What is the appropriate radiation volume for a stage III, group III unresectable embryonal rhabdomyosarcoma originating from the bladder?

When the original tumor extent is substantial (displaces normal abdominopelvic organs), but responds well to therapy (i.e. as of week 12) - How would you target the pretherapy extent without unnecessarily overtreating the previously displaced organs? i.e. Should your CTV1 encompass the current bladder tumor plus a 1 cm expansion w/o the displaced bowel volume, OR should it include the entire pre-chemotherapy bladder tumor extent (which necessitates irradiating large amounts of normal bowel)? Are there potentially infiltrated spaces which should be covered beyond the 1cm expansion on the post-therapy extent which were likely involved pre-therapy?