Random Questions Flashcards
Prostate: what image guidance, what are you matching to?
Daily CBCT, then match to bone (pelvic bowl, not femurs as these can flip flop), and then to rectal/prostate interface (or if we have fiducials, match to these). If post-op, we have clips and can match to these.
USE CLIPBOX TO Make sure majority of target is within field of the original contours.
Define penumbra. Discuss the effects of penumbra and field size: does it increase or decrease . Does it increase or decrease with energy? SSD?
Penumbra increases with field size because of increased scatter from increased volume of irradiated tissue. Penumbra is distance from 50-90% IDL.
Penumbra also increases with energy and SSD (bc the deeper the PDD)
Does lung inhomogeneity cause larger or smaller penumbra?
A: larger penumbra, because in lower density the electron travel further.
-> this effect is greater with 18Mv photons. So for lung we treat with 6Mv.
If you have a lung tumor and the inhomogeneity correction is not turned on, what will happen to the dose to the outside of the tumor and the inside?
A) Inside = hotter because when the beam moves through air there is not as much attenuation as if it moved through water.
B) periphery = colder: build-up effect and build down effect!
When analyzing a PDD of photon beam passing through higher density material (such as titanium implant or through lung tumor), what happens?
A: Will have increased dose on the less dense tissue at the interface! and decreased dose on the more dense side.
ONCE AGAIN: these effects will increase with beam energy. DO NOT USE 18MV beam with prosthesis!!!! Effects will be much worse!
What are the issues in treating with a prosthesis?
1) If obese and treating with 4 field, avoid beams through lateral but this could be difficult with 6Mv because of large ant/post separation so you could get hotspots on the skin! Reduce this with IMRT or VMAT (with avoidance structure for prosthesis to prevent beams going through here). You also cannot use energy greater than 10Mv bc nutron production.
2) There will be increased dose to softtissue/bone adjacent to the the implant due to electron scatter. This can increase tox/ fracture risk
3) prosthesis can cause artifact on CT which prevents appropriate dosimetry.
What size applicator do you want to use for vaginal vault brachytherapy?
Use the largest possible cylinder because larger SSD means more homogenous dose to target due to inverse square law
Why do we create an eval_PTV 5mm cropped from skin?
PTV_eval is used to more accurately determine dose by accommodating for buildup (ie 5mm near surface will be underdosed).
Troubleshooting if clipbox is off? What do you do to determine if resim needed?
Trial setting the patient up again, take another CBCT.
If still off, try to coregister CBCT to CT sim and do dose calc to see how differences affect dose to OARs. If significant difference, then we resim and replan.
- IE head and neck: need new mask, DO NOT DO CT sim with CONTRAST BECAUSE LIKELY AKI. Need to resim fast because worse outcomes with tumor repopulation if not!
SBRT: how do we do image guidance?
IGRT: daily CBCT: match to TUMOR ie in lung, and then see if bone match. Couch shift to match clipbox tumor with PTV target, if have to move more than 1cm then get patient off table, reposition, and re-CBCT. Call MD.
A 30 year old woman with small children receives 100 mCi of 131I as treatment for thyroid cancer. What precautions are necessary during treatment? What factors determine when can she safely be discharged from hospital? What would you tell her about the contact with children after discharge?
- Avoid shared spaces
- Admit patient for 48 hours once given treatment
- Avoid contact with children for 2 weeks
- Patients should have greater than 3 months life expectancy.
Asked why dmax is deeper with increasing MV energy.
The secondary electrons (created when photons interact with tissue) are higher energy and thus travel further and take longer to reach their electronic equilibrium (where Dmax is)
(dmax 6mv 1.5cm vs 3cm)
What are the pros and cons of high energy (MV) vs low energy photons (KV)?
High energy: deeper penetration for deeper lesions but increased exit dose, need for bolus.
Low energy: 100% dose at skin, less exit dose, sharp penumbra but need specialized machine and risk of bone necrosis.
If treating with an electron beam, what beam modifiers will you use?
Will need a cone/applicator (for field size, either 6x6 or 10x10), a lead cutout to insert into the applicator to match the PTV + Penumbra, 1cm bolus, external tungsten eye shield (can create a lead cerrobend insert)
Ortho: for 100kv and 250kv:
a) 90% depth
b 50% depth
c) HVL
d) lead shielding thickness required
a) 90%: 6mm, 10mm
b) 50%: 24mm, 40mm (x4)
c) 3mmAl, 2mm Cu
d) 1mm, 2.5mm
