AbstractID: 7631 Title: Evaluation of Dosimetric Issues related to IMET for a Prototype eMLC
A prototype electron multileaf collimator (eMLC) has been developed for a Siemens
Primus accelerator. The eMLC has 21 leaves that project 1-cm width at isocenter. The
downstream edge of the leaves are 10 cm above isocenter, and each leaf can cross the
entire field, allowing any field shape within a 21 x 20 cm2 area. The aim of the present
work was to perform preliminary dosimetry measurements that can assess the potential of
this prototype eMLC for intensity modulated electron therapy (IMET). Film dosimetry
was used to study the shaping resolution and radiation leakage (interleaf leakage of
electrons and transmission leakage of photons) for the eMLC. Results showed that the
shaping resolution (FWHM of pencil beams) was 0.76 cm at 15 MeV and was
independent of leaf position across the field. Slit beam measurements showed that
electron leakage should be less than 0.1% of given dose, assuming easily achieved
machining precision. X-ray leakage is less than 2% at 15 MeV for the fixed beam;
however, this can become significantly greater as monitor units 3-9 times greater may be
required for IMET. We concluded: (1) as electron leakage is not an issue, stepped edges
on the sides of the eMLC leaves are not required; (2) x-ray leakage may restrict the full
utilization of IMET, and (3) the shaping resolution for electron intensity patterns with the
eMLC should allow sufficiently sharp gradients for IMET.
This work is supported in part by a sponsored research agreement SR00-327 with
Siemens Medical Systems, Inc.