Analysis of extended sources is complex, challenging and time-consuming. There is currently neither an official SAS recipe, nor a simple thread. Several groups are independently working in this field. The XMM-Newton EPIC Background Working Group (BGWG) was therefore founded in 2005 as a steering and supervising committee to provide the users with clear information on the EPIC Background and (SAS)-tools to treat the background correctly for various scenarios.
A dedicated Background Analysis web page is available at:
which contains information and tools collected and documented by the Background Working Group. A summary of the contents of these pages is collected below:
One of the main problem in the analysis of extended sources is that often no statistically useful blank background region can be defined in the observational field-of-view. A workaround is to make use of the provided 'blank sky' background files to generate background spectra corresponding to the camera/mode/filter combination rescaled to the actual observation.
An alternative approach is to model the background spectra based on the background conditions of the individual observation under study. The recommended method is to make use of the Extended Source Analysis Software (XMM-ESAS) package. As of SAS v9.0, this package is integrated in SAS. The list of XMM-ESAS packages integrated in SAS can be found here:
The XMM-ESAS package allows to model the quiescent particle background both spectrally and spatially for the pn and MOS detectors. XMM-ESAS produces background spectra for user-defined regions of the detectors and background images. There is an XMM-ESAS Cookbook  and two SAS Analysis Threads that describe how to use XMM-ESAS step-by-step.
Suggestions and caveats on the analysis of extended sources with the EPIC cameras have also been addressed in a SAS workshop presentation (available from the SAS Workshop page).