Radiation therapy

Radiation is often used to treat NHL (and many types of cancer) when there is a localized tumour that can be easily targeted without any other major organs in the way, or for bulky tumours that do not fully respond to chemotherapy.

There may be other reasons why your doctor chooses radiation treatment as well. For example even some of the low grade lymphomas which are normally considered incurable, can be cured if they are truly localized to one site and treated with radiation.
Lymphoma tumours are generally considered to be more sensitive to radiation therapy than many other types of cancer. This makes them good candidates for Radiation therapy. 
Before beginning radiation therapy you will probably make a trip to the clinic for a simulation. During this simulation they position you on the table and measure exactly where they will be aiming the radiation. If often involves a CT scan to locate the tumours, and you will probably even get some tattoos. These tattoos (the size of a small pencil dot) are so they can line you up on the treatment table exactly in the right position.

Radiation techniques differ somewhat from those used in the treatment of Hodgkin's disease. The dose of radiation therapy usually varies from 2500 to 5000 cGy and is dependent on factors that include the type of lymphoma, the patient's stage and overall condition, the goal of treatment (curative or palliative), the proximity of sensitive surrounding organs, and whether the patient is being treated with radiation therapy alone or in combination with chemotherapy. Treatment with radiation therapy requires the use of linear accelerators with energy of 4 to 15 MV and treatment planning simulators. Simulators are used to obtain detailed x-rays of patients in the treatment position so that radiation fields can be tailored to conform to the patient's anatomy, and individually shaped protective blocks can be fabricated to shield normal tissues for the individual patient. Radiation treatment fields are tailored to individual patients and are usually limited to involved regions or extended to the immediate adjacent sites. The majority of patients who receive radiation are usually treated on only 1 side of the diaphragm.  

Below is a diagram of the typical radiation fields used, and the names of the various lymph node regions.

Additional information

From the U.S.A. Environmental Protection Agency
Understanding Radation -Alpha, Beta and Gamma