Procedure for prostate brachy Flashcards
pre-US patient prep
patient under anesthesia patient positioned with legs on stirrups catheter in rectal suctioning disinfectant
distance between holes in template
5 mm
sterile area
cannot touch anyone or anything within this region
virtual plan in oncentra
put on rough contours that encompass entire area where doctor puts needles
- needles are assigned to grid positions in TPS according to where RO wants to put the needles
- o Virtual planning may be used to investigate different needle arrangements by checking which give the best dose distribution when inverse optimization is performed.
Once needle placement is finalized, remove obturators and acquire final image of the prostate
how does doctor load needles
peripheral (outside on top 2 rows)
remaining rows- put needles in middle
skip a row
another row on bottom
too few needles- have to fire source for too long and get hot spots
too many needles - shadows from needles can make it difficult to see all of them (this is also why we load from top to bottom so doctor can see that
measures distance from template to ring
template position in oncentra = 193 - (template to ring)
base vs apex of prostate
base = sup portion apex = inf portion
how does the US give a 3D image?
- US acquires single axial plane at a time
- pull US out and acquire whole time to get 3D image
prostate PTV margin
- only to allow souce to fire just outside prostate, not trying to cover the PTV
- 3 mm in each direction except post (rectum) and sup (bladder)
recording needle positions for Oncentra
doctor reads out positions of each needled
later therapists measure free lengths (2 therapists measure and if they disagree a third confirms)
knowledge of total needle length is used to determine lenght of needle inside patient. Software will update needle contours once free needle lengths are entered
o In order to have the measured free length correspond to a location on the US image, must also measure distance from the outer surface of the template to a reference point on the probe. Knowing the distance from this reference point to the base plane (the plane of the transverse crystals) allows distance from the template to the base plane to be known; and from this the needle tip can be assigned to a particular location in the US image if the free length is known.
needles in Urethra?
normally no
want 100% of dose to urethra, no more (urinary retention)
what does physician do with the needles once Oncentra is switched to live plan?
pushes in needles further to base of prosate
how far do we want the needles to extend?
Needles have 1 cm deadspace at end
Want them to extend 1 cm beyond prostate (tough to do at sup end as run into bladder)
do we use plastic or metal needles
metal
-metal adds rigidity but plastic is more flexible (less predictable but can shape it a bit more)
what contours does the doctor draw in Oncentra
prostate (apex and base most important) - doc remembers where they put needles
- urethra
- rectum
US slice thickness
1 mm
issue with large prostate
more risk of urinary retention
pubic arch interference - appears as black triangles at edges of US field (pubic arch blocking prostate)
advantage of brachy vs just EBRT
- less dose to rectum compared to EBRT, thus can deliver higher dose
- don’t have PTV
- US probe actually creates space between rectum and prostate
what % of prostate patients get brachy
20%
what is done to needles in Oncentra?
reconstructed to line up with actual needle positions in all slices
3-4 pts per needle
-if needle at different positions on different sliced, go with midpoint
rectum constraint
V80% < 500 mm^3
target requirements
V90 ~ 100%
V200 < 11%
V150 < 33 -35%
V100 > 95%
How can you cool or warm up hot/cold spots
change local distribution function
pts to choose for RadCalc
10 pts, 2 planes
overview of steps in prostate brachy
acquire images
tell system where needles are
reconstruct needles
inverse optimize (IPSA)
when is dose not 15 Gy?
patient has IBS or collitis - patient gets 2 brachy sessions instead
gyne vs prostate step size
5 mm vs 2.5 mm
agreement between source calibration and activity
+/- 5 %
needles used at NSHA
1.9 mm trocar
with obturator inside to give rigidity when putting in the needles
what is checked during physics 2nd check
o Checking patient identity and other input data (e.g., correct source model, afterloader, step size, template, catheter type, indexer length)
o Correct US probe is used
o For prostate: tip to first dwell = 1 cm, free length agrees with values written down
o Checking that the correct applicator is used in the correct location
o Check that catheter reconstruction is correct and starts at the appropriate end; no bent needles etc
o Checking that the date and time are correct (check that source strength agrees with printed out table within 0.5%)
o Checking that the correct optimization process was used
o Correct prescription, F-factor
o Verifying that the doses at various points agree with an independent dose calculation within expected tolerance (e.g., RadCalc, 5 %)
o Checking that the treatment plan agrees with policies of treatment for patient’s disease (i.e., does this treatment make sense given patient disease)
o Acceptable DVH
o Checking that plan objectives were adequately satisfied
o Ensuring that there is a record of treatment.
o Care path updated
o ARIA EMR documents approved.
can you put a RadCalc point in a needle?
No, results won’t agree
what does physics do with therapists regarding after-loader tubes?
ensures number goes into right location
therapists survey afterloader to make sure source is inside
radius of urethra
3.5 mm
what does oncentra do to holes within proximity of urethra?
blocks them- cannot put needle here unless you free them
when is patient surveyed?
survey patient before and after treatment
medical prep for patient
general anasthetic
dorsal lithotomy position
foley catheter in urethra (to collect urine and also make urethra visible)
how is US and template kept stable?
• US is fixed relative to the patient bed, and the template is rigidly fixed to the US equipment. In some centres, the template may be sutured transperineally.
-a grid is superimposed on each US image slice to represent template coordinates
How does IPSA work?
inverse optimization simulated annealing
changes dwell positions and times iteratively
QA done by therapists on the plan before treatment
o They check that the information on the TCS agrees with the report created by the physicists (e.g., spot checking dwell times).
o They also verify patient information (name, ID)
what is dummy source used for?
Thicker than actual source
Checks that there aren’t any blockages for any of the needles or tubes (checks all positions) before whole treatment
Also checks indepedent needles before the source goes into that particular needle
How is the 3D US image created?
scan probe longitudinally longitudinally (manually – must go very slow to avoid lag in software due to frame grabber and image refresh rate) or by rotating the probe (can be done automatically – software automatically corrects for shift due to lag since rotation is at a constant speed; however, this method is limited to prostates small enough to fit in FOV)
what speed of sound does US assume?
1540 m/s
how to get speed of sound of 1540 m/s in water?
add 43 g to 1 L of water
where do you avoid placing needles?
under urethra as it makes it hard to view and contour urethra
- avoid plaing in grid locations that are directly adjacent (uncecessary and may result in crossed needles)
- avoid needles that block other needles - look at radial path in transverse view
urethra constraint
Dmax < 125%
D10%< 118%
permanent implant prostate brachy dose
125-145 Gy (monotherapy), 125 Gy with Pd-103, 145 Gy with I-125
If with 40-50 Gy EBRT, then 90 Gy Pd-103 or 110 Gy I-125
-I-125 dose rate is 7 cGy/h
dose for prostate HDR with EBRT
15Gy/1
9.5 Gy/2
constraints and objectives for permanent implant prostate PDR
CTV: V100%> 95% so D90% will be > 100%
CTV: V150% < 50%
rectum: D2cc < Rx, D0.1cc < 150%
urethra: D10< 150%, D30<130%
Why is Pd-103 dose lower than I-125?
lower energy (and therefore increased biological effectiveness) and shorter half-life (less opportunity for normal tissue repair)
The Pd-103 dose rate [due to its shorter half-life] as well as its relative biological effectiveness (RBE) [due to shorter half-life/higher dose rate less chance for tissues to recover and lower energy higher LET higher RBE] is higher than that of I-125. Thus, the prescription dose for Pd-103 seeds is lower than the recommended dose for I-125 seed implants, 125 and 144 Gy, respectively. The value corresponding to 80% of the dose deposited by I-125 seeds is delivered in 140 days, while the Pd-103 seeds deliver such value in 39 days.
Short half-life is particularly important for cancer strains with a high proliferation index.
homogeneity of plans with I-125 vs Pd-103
Because of their lower energies, Pd-103 photons have a lower range in the tissue than those emitted by I-125. Thus, for the same distribution of seeds, the spatial dose distribution obtained by Pd-103 tends to be less homogeneous that that obtained with I-125. On the other hand, and for the same reason, Pd-103 seeds will produce lower absorbed doses at longer distances, particularly in the surrounding healthy tissues [also it is easier to protect medical personnel from a radiation safety point of view].
How is LDR prostate usually done?
US done of patient
plan made, seeds ordered
US of patient- put in seeds- can adjust positions but generally don’t replan
-some studies may MRI or CT patient afterwards to assess seeds based on prostate swelling etc over time
difference between LDR and HDR plans
LDR is more densely packed because it is a lower energy source
HDR distribution is better as you can spare the structures better, LDR has inherent randomness
good trick when graphically cooling hot spot
adjust on slice where dwell position is
US mode used for prostate
B mode
Brightness mode
Commonly referred to as B (brightness) mode, the use of grey scale imaging in ultrasound renders a two-dimensional image in which the organs and tissues of interest are depicted as points of variable brightness. The formation of a B-mode image relies on the pulse-echo principle; assuming the speed of sound remains constant, the position of a target of interest may be inferred by the time taken from emission to its return to the transducer. In order to construct a cross-sectional image, the pulse-echo sequences from a multitude of neighboring scan lines are sequentially summated in real-time, generating a moving image
US modes
A-mode: A-mode is the simplest type of ultrasound. A single transducer scans a line through the body with the echoes plotted on screen as a function of depth. Therapeutic ultrasound aimed at a specific tumor or calculus is also A-mode, to allow for pinpoint accurate focus of the destructive wave energy.
B-mode: In B-mode ultrasound, a linear array of transducers simultaneously scans a plane through the body that can be viewed as a two-dimensional image on screen.
M-mode: M stands for motion. In m-mode a rapid sequence of B-mode scans whose images follow each other in sequence on screen enables doctors to see and measure range of motion, as the organ boundaries that produce reflections move relative to the probe.
what is doppler mode in US?
employ the Doppler effect to assess whether structures (usually blood) are moving towards or away from the probe, and its relative velocity. By calculating the frequency shift of a particular sample volume, for example a jet of blood flow over a heart valve, its speed and direction can be determined and visualized.
what does pubic bone do?
pushes needles down
what “determines” the urethral dose?
2 ant needles at apex
how can you use TRUS to manipulate the prostate shape?
fill balloon with water to push it around
what does physician check during TRUS before deciding if a ptient can have brachy?
- prostate doesn’t protrude into bladder
- prostate size < 55 cm^3
- no pubic arch interference
patient is in same position as for treatment (dorsal lothotomy)
yellow boots
fins
why is D10% < 118% for urethra a bit of a silly constraint?
depends on how far out you contour the urethra
frequency of US
6-12 MHz
here we use 12 MHz
stranded seeds
in permanent prostate
prevents seed migration
ribbon of seeds
what does template do?
provides a consistent, structured grid for implanting needles
DVH for prostate brachy
remember the point 0,100 is on ALL DVHs
then plot 90, 100
100, 95
make sure drop off from 100 is not too fast or it is a bad plan!
put 150, 33
200, 10
in brachy, doses theoretically extend to infinity
stepper encoder test
tests integrity of stepper- bypasses TPS so that if steps are not appearing in Oncentra we can verify if it is due to communication with TPS or the stepper itself
who is not a candidate for permanent prostate brachy?
also for HDR prostate brachy
patients who cannot undergo anasthesia
patients with mets
limited life expectancy
lack of rectum (cannot do TRUS)
-patients with high risk of urinary irritative and obstrutive symptoms are at higher risk of urinary retention after brachy
Should be considered but are not deal breakers:
- prostate > 60 cc
- inflammatory bowel disease
- previous pelvic radiotherapy (especially if > 50 Gy)
- prior TURP (transurethral resection of the prostate)
- pubic arch interference
low risk, intermediate risk and high risk
low risk: gleason = 6, PSA < 10 ng/mL and tumor classification T1, T2a
intermediate: gleason 7, PSA between 10 and 20, T2b, T2c
high: geason > 7, PSA > 20, T3a, T3b
treatment for low, intermediate, and high risk
low: brachy alone, no EBRT or androgen deprivation
intermediate: brachy alone or with ABRT, maybe androgen.
high: brachy and EBRT, androgen depravation is favoured
CT vs MRI vst TRUS for pre-implant peranent proatate planning
MRI can be used for pre-treatment planning
CT is less reproducible than TRUS
lingo for the way prostate is loaded
modified peripheral or modified uniform
what is somtimes used in addition to TRUS for seeing the seeds being implanted?
fluoro
types of seeds
stranded and loose
dose for permanent prostate brachy if combined with EBR
I-125
41.4-504 Gy (1.8 Gy fx) and 108-110 Gy brachy
Pf-103
41.4- 50.4 Gy EBRT and 90-100 Gy brachy
may prescribe hihger dose 9upw9
why is a higher permanemt orostate dose sometimes prescribed? (up too 200 Gy)
to compensate for edema
usually toxicity is still tolrated
seed activity
0.2-0.4 mCi for I-125
1-2 mCi for Pd-103
when is permanent seed brachy done- before or after EBRT?
unknown what is better
doing brachy before may lead to more toxicity (exposure to both treatments at same time) but also allows you to vary EBRT depending on response of cancer to the brachy
when should post-operative CT dosimetry be performed for seed prostate patients
within 60 days of implant
optimal is 16 days for Pd and 30 days for I125
ABS dose constraints for seed prostate
urethra V5 < 150 %
urethre V30 < 125 %
rectal V100 < 1 cc
is HDR brachy used as monotherapy in prostate?
yes, for early and intermediate risk prostate cancer
HDR prostate doses
all over the map 9-15 Gy in 1 fx to 26 Gy in 4 fractions with many intermediary fractionation protocols
absolute contraindications for HDR prostate brachy
rectal fistula
cannot handle anasthesia
no proof og malignancy…
examples of monotherapy HDR brachy doses
- 5Gy X3
9. 5 Gy X 4
toxicities from brachy
erectile dysfunction
rectal irritation
urinary irritation
rectal fistula is rare
is brachy used in salvage therapyt and what is salvage therapy
yes it is
salvage: given after cancer doesn’t respond to standard therapy
who is eligible for SABR prostate with fiducials?
low-intermediate risk
-36.25 Gy/5 to PTV, 40Gy/5 to CTV, on alternate days
-if patient has hip prosthesis, there would be too much artifact in the CT
