Special Techniques Flashcards
Describe a suitable radiation therapy technique and dose fractionation schedule for TBI. Justify your answer
DOSE PRESCRIPTION
* 12Gy in 6 fractions, 2 Gy per fraction, 2 fractions per day, over 3 days
* Prescribed to a single point at midline of the patient (usually umbilicus)
* (Other dose options: 2Gy/1#, 13.2Gy/11#, 4Gy/2#)
Pre-SIM
* MDM discussion
* Fertility preservation referral
SIMULATION
* Position: supine, upper arms on side resting on 4cm polystyrene (maximise lung shielding from lateral beam),
and hand resting on abdomen
* Planning CT: 3mm slice covering the entire body length (from vertex to mid-thigh)
TARGET VOLUME
* Entire body contour
TECHNIQUE
* 4-field equally weighted MV photons AP/PA and opposing lateral, with extended SSD (4m) and largest practical
field size
* 6/10/18 MV (avoid 18MV if possible)
Alternate APPA and oppose lateral for each fraction (e.g. APPA for fraction 1, 3, 5, lateral for fraction 2, 4, 6)
Lateral field Position:
* patient lie supine, small sponge under head, knee fix,
* Upper arm resting on 4cm polystyrene (to reduce lung dose)
* Hand resting on abdomen
* Trolley turn around for the opposing lateral field treatment
Compensator/ bolus (‘beam spoiler’) – (because of skin-sparing effect of photon)
* Super-flab – on lateral and anterior surface of neck + chest (to reduce dose to lung)
* Perspex – as head frame compensator, and from mid-thigh inferiorly (thicker from mid knee
inferiorly)
AP/PA field Position:
As above but:
* patient lie on side, head sponge under head, patient’s torso support with pillow (on beam exit side),
towel and pillow between legs
* Patient facing linac head for AP, and turned around, facing away from linac head for PA
Verification:
Physics QA prior to treatment
- Light field – TBI laser run along the incident surface of patient’s pelvis, coverage of whole body within the light
field
- TLD for dose verification on each body site of interest
Indications for TSET
Dose for TSET and benefit:
Mycosis Fungiodes or Sezary Syndrome, typically where symptomatic, topical therapies offering limited control and >10% Total body area.
All data limited to phase II at most.
Classically 30-36Gy over 10weeks - >90% response, 80% complete response. 50% relapse 1 year - tox - permanent allopecia, temp finger/toenail loss,infertility (unless shield).
12Gy (more modern). Much less tox. 88% respond, 25% complete response, 6 years of clinical benefit.
TSET dose technique:
12Gy/6#s over 3 weeks, 2#s/week. 6Mev prescribed to approx 5mm depth, SSD 4Meters, Stanford Technique = 6 dual fields (pointing to upper and lower body matched at umbilicus), 6 positions circling the body separated by 60deg. Pt standing, elevated off floor to avoid floor scatter. 1cm acrylic scatterer.
Eye-shield, finger/toenail
Boost to shadowed areas (e.g. the taint and the underboobs)
HDR prostate
Vault brachy for endometrial:
Which other treatment approaches may be used to achieve the same benefit and conventionally fractionated TBI? What are the advantages and disadvantages of these approaches? (3.5)
1) Systemic therapy/additional chemo (e.g cyclclophosphamide or mephalin for Myeloma). Advantages: More available than TBI, avoid acute and late toxicities of radiation. Treat pts with contraindications to RT (e.g. connective tissue disorders). Disadvantages: Increased side effects (including death) from further cytotoxic therapy, systemic therapy may not penetrate sanctuary sites as well as XRT and may not overcome chemo resistant cells.
2) Lower dose TBI – e.g. 2Gy/1#, 4Gy/2 – Advantage: May achieve acceptable immune suppression/suff while sparing significant toxicity. Disadvantages: May not achieve tumour cell elimination.
Dose and technique for adj Brachytherapy component of definitive Cervix:
24gy/3#s, 3days apart, 2#s/week. Timed with final 2 weeks of RT (aim OTT <49 days), no chemo on brachy days. HRCTV D90 to receive 100% of prescribed dose.
Pre-treatment
- Anaesthetic review
Applicator insertion
- Under GA
- Lithotomy position
- Examination under anaesthesia (EUA): to assessment of tumour response (from external beam radiotherapy),
cervix and fornix anatomy (for selection of appropriate applicator/ ovoid size)
- Prep and drape
- 18F 3-way IDC catheter inserted, 7-10ml balloon, bladder filled to 300-400mL normal saline
- Uterus sounded to determine length of tandem applicator
- Cervical os dilated
- Insert applicator (tandem and 2x ovoid) under ultrasound guidance
- Vaginal packing with gauze to hold applicator in situ
- Position checked with ultrasound
Planning Imaging: MRI pelvis performed with applicator in situ Target volume
- GTV = macroscopic tumour (based on EUA at applicator insertion and MRI)
- HR-CTV = GTV + whole cervix
- IR-CTV = HR-CTV + 1cm expansion Plan review
- Aim HR-CTV D90 of EQD2 85-90Gy with / of 10 (with assuming the entire volume received 45Gy in 25
fraction on EBRT)
- Aim IR-CTV EQD2 65-70Gy
- Review OAR dosimetry
o Bladder D2cc<90Gy
o Rectum/ sigmoid/ bowel D2cc<75Gy Treatment delivery - Treatment delivery in bunker, with patient in supine position
- Treat with Iridium-192 radiation source
- Applicator removed after each brachytherapy fraction
In general terms, list the steps in treatment planning LDR prostate implant (3 marks)
Planning:
- Based on TRUS volume
- Contouring
o CTV = prostate; PTV = CTV + 3mm (except 0mm posterior)
o Urethra and rectum
- Placement of seed ~ 100 through the prostates
- Review DVH
o PTV: V100>98%, V150% ~50%, V200%~ 13-18% , D100%>95Gy
o Urethra: D10<210Gy, no 150% isodose line encircling/ in urethra
o Rectum: V100<1cc, 70% isodose line away from anterior rectal wall
- Seed ordered from overseas
In general terms, list the steps in treatment that are involved with delivering an LDR prostate implant (3 marks)
- GA, lithotomy position
- Prep and drape
- IDC and injection of ~120mL mixed normal saline/ contrast
- TRUS probe per-rectum, ensure image closely resembles volume study
- Interstitial needle inserted using template and under ultrasound guidance, and position
checked - Stylet (with radioactive seed) deployed
- Interstitial needles removed, leaving stylet (with radioactive seed) in situ
- TRUS probe removed
- Cystoscopy – to check bladder and urethra of any seed, trauma, obstruction
- 2 weeks post CT check for seed migration
Stg II endometrial Ca what get brachy? outline the technique.
If Stromal involvement: 10Gy/2#, 1-2#s/week not on chemo days. Aim towards end of RT.
Vaginal vault brachytherapy
* Position: supine, Examination for sizing and any EBRT toxicity. Select largest vaginal cylinder that will fit comfortably to minimise air pocket. Fix in place with support arm and details of cylinder and depth recorded.
* Target volume: upper half of vagina.
* Select plan from plan library
* Technique: HDR brachytherapy with Iridium-192 radioactive source, prescribed to 5mm from surface.
OARS EQD2: Bladder D2cc<90Gy, rectum/bowel/sigmoid < 75Gy
Systemic radiation for metastatic prostate cancer:
Name the studies and benefit, describe the agent
1) Radium-223 (Parker Study). Alpha emitter. Increases OS by 3 months
2) Lutetium-177-PSMA-617 (VISION). beta-gamma increases OS by 3 months.
Pre-LDR brachy considerations:
LDR goals:
- “I would perform a volume Study to determine: Pubic arch interference, prostate size.
- Previous TURP - large urethral defect prevent goals being achieved.
- Urine flow study.
Goals:
D100>98% PD
D50 > 150%
V200% aim 10%
Urethra not encased by 150% isodose.
Describe the Stanford technique:
The Stanford technique has the patient assume 6 standing poses at 60° increments: anterior, posterior, right anterior oblique, right posterior oblique, left anterior oblique, and left posterior oblique At each angle, 2 fields are treated, one for the upper body and a second for the lower body, resulting in a total of 12 fields. A thin polycarbonate scattering panel was used at approximately 212 cm from the isocenter. A 6-MeV
