There are two main types of testicular cancer: seminomas and teratomas.
Following surgery to remove the affected testicle (the operation is called an orchidectomy) patients with early teratomas are usually offered a choice between chemotherapy or careful check-ups. The situation in seminomas is slightly different.

Radiotherapy is the standard precautionary treatment (often called adjuvant treatment) after an orchidectomy for an early seminoma. The treatment is given as an out-patient and usually involves fifteen sessions, Monday to Friday, over a three week period. The radiotherapy is given to treat the lymph glands in the abdomen, which are the most likely place for microscopic seedlings of seminoma to have spread to. Only relatively low doses of radiotherapy are needed for the treatment to be effective and so side-effects are normally not troublesome.

Doctors are looking at the alternative of giving chemotherapy, after surgery, instead of radiotherapy, for men who have had an early seminoma removed. The aim of this treatment is to prevent a recurrence of the growth in other parts of the body (secondary cancer). The use of chemotherapy in this situation is being tested in a clinical trial but the results are not yet known. So although chemotherapy may prove to be as effective for early seminomas the benefits are not yet certain so it is still considered an experimental treatment.

The policy of offering very regular check-ups, with blood tests, scans and x-rays, instead of further treatment after surgery has been tested for men who have had an early seminoma removed. But to be safe these check-ups have to go on for far longer than for patients with teratomas (where this approach is often used). For this reason the use of regular check-ups as an alternative to radiotherapy is not usually recommended.

So the advice your doctors have given you is correct.