the time of insertion is 8.74 Ci (13 July 2011)
Need to determine time t
Use definition of dose rate

(units are cGy/secCi)

From the table, we see that for the 6 cm tandem, 7 dwell positions at locations 1, 7, 10, 13, 16, 19 and 22 should be loaded with dwell times of 46.94 seconds each
The source (ie dwell position) spacing is 2.5 mm giving a dose rate at point A of 1.95 cGy/secCi
Colpostats 3, 4 and 5 should also be loaded with dwell times of 46.94 seconds each (assuming they lie laterally across the tandem. If they do not, then the positions that do lie laterally across it should be loaded)

Notice that the tandem and colpostat source positions are 2.5 mm apart.

Tandem and Colpostat Treatment - Manual Calculation

Following the manual calculation, the treatment is setup on the treatment console computer

Essentially the dwell positions, as determined from the table for the tandem, and as appropriate for the colpostats, are set and given the determined dwell times

Following delivery of the treatment, a post-treatment 3D planning is performed on the CBCT using Oncentra

The purpose of the 3D post-planning is to determine the dose distribution that was actually delivered and to determine the dose that the organs at risk (rectum and bladder) actually received.
This information can then be factored into the external beam planning for the external beam remainder of the treatment

Manual treatment example (another example done already above)

Tandem and Colpostat Treatment - Oncentra Post-planning Example

Import the patient’s CBCT into Oncentra

Open Oncentra
Choose the patient and click the + to show the series available
Highlight the appropriate series and click Import

It should detect the patient ID and name

Click New Case
Write in a Case Label and click OK

Eg GYN 28JUNE2011

Click OK when it asks to open the case
Close the CM Import dialog
Do not click Close on the Non-Active Case window

Click on the red Brachy icon on the left hand side

Brings up the CBCT images

Click anywhere on an image to start a new plan

Label the plan as appropriate
Choose the machine, through a series of double clicks, and click finish

The right side of the patient’s body is marked using a CT marker

Note that the patient is imaged feet first supine

Eclipse will show the image as such but Oncentra will flip it to appear head first supine
Be careful of this by examining the CT marker location and by comparing Oncentra image with Eclipse image

Reconstruct the catheters

Click on Catheter Reconstruction
Click Remove All
Add three new catheters

Don’t add them together, rather add one, contour it and then add the others - avoids mix-up

1 and 2 will be the colpostats, 3 will be the tandem

1 will be the colpostat on the right-hand-side

Ie the side of the patient’s body containing the marker

For each catheter add the following information

Offset: -6.0 mm for the metalic (non-MRI) colpostats

See memo by B. Reniers 03 Jan 2008

Source Step: 2.5 mm
Start At: Tip End

Draw the catheters

Don’t need to draw on all slices
However, should start at tip end and work backwards
Keep the catheter points in order, to not go back a slice
Be careful with the Activity gadget

It allows the points to be drawn and it tends to be turned on by default - make sure it is off before clicking

Adjust the window and level as appropriate

Find the adjustment tools by clicking on the Toogle window icon

Zoom in using CRTL-scroll
Pan around using Shift-scroll

Activate the dwell positions

Click the Activation tab - if it is not lit up then check that all catheters are drawn, even catheters that will not be used (such as catheter 2 in tandem and ring)
For the appropriate tandem points, as per the manual calculation, click on the points to activate them. Remember 1 and 2 are the colpostats and 3 is the tandem

Add calculation points

For the prescription and the OARs (bladder and rectum)
Click on the green +

Choose Patient points

Add bladder point (using the little green and red pencil highlighted)

Go to the Foley and find the bottommost point on it, add the bladder point there
If, after the dose is calculated, it looks like there may be another point on the bladder with a hotter dose, the bladder point should be moved to it, rather than simply leaving it at the lowest Foley point
Add rectum point

Go to the axial slice where the tandem and colpostats cross - ie the slice where all three are in a horizontal line
Right click -> Measures -> Distance
Go 5 mm below the packing and add the rectum point
Again, as for bladder, err on the side of caution and make the rectum point hotter, if in doubt

Rename the points by clicking on Points in the list at bottom left and double-clicking on P1 and P2 to change them

Reset the coordinate system

Using toggle window somehow get to a right-handside panel that gives a tab called ECS Views
Click on the ECS Views tab
Drag the axial, sagittal and coronal views into the screen
In the sagittal view, drag the pink crosshairs to the flange

The flange is the ring point of the applicator at the cervixal os

Rotate the crosshairs until tangential to the tandem
Can check using coronal view and fix the rotation if necessary

Add the applicator (prescription) points

Go to the tandem slice on the coronal view
Turn on the cm grid from the view menu
Add points A1 and A2 2 cm superior and 2 cm left and right of the origin
Set the exact distance using the table at the bottom of the screen - ie 20 mm
Can re-adjust the coordinate system if needed such that the points A are symmetric about the tandem and the two colpostats lie along “x” axis.

Normalize the distribution

Click the normalization tab
Normalize to points -> applicator points
Should provide a dose distribution

Prescribe to points A

Click the prescription tab
Insert the prescribed dose in cGy (eg 800 cGy for 8 Gy)

Examine the planned dwell times and compare with the actual dwell times

In the explorer at the bottom of the screen, click on Applicator and choose Active Dwell Positions and look at the time shown in seconds
Be sure that the apparent source activity corresponds to the actual source activity on the date of the actual treatment
If it is different, then to compare the post-planned and actual treatments, the actual treatment time should be scaled by the ratio of the source activities

Say, the activity during the actual treatment was 5.12 Ci and the manually calculated treatment time was 80.1 sec per dwell position
Say, that the apparent activity in Oncentra, at the time of planning is 9.95 Ci, then, if the manual calculation had been done for the Oncentra activity it would have been given a duration
80.1 sec x (5.12 Ci/9.95 Ci) = 41.22 sec (remember, since the apparent source activity is greater the time of treatment will be less)

This compares very well with the calculated time of 42.7 sec per dwell position

Put the time that was actually delivered (or its equivalent scaled time) into the treatment planning system to see the actual dose value that was delivered to point A and to the OARs

Click on the Optimization tab and chose Manual dwell weights/times
Highlight all the dwell positions and change their times
Go to the Patient and Applicator points in the explorer and see what the actual dose values delivered were. Tell the physician as appropriate.

Butterfly Treatment Example

Two tandems

Draw both as a single applicator until they clearly separate

Used for treating disease that is confined to the uterus - ie no cervical involvement
No manual calculations done (yet) for this at the MUHC as tables not yet verified
Start at the tip end
Load every other dwell position
Physician should say how far down to load - eg 5 cm from the top - see figure

Physician should also say how far out from the applicator to prescribe - eg 2 cm (in which case, position prescription pt 2 cm out)

Vault Treatments

Two colpostats - no tandem
Used to treat disease which is confined to the cervix or situations in which the uterus has been removed.
Tables are provided
Can measure the separation between the colpostats on the radiograph to be sure that the correct tables have been used.

Tandem and Ring GYN Treatments

Manual calculation is similar to tandem and colpostats

Count distance along tandem to centre of ring to be sure of correct tandem to load

1 cm apart dummies

Ask physician which ring was used - small, medium or large

Just a few things to note for the post-planning

When starting the post-planning, prepare ECS views and do the catheter reconstruction in the ECS views, aligned along the tandem and perpendicular to the ring centered at the centre of the ring

Will be easier - just a few points for the tandem and a circle of points for the ring

Use offset of -6 mm as per the tandem and colpostats
The tandem goes into channel 3, the ring goes into channel 1
Need to create a short two-point catheter for channel 2, otherwise Oncentra will complain

It will not be loaded and can be drawn anywhere

The ring is at the same point as the flange, ie at the cervical os, and so it should be used when determining point A

Ring contoured in Oncentra

Prostate Cases

Physics Role at the CT Sim

Want to be sure of two things

The CT scan does not cut off before the catheters end

Both at the template end and at the tip end (should be able to see all the catheters as black air and then eventually no catheters, as move superiorly)

The catheters do not end within the dyed-out bladder

If they do it will be impossible to see the actual end of the catheters as the dye is too bright. The catheters should end before the bladder

Other things to check include: (from Devlin, on ipad)

Catheters deep enough to cover the PTV (ie prostate)
Foley pulled down toward bladder neck
End of all catheters included in scan
All fiducial markers and volumes of interest in scan
Identity of each catheter is clear - ie they don’t overlap or twist around one another

Checking for coverage

Important that all the catheters are well covering the prostate. If the edge of the prostate is too far from the catheters then in order to cover the edge of the prostate with the prescribed dose, a symmetrical increase in radiation will need to be given, such that in the opposite direction a large (and possibly detrimental to the urethra) dose will be given - see figure below

In order to give the prescribed dose to the edge, an equal dose will be given to the urethra for the setup shown here. If the sources are closer to the urethra than to the edge,the urethra will be overdosed relative to the edge.

Planning Notes

Export plan from Eclipse to Oncentra
Open plan in Oncentra
Turn off the body
Change the contrast as desired
Use 17 catheters
Offset is 0 for the comfort catheters
Use 5 mm spacing
Start at the connector end just below where the contours of the prostate begin
Note the start slice and use it for all
Contour to the tip end of each individual catheter

Contour just a few points on each catheter
At the tip end, if there is any hint of the catheter then contour it in
Don’t use the template (1, 2, 3,4 on left top to bottom, then 5, 6, 7, 8), 9 is by itself in the middle. (10, 11, 12, 13 to the right of 9, then 14, 15, 16, 17)

When finished contouring catheters, check them in the 3D view
Autoactivate (the A icon in the Activation tab)

Autoactivate within 3-4 mm of the CTV
Fill in any holes within the activated catheters

Length is 1240 mm - double check with physician/techs

Select all catheters and change the length

Do IPSA optimization

Click on IPSA icon
Load solution Postate_ID and fill in organs as appropriate
If names don’t correspond then change in Target definition

Change isodose lines

Under Plan - Define Isolines
Choose autospectrum - get red (100%) covering red (CTV)

Graphical optimisation

Under Optimisation tab - Graphical, all the way to local

Aim for complete coverage

Don’t want the 100% line going inside the prostate. It should cover it from the outside
Also want no hotspots within the CTV, so reduce the white where possible, without compromising coverage

Fix notifications

Generally make equal zero anything that is close to zero

Check the DVH

Want 100% of CTV volume to get 100% of the dose
Don’t want the prostate to be too hot, so no more than about 30% getting 1500 cGy
Want 1 cc rectum getting < 7 Gy
Keep the urethra to less than 1250 cGy

A small volume can go above this if needed

Putting Out an Oncentra Plan

Approve

Under Plan menu

Print out treatment printout

Under Plan menu
To paper and pdf

Take screenshot

Show 3 D views and DVH

Export plan to RadCalc

Browse the DICOM export folder on the desktop and move the RP1 file tot he desktop, then import it from the desktop into RadCalc
Do radcalc - should be within a few percent
Print radcalc report to pdf

Export plan to TCS

Make sure name correct

Import the following documents into ARIA

Treatment printout
Screenshot
RadCalc report

Superficial Lesions

Example - Penis - Oncentra Planning

Patient with penis cancer had three interstitial catheters inserted as per figure below

Catheters to remain in during course of treatment

Dose of 28 Gy in 8 fractions (3.5 Gy per fraction) prescribed
Patient CT’ed after catheters inserted, in position shown in figure above
Physician contoured, in Eclipse, the treatment volume (only) on CT slices
CT image loaded into Oncentra from Eclipse
CT projection planes in Oncentra re-orientated to be perpendicular to the direction of catheter travel, so as to better scroll and draw the contour the catheter
Catheter was loaded with dwell positions
Optimisation done in a box
Physician graphically adjusted the isodose lines to provide best coverage
Plan was approved, documents printed and plan sent to treatment planning system
Plan was checked at console and patient was treated

Example - Penis - Manual Calculation (ie sanity check)

Using John’s and Cunninghan’s tables (for Radium) as a sanity check on the total treatment time for the penis patient

Oncentra gave the following:

Total treatment time of 47.9 seconds
Source initial activity: 9.52 Ci

Source decay factor on date of treatment: 0.89
Source activity on date of treatment: 9.52 Ci x 0.89 = 8.47 Ci

Volume receiving the prescribed dose (from the DVH): 5.85 cm3
Treatment volume roughly rectangular, about 1.0 cm deep, or 0.5 cm either side of central plane - ie either side of catheters.

Can thus be imagined as a rectangular source of 1 cm thickness and 6 cm length (thus 6 cm2 area along the long dimension)

Looking at the table for planar interstitial implants in Johns and Cunningham gives 188 mghr of Radium for 10 Gy - treatment area of 6 cm2 and prescription distance of 0.5 cm

But, we want to give 3.5 Gy (per fraction)

So we have 188 mg.hr x = 65.8 mg hr of Radium for 3.5 Gy

Converting into Ci we have approximately 65.8 mCihr of Radium for 3.5 Gy (remember 1 g Radium = 1 Ci, so 1 mg Radium = 1 mCi)
But we are actually using Ir-192, so we need to convert from Radium to Ir-192 - use table 13.2 of Johns and Cunningham to get ratio of exposure rate constants

Ratio is 0.825/0.46 = 1.79 (using 0.46 for Ir-192 as it is the value that is accepted today according to Michael. Actually, according to the table by Cember, at the top of this document, it should be 0.48 )

We know that the ratio must be larger than 1, since when we multiply by this factor we should increase the treatment time
Iridium has a lower exposure rate constant than Radium, so it should require longer treatments, a little less than twice as long

Converting from Radium to Iridium gives

(65.8 mCihr)Ra x 1.79 = 118 mCihr for Iridium

Our source is 9.52 Ci
So our treatment time is = 0.012 hr = 44.62 seconds
This compares very well with the 47.9 seconds given by Oncentra (about 7% different, only!)

Top

Lung/Esophagus Cases

In an actual case, Dr. X wanted to treat a patient with 8 Gy in 1 fraction, using esophagus cylinder loading

Dr. X’s patient was actually a lung cancer patient that he chose to treat with a line brachy treatment

Here two examples are shown

First the standard example given in the brachy binder
Second, the actual case that Dr. X was treating

Dose Rate Formula

The Brachy Binder Example

The example from the brachy binder

The loading table from the brachy binder - note that the last source at each end is boosted by 8/3 times the treatment time calculated for the other dwell positions

Dr. X’s Actual Case - Lung Cancer Line Treatment

Diagnosis: Lung cancer
Prescription: 8 Gy in 1 fraction
Application: Esophagus cylinder loading - ie line treatment
Length of the treatment

Determined by the physician and marks placed on radiograph by the physician to indicate the start and end points of the treatment - ie the start and end of the actual loading of the sources. It is understood in this clinic that the sources are loaded from the start until the end - confirm with the physician if there is any doubt
Need to determine the dwell positions of the start and end points and determine from them the length to be treated

Prescription distance: 1 cm from the central axis
DosePerCi: Determine from the tables for the given length and prescription distance
Source activity on the day in question: 5.473 Ci

In this figure the start is at the tip, thus at 995 mm

Counting dummies back from the tip we have: 995, 985, 975, 965, 955, 945, 935

start end

length = 995 mm - 935 mm = 60 mm

Quantity	Value	Unit	Note
Date	1 Sept 2011
Patient	XXXX
Source Activity	5.473	Ci
Prescription Dose	800.0	cGy
Prescription Distance	1.0	cm
True catheter length from afterloader to tip	995	mm
Dummy separation	10	mm
Source loading start point	995	mm	from radiograph - starting at the tip
Source loading end point	935	mm	shown on radiograph - counting dummies back from tip
Catheter length to use in TCS	995	mm	Use the position of the furthest dwell position as the catheter length in the TCS. As such, the tip dwell position is at number 1 in the TCS and loading is 1,2,3, etc as opposed to having to figure out where in the centre of the catheter they should fall
Source loading length	60	mm	Distance
Number of sources to load, 5 mm spacing (from table)	13
Catheter length to use in TCS	995	mm	See note below
Dose rate per Ci (from table)	6.7
Dose rate at time of insertion			Basically the dose rate accounting for the true activity
Calculated dwell non-end positions		sec
Calculated dwell time end positions	= 58.2	sec	End positions are boosted by 8/3

Note: For the catheter length in the TCS, use the value of the start loading point, this tricks the TCS into thinking that the tip of the catheter is at the start point and so source loading can start at the tip in the TCS - ie 1, 2, 3, etc rather than loading dwell positions further back in the catheter in the TCS.

Basically, the loading starts at the furthest point and works its way back to the least furthest point. Thus the length of the catheter to put into the TCS is the furthest point.

Brachytherapy Catheters

Catheters for Interstitial Brachytherapy

Comfort catheters

Inner catheter holds the source and goes inside outer catheter that stays in patient

Easy to remove in an emergency - just pull the inner catheter out

Head and neck catheters

Just one catheter

Tricky in an emergency as tip button may not be easy to remove and cannot be pushed back through the tongue

Prostate catheters (needles)

Can be pulled out directly

Brachytherapy Problems and Issues

Dummy source gets stuck on its way out

The TCS error message will say where the source got stuck, examine it and think about it
Check that the correct catheter length was specified in the TCS

If a length too short was used the source will jam into the end and immediately return to the safe with an abort
This happened for an esophagus case where a length of 1500 mm was used when it should have been 995 mm. The dummy source jammed at length ~1000 mm and returned to the safe with an abort.

If the catheter goes into an applicator check that the connection is correct. Some catheters (eg those used for esophagus) must be inside the appropriate applicator or the source will get stuck at the connection

This was an issue when checking why the esophagus case was jamming. The catheter was laid out on the table and an attempt was made to run the source into it. It jammed at length about 450 mm, corresponding to the location of the connector.

Most common error is that the lock ring is not turned to lock the catheters in place at the afterloader and the procedure won’t proceed.

Figure: In this situation where the catheter is supposed to go into an applicator, the connector is designed such that the source will jam if the catheter is not inside the applicator

Brachytherapy Best Practise

Critical that the correct catheter length is used. No harm to double check.
Always good to have radiographs to be sure that things are correct and to document.

Radiographs are your friend!

Take good photographs of the setup. Especially important when identifying catheters on the CT for interstitial cases (head and neck, breast, sarcoma, etc).