AAPM TG 158 Flashcards
measurement and calculation of doses outside the treated volume from EBRT
guidance for assessing and managing non-target doses
aims of report
- highlights major concersnw ith non-target radiation
- gives estimate of dose associated from different treatment approaches
- discusses dosimeters for measuring photon, electron, or neutron doses
- discusses calc techniques for dosimetry
- highlights techniques that may reduce non-target doses
- discusses dose reporting
- makes recommendations for clinical and research practise
concerns regarding non-target radiation
-secondary cancers
-cardiac toxicity
-cataracts
-pacemakers
-fetus
-skin dose
neutrons with pacemakers
-neutrons cause single-event upsets
-risk is stochastic; there is no “safe” dose
average energy outside of treatment field
for 6 MV, ~ 0.2-0.6 MeV vs 1.6 MeV in the treatment beam
how does out-of-field photon dose change with distance from field edge?>
decreases exponentially with distance from field edge
hoe does dose outside treatment field change with field size?
increases with field size because there is more patient scatter
Farther from the field edge, there is lessfield size dependence53because this dose is dominated byhead leakage. For modulated treatments, the dose outside thetreatment field also depends on beam modulation. The doseincreases with increasing modulation because more head leak-age and more collimator scatter is generated as the number ofmonitor units (MU) increases to deliver the modulatedfields. The dependence on modulation is most noticeablefarther from the treatment field; near the field edge the dose isdominated by patient scatter and patient scatter depends onlyon the volume of tissue irradiated.
how does dose outside tratment field change with depth and beam energy?
-varies little with beam energy
-pretty constant with depth, except for surface, which has electron contamination
what does neutron dose increase with
MU
beam energy
neutrons in patient decrease with depth
IMRT out of field dose vs 3DCRT
IMRT treat-ments typically have lower doses near the edge of the treat-ment field but higher doses far from the treatment field
Near the treatment field, IMRT provides better conformality,which constricts the field edge and thereby reduces the vol-ume of tissue receiving high doses.76However, far from thetreatment field, beam modulation leads to increased headleakage than can result in higher doses.
that the majority of second cancers were in organsnear the treatment field as opposed to organs far out-of-field,indicating the importance of the high-dose region. However that the majority of second cancers were in organsnear the treatment field as opposed to organs far out-of-field,indicating the importance of the high-dose region. However
VMAT vs IMRT
VMAT spreads out the dose deliveryover more angles. Consequently, while the average dose willbe similar between VMAT and IMRT, more tissue will beirradiated to a lower dose with VMAT.
out-of-field doses for cyberknife
higher, high modulation
out-of-field doses for tomo
comparable to IMRT
out-of-field doses for gamma knife
between linac and cyber knife
Because Gamma Knife units have a relatively low-energyspectrum, doses very close to the treatment field are oftenhigher than those of CyberKnife or linac systems.126,139Formost distances from the field edge, Gammaknife treatmentshave been shown to produce intermediate doses—higherthan linac-based therapy but lower than those achieved byCyberKnife
FFF
reduces out-of-field dose
It reduceshead leakage because less target current is required. Simi-larly, collimator scatter is reduced because the flatteningfilter is no longer a source of scatter. However, patientscatter may be increased in FFF modes.
electron out-of-field dose
majority is from scattered electrons
The increase in dose at~20 cm from the field edgearises from scattered electrons originating from the roundedsurface of the MLC
While out-of-field doses gener-ally decrease with distance from the edge for electron therapy,this is less pronounced than for photon therapy
-high compared to photons
-increase with energy and obliquity
-little variation with applicator size
non-target dose from brachy
-similar to EBRT at close distance
-At distance > 35 cm from target, BT nontarget dose is much less than EBRT
photon distance from target
remember about 1% at 10 cm from field edge but varies with FS, modulation- for 3DCRT
for imrt, closer to 5%
why cover dose detectors with a bolus?
surface dose will be greater due to electron contamination and will overestimate photon dose at depth. For better estimate of dose at deoth, put bolus over detector
issue with choosing detectors for measuring out=of-field dose
-beam energy is lower outside of field. Forexample if using diode, dose will be overestimated outside of field
-dosimeter dynamic range (dose outsode pf field may be too low)
-presence of other particles like neutrons can cause different detector repsonse
limitation of using film for out=of-field dosimetry
-usually film is good for D> 1cGy- dose may be much less than that
-dose can span orders of magnitude- difficult to get calibration curve that accounts for this range
most common passive thermal neutron detectors
TLDs (for neutrons) and activation foils
The TLDs are used in pairs, onesensitive to both photons and thermal neutrons (TLD-600)and the other sensitive only to photons (TLD-700). The neu-tron component is determined by the difference in theresponse in the two types of materials.
-require moderators
thermal neutron vs fast neutron dose deposition
he fast neutron peak (which depos-its the dose) decreases sharply with depth, while the thermal neutron peak (which is often responsible for depositing signal in the detector) shows a very differentdependence on depth, increasing up to 4.5 cm depth and still being notably present at 19.5 cm depth.
therefore have to use thermal neutron detectors with moderators in order to get info about the fast neutrons that actually deposit the dose
why not use a rem meter?
Because Rem meters have activedetectors, pulse pile-up is a concern and these detectors aregenerally not well suited to measurements in or near the pri-mary beam.
why shouldn’t bubble detectors be used in beam?
These detectorsshould not be used in the primary photon beam because spu-rious signal has been observed within the polymer, overesti-mating the true result
The advantages of bubble detectors over otherneutron detectors are that they are very easy to use, are reusa-ble, and can be read instantaneously. Disadvantages of bubbledetectors include energy dependence, loss of linearity at highdoses, and the potential for spurious bubbles.
why not use track etch detectors fordosimetry for neutrons in proton therapy?
Most track etch detectors are not suitable for measurementaround proton (or carbon) beams because of the lack ofresponse to high-energy neutrons.
Track etch detectors are very sensitive and are, therefore,suitable for use outside the primary beam in high-energy pho-ton therapy.
-can measure fast and thermal neutrons
tissue-equivalent proportional counter
The absorbed dose contributions from vari-ous types of secondary particles can be differentiated via themicrodosimetric spectrum because different secondary parti-cles have different lineal energies in the active volume.
Given that the signal from this type of detector depends onthe measurement of each energy deposition event, the flux isa major concern when using this detector in radiotherapyenvironments because of pulse pile-up and dead time effects
does TPS calc under or over estimate out of field dose
usually overestimates it
Peridose
software developed to calculate out of field dose
proton therapy integral dose vs IMRT
proton therapy is 2-3 X less integral dose
where do TPS systems dhow large errors
even as close as 3 cm to field edge
