Heterotopic ossification (HO) is the process by which bone tissue forms outside of the skeleton in muscles and soft tissue. Symptoms
In traumatic heterotopic ossification (traumatic myositis ossificans), the patient may complain of a warm, tender, firm swelling in a muscle and decreased range of motion in the joint served by the muscle involved. There is often a history of a blow or other trauma to the area a few weeks to a few months earlier. Patients with traumatic neurological injuries, severe neurologic disorders or severe burns who develop heterotopic ossification experience limitation of motion in the areas affected. Causes
Heterotopic ossification of varying severity can be caused by surgery or trauma to the hips and legs. About every third patient who has total hip arthroplasty (joint replacement) or a severe fracture of the long bones of the lower leg will develop heterotopic ossification, but is uncommonly symptomatic. Between 50% and 90% of patients who developed heterotopic ossification following a previous hip arthroplasty will develop additional heterotopic ossification.Heterotopic ossification often develops in patients with traumatic brain or spinal cord injuries, other severe neurologic disorders or severe burns, most commonly around the hips. The mechanism is unknown. This may account for the clinical impression that traumatic brain injuries cause accelerated fracture healing.There are also rare genetic disorders causing heterotopic ossification such as fibrodysplasia ossificans progressiva (FOP), a condition that causes injured bodily tissues to be replaced by heterotopic bone.
Lymphoid leukemias are a group of leukemias affecting circulating lymphocytes, a type of white blood cell. The lymphocytic leukemias are closely related to lymphomas of the lymphocytes, to the point that some of them are unitary disease entities that can be called by either name (for example, adult T-cell leukemia/lymphoma). Such diseases are all lymphoproliferative disorders. Most lymphoid leukemias involve a particular subtype of lymphocytes, the B cells. Classification
Historically, they have been most commonly divided by the stage of maturation at which the clonal (neoplastic) lymphoid population stopped maturing:
Acute lymphoblastic leukemia
Chronic lymphocytic leukemiaHowever, the influential WHO Classification (published in 2001) emphasized a greater emphasis on cell lineage. To this end, lymphoid leukemias can also be divided by the type of cells affected:

B-cell leukemia
T-cell leukemia
NK-cell leukemiaThe most common type of lymphoid leukemia is B-cell chronic lymphocytic leukemia. B-cell leukemias
B-cell leukemia describes several different types of lymphoid leukemia which affect B cells. Other types include (with ICD-O code):

9826/3 – Acute lymphoblastic leukemia, mature B-cell type
9833/3 – B-cell prolymphocytic leukemia
9940/3 – Hairy cell leukemia

T-cell leukemias
T-cell leukemia describes several different types of lymphoid leukemias which affect T cells.The most common T-cell leukemia is precursor T-cell lymphoblastic leukemia. It causes 15% of acute leukemias in childhood, and also 40% of lymphomas in childhood. It is most common in adolescent males. Its morphology is identical to that of precursor B-cell lymphoblastic leukemia. Cell markers include TdT, CD2, CD7. It often presents as a mediastinal mass because of involvement of the thymus.