verfication Flashcards
two types of verification
dosimetric
geometric
what is the aim of geometric verification
to deliver RT as it was planned
how is geometric verification achieved
comparing images acquired in treatment with images of treatment planning position(references images)
Gross Tumour Volume
gross palpable or visible extent of the tumour
- NO GTV if patient has had surgery
Clinical Target Volume
tissue volume that contains the GTV and subclinical microscopic malignant lesions.
Internal Target Volume (ITV)
-CTV plus a margin taking into account changes in shape size and position of CTV of patient
-Lung and cervix uses ITV
SET UP MARGIN:
accounts for
uncertainties in patient positioning and position of beams during tx planning and tx sessions. (geometric uncertainties)
Image Guided Radiotherapy
-starts before treatment
-fusion of added diagnostic or simulated scans to the planning CT to facilitate target definition
-ideally carried out in tx position
CTV-PTV MARGIN:
too tight
too generous
-risk of geometric risk
-risk of irradiating excess normal tissue
margin is critical for understanding the precision required at verification
what is a set up error
any geometric displacement in the patients position compared to the reference image.
calculated as a shift in the tx position when the tx image is being compared to the reference image.
how to reduce PTV margin
eliminate geometric uncertainties as much as possible before delivering your beam
eliminating geometric uncertainties by
-ITV = bladder filling protocols and motion management techniques
-Robust immobilisation= reduces set up errors
-IGRT=daily imaging of tx volume (IGRT has the potential to remove nearly all geometric uncertainties)
interfraction definition
motion observed between fractions.
quantified and corrected by online daily imaging
-intrafraction defintion
-how does it happen
-treatment planning approach
-motion observed during a single tx fraction
-due contracting muscles and organ motion
-real time imaging needed to account for it
Gross Errors:
-what is it
-which can cause
-unaccepatly large set up error (more than 1cm)
-underdose of GTV/CTV and over dose of OARS
planning margins do not account for gross errors and must be corrected before tx.
Gross errors can be caused by
-incorrect patient, site or ortentaion
-incorrect immobilisation or shifts implemeneted
-incorrect plan
definition of systematic set up errors(SSE)
deviation from the planned position that occurs in the same direction and similar magnitude for each traction throughout the course
pattern detected when imaging trends are assessed
systematic error issues
-data transfer issues=immobolisation details , shifts from tx plan, couch sag
-patient issues=positioning , bladder/rectal volumes
Random Errors definition
deviations that vary from direction and mangitude from every tx fraction. daily fluctuations
how do random errors occur
variations of positioning or organ motion and can be attributed to patients anatomy and compliance
Random and Systematic Error & Dose Impact
systematic errors cause a shift in a cumulative dose distribution
random errors cause a blurring in dose distribution
dosimetric impact will entirely depend on magnitude of shift
What are the target volumes in prostate RT?
-prostate only
-prostate +/- sv
-prostate + sv +/- lymph nodes
-prostate beds
What lymph nodes do we treat in high risk patients?
-common iliac nodes (starting at L5-S1),
-internal and external iliac nodes,
-pre-sacral (sub-aortic nodes )
-obturator nodes
nodal coverage and imaging
when imaging never comprise treating the prostate and sv to mantain nodal coverage !!
Prostate Motion
prostate moves independently of bone (must be imaged correctly using 3D imaging or a apt surrogate.
Target Motion
-SV larger motion than prostate but motion can be independent to prostate
-bone is a reasonable surrogate for nodes
