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.
