On call Flashcards
at the unit, patient contour doesn’t jive with body contour
-if body too big, dosimetry is ok (just air), but make sure body-generated structures like PTVeval are appropriate
-if body too small, dose distribution will be different
-investigate if setup issue, bowel prep issue etc., immobilization issue
3 P’s
check these for each patient
-pregnancy, pacemaker, previous tx
options with pacemakers/defibrillators
TG203
Consider:
-assess patient dependency (class 1 is most dependent, class 3 least dependent) and also risk category
-beam energy </= 10MV
-optimize beam geometry
-assess device functional before and after tx, monitor device if required per dependency protocol
-in vivo dosimetry (bolus on top of detector for buildup)
-Be aware of dose limits (use manufacturer limit or use 2 Gy)
-can consider moving pacemaker to other side if cannot get it out of beam path…
-protons and heavy ions also produce neuttrons; electrons produce very few
-MRI effects depend on field strength and imaging conditions
-stereo- higher dose/fraction but potentially less to device given high conformity
-what we measure is the dose to water (not to the actual device, which is typically SiO2)
-lower dose rates are preferred
on exam, don’t ever say yes to treating if machine is out of tolerance
-some people will only tune beam by < 1% with solid water, do full TG51 after hours
ways to assess energy
-profiler
-PDD10 in solid water
why would machine beam energy drift
-problem with target, FF, bend magnet issues (scrubbers that only allow elexctrons at a given energy to pass could be malfunctioning)
hip prosthesis
-consider MV image
-contour artifacts (streaks, or starvation)- assign HU value
-even if no obvious streaking in prostat,e likely HU value is wrong
-extent of error depends on how different the HU value is from what it should be
-don’t enter through prosthesis with beam
can we use TPS to assess dose to pacemaker?
-only if device is within 3 cm of tx field; or else calc is inccurate
-use specific models like Peridose or Tg36 to calculate dose to device > 3 cm out of field
-for dosed of 50-60 Gy photons, dose should be < 2 GY to device if it is > 5 cm from beam edge, 7 cm for IMRT- use 10 cm for rule of thumb- determine dose with in vivo dosimetry if device 3-10 cm from beam edge
-if non-coplanar beams, use in vivo dosimetry even if d>10 cm
what can be used to measure dose to pacemaker
-TLD, OSLD, film, diode, MOSFET, ion chamber
risk levels for pacemaker
-dose > 5 Gy or neutron producting therapy = high risk
-dose< 2 Gy and patient is pacing independent- low risk
-otherwise, medium
devices other than pacemakerss that should be considered
-chochlear
-epidural pumps
-nerve stimulators
- Explain how you would go about commissioning IMRT
See TG-119: IMRT commissioning with multi-institution comparison. Use phantom studies to verify that treatments can be planned, prepared for treatment, and delivered with sufficient accuracy. Gamma criteria of 3%/3 mm are used. It is common to only analyze pixels with doses > 10% of maximum dose. Alternatively, rectangular ROI may be set to jaw settings. Typically require 95% pass rate (2 sigma confidence interval). Commissioning studies should mimic the types of target and structure geometries and target doses and dose constraints that are likely to be encountered in the clinic.
* -QA MLC leaf positioning and speed
* -E2E
* -anthropomorphic phantom
* IROC
o –use test plans in TG reports for comparison
- Explain how you would commission VMAT: Because VMAT involves simultaneous use of DMLC and gantry arcing, need to ensure that system is able to properly perform these motions in synchrony, along with dose rate modulation
- -additional QA: output as function of dose rate, gantry angle, profiles for other gantry angles, MLC leaf speed and positioning
- -E2E, verification plans, anthropomnorchic phantom
- IROC again
- Explain process for commissioning and clinical implementation of SRS/SBRT on conventional C-arm linac. What equipment/measurements are required?
o Beam measurements with small fields are challenging due to charged particle non-equilibrium, non-negligible detector perturbation effects, enhanced influence of finite source size. Challenges in defining field size.
Typically stereo commissioning data must go to field sizes smaller than for conventional RT. Extrapolation of commissioning data potentially leads to errors in dose calculation.
Requires special small field dosimeters
o Some TG-142 QA test tolerances are more stringent (e.g., coincidence of mechanical and radiation isocentres, measured using Winston-Lutz test)
o The commissioning of MLC commonly includes mechanical stability checks [e.g., Winston-Lutz test with MLC defined field, spoke shot], leaf position verification [accuracy/reproducibility using picket fence test, log file analysis], beam data acquisition, leaf transmission, leaf leakage, verification of beam penumbra, and the dynamic leaf gap test [I.e., dynamic picket fence test]
o IGRT system: image quality, isocentricity
o End to end test with anthropomorphic phantom with embedded target to test full workflow including e.g., target localization ability
o TPS may be finer grid- have to commission this as well
o Any additional imaging, motion or immobilization have to be commissioned
- Explain how to commission a new dose calculation algorithm (e.g., Acuros if you already have AAA)
o Can use same commissioning data that was used for original algorithm beam commissioning (there is no reason to remeasure the data).
o Comparison of results from the two algorithms: if new agrees with old and old agrees with measurements that were done originally, then new agrees with measurements (there is no reason to redo measurements)
o Use measurements (e.g., with film in anthropomorphic phantom) or Monte Carlo to investigate cases where they disagree
- Give example situations where you would and would not include skin in the evaluation structure.
o Want to include skin in evaluation structure if the intent is to treat the skin, and in these cases you would want to use e.g., MV photons with bolus, kV photons or MV electrons.
o Example where bolus is typically used: post-mastectomy chest wall radiotherapy
- You are called to a treatment unit where you determine that the kV imaging system is not functioning properly and your service personnel inform you that they can’t fix it until parts arrive tomorrow. Should you take the machine out of service in such a scenario? Discuss the reasons for you choice.
o Yes, kV system needed for IGRT. No substitute for this exists. Even if MV imaging system is available (EPID), there is much less soft tissue contrast so this is not a reliable substitute.
- Compare and contrast the plan QA methodology when checking a cranial, single fraction, stereotactic plan and a cranial 3D conformal plan.
o Rx to 100% for conventional. Typically prescribe to lower value isodose for stereotactic (so that dose fall off outside of target it faster).
o Dose calculation grid and CT sim voxel sizes smaller for stereotactic.
o Stereotactic typically requires multiple non-coplanar arcs or beams.
o OAR tolerance doses are much lower for stereotactic due to hypofractionation.
o Patient immobilization equipment more stringent for stereo. Need thermoplastic mask plus IGRT or head frame for SRS.
o FFF beam likely appropriate for SRS.
o Arc type needs to be “SRS” to allow for more MU per arc.
- A patient has lost weight during the course of their head and neck treatment and their mask no longer fits. The oncologist has ordered a new mask and CT scan for the patient and would like to continue treating with the current plan until a new plan is ready. He has contacted you to determine the dosimetric implications of this. Describe what you would do in this scenario to help the oncologist make his decision.
o Discuss potential for intra-fraction motion given that mask is now too loose.
o Can deform planning CT to match daily treatment CBCT (use extended range CBCT ideally). Then recalc current plan on deformed CT to assess dosimetric implications. Discuss with RO, noting that deformed contours may not be accurate (don’t just blindly look at DVHs). Tell RO that they must assess accuracy of deformed contours, image registration and dose distribution.
- Patient abdomen body contour consistently smaller than it was at CT sim. RO asks for help deciding how to proceed. What do you do?
o Follow same procedure as point #2 above.
o Assess whether it is weight loss or gas.
Can argue that gas won’t be a problem since amount of tissue traversed by beam is unchanged. This is valid as long as the gas is not directly adjacent to target.
o Assess patient setup (have they been setup wrong, with a rotation)
o Assess beam entry points. Contour change that does not occur at beam entry point is generally less of an issue.
- You are called to the treatment unit because a patient being treated with a 6MV POP, 10x10cm field (has lost weight – separation has changed from 30cm to 25cm). What do you advise? Would your advice change if this were an ENT IMRT patient (determined from CBCT that external has shrunk by 1cm near tumour)?
1 cm change vs 5 cm is significant
- The dosimetrist calls you with a patient who has bilateral hip replacement as seen on a CT scan. The physician wants to treat the pelvis with a standard 4-field beam arrangement. What advice would you give the dosimetrist in the case?
o Consider acquiring planning CT with higher energy beam, if available (e.g., MV CBCT using EPID, tomotherapy)
o Regions of artefact (e.g., dark streaks) that are outside of prostheses resulting in non-representative HU values should be contoured and set to HU=0 (assuming water equivalent region is more appropriate than streaky artefacts).
o Instead of ant/post/right/left beams, should use oblique beams that avoid entering or exiting through prostheses since this may result in strong attenuation and may not be properly modelled in TPS.
- Create a plan QA checklist for standard 2 field tangent-breast technique.
Check that there is 2 cm flash.
Isocentre should be near chest wall-lung interface to minimize divergence into lung.
Check documentation to see if it is a breath hold patient (for left sided treatments). If so, this should be indicated appropriately (for example in the plan name, consistent across all documents, etc.)
Arms should be above head using breast board immobilization equipment (unless particular reason why patient can’t do this)
In evaluating treatment plan, look for intensity modulation using FiF technique or wedges.
-location of norm point
-hot spots acceptable
-coverage
-dose to OARs, don’t enter through contra breast
-lung < 2 cm in field
-no heart in field
- A large patient, being simulated for a right breast treatment, undergoes a CT scan and her anatomy does not fit within the standard 50 cm field-of-view. The patient has missing anatomy on both the right and left sides. The therapists call you for advice; describe what options you have at the CT simulator in this case.
o Breast typically treated using partial arcs or tangents so beam enter/exit on one side. So it is more desirable to have more missing anatomy on the not treated side than a smaller amount of missing anatomy on both sides (which will have a larger clinical impact in terms of calculating dose accurately). Therefore, shift patient so that right breast (being treated) is closer to centre of CT bore [this means user origin closer to right breast I think].
o Otherwise could manually add in extrapolated contours set to HU=0 where there is missing tissue. However, this is an approximation since don’t actually know where body contour is and won’t be able to account for heterogeneities.
- Refer to image: 3-field rectum plan and the 95% isodose is not covering the PTV. What would you do to get coverage?
increase weighing of PA beam and use wedges
kV vs MV image for reference
