PET scan is a sensitive investigation for many tumors. Angiosarcomas can also be picked up by PET scan. However, a PET scan (about Rs 20000) is relatively expensive compared to a CT scan
(5000 - 10000) or MRI
(~10000). If the sequential CT scans are normal and if your brother does not have any complaints on regular follow up then he might not require a PET scan. If on the other habd there is some abnormality on the CT which is difficult to make out then you can go for a PET scan especially to see if there is active tumor or fibrosis
. Lot of places now offer PET scan. An idea of your location would help give a specific answer.