There are two main radiation therapy techniques: external beam and internal beam (brachytherapy).

• External Beam

Radiation delivered by an external beam aparatus.

Its goal is to deliver a tumoricidal radiation dose to a known tumor while sparing normal tissue as much as possible, subject to the laws of radiation physics and radiobiology [3]. 

In the last years, the development of alternative kinds of radiotherapy, such as conformal radiotherapy and intensity modulated radiotherapy based on 3D imaging allowed a reduction of normal tissues in the high-dose region, while maximizing the radiation dose delivered to the target volume [1,4,5].

• Internal Beam (Brachytherapy)

Involves the implantation of small radioactive particles (“seeds”) within malignant tissue.

These particles release high doses of radiation to local tissues over a known period of time [5]. 

The great decrease in radiation intensity at a short distance from the source diminishes side-effects on adjacent organs [1].

There are three main subdivisions: intersticial implantation, intracavitary treatment and radiation moulds [1].

• Post-radiation imaging appearence: uterus and ovaries

The effects of radiation therapy on the premenopausal uterus are two-fold: direct radiation changes are induced and ovarian function is supressed [6]. This leads to changes in uterine volume and anatomy, similar to those seen in postmenopausal women, such as endometrial thinning, loss of myometrial zonal anatomy, and reduction in uterine volume [4,5]. (Figure 1)

In postemenopausal patients, there are no significant changes in the appearence of the uterus at follow-up MR imaging [2,6]. 

The effect on the ovaries depends on patient age and the radiation dose delivered. The ovaries decrease in size and signal intensity, with loss of physiologic follicles, which results in permanent sterility and substantial ovarian dysfunction [2,4]. (Figure 2)

• Post-radiaton imaging appearence: bowel

Rapidly proliferating cells makes the small bowel mucosa the most radiosensitive tissue in the gastrointestinal tract [5,7].

Radiation may cause acute inflammation and cell death of the bowel mucosal layer, along with changes in the vasculature [4]. 

At T2-weighted MR imaging, affected bowel loops show increased signal intensity (edema), mural thickening and increased signal intensity in the perirectal space [4,8]. (Figure 3)

Increased perirectal space due to fatty deposition may be found [2].

• Post-radiaton imaging appearence: bone

Hematopoietic elements undergo necrosis and fatty marrow replacement [4,7,9].

The areas of fatty replacement appear as well demarcated high signal intensity on T1 and T2-weighted MR imaging, strictly confined to the radiation field [4]. (Figure 4)

Occasionally, however, during conversion from hemopoietic to fatty marrow, patchy differences in signal intensity may simulate metastatic bone disease [2,10].

• Complications: cervical

In cervical cancer patients, cervical os stenosis is a well-recognized intermediate to long-term complication [5].

It is seen as narrowing of the cervical canal resulting in distention of the uterine cavity with blood or fluid [4]. (Figure 5)

• Complications: fistula

Radiation therapy can cause fibrosis, loss of soft-tissue planes, and, in some patients, necrosis, leading to the formation of fistulas in the pelvis [2].

Fistulas are late complications of radiation therapy and most commonly occur between the bladder and the vagina or rectum, but may occur in the setting of recurrent disease as well [2,4,10]. (Figures 6 and 7)

Imaging characteristics depend on the content. Look for indirect signs (gas in the bladder, for example).

Sagital T2-weighted MR plane is the best to detect fistulas.

• Complications: gastrointestinal

The small bowel more radiosensitive than the large bowel, due to its higher cellular turnover [4,5].

Chronic changes can lead to separation of bowel loops, tethering of other loops and stricturing, which can result in small bowel obstruction [2,5].

Similar findings may occur in the colon.

• Complications: bladder

Due to the rapid proliferation of its cells, the bladder is the most radiosensitive organ of the urinary tract [10].

Long-term effects are dose dependent. Lower doses of radiation may cause mild cystitis with higher doses causing more a severe disease [2,4].

In the chronic phase, the bladder has a small volume and cannot be fully distended because of fibrosis [2]. (Figure 8)

• Complications: musculoskeletal

Insufficiency fractures are caused by normal stress on an abnormal bone, with low signal intensity representing the fracture on T1-weighted images and the bone edema having high signal intensity on T2-weighted images [8].

Osteonecrosis and osteomyelitis are other possible complications [4].

• Complications: radiation-induced malignancies

Secondary malignancies following radiation therapy are rare, and have been described to arise from soft tissues (sarcomas) and bones (osteosarcomas) [4,8].

The latency period between the completion of radiation therapy and the development of a secondary neoplasm may be many decades [5].