Acute Leukemias - Republican Scientific Medical Library

120 Chapter 8 · Diagnosis of Acute Lymphoblastic Leukemia
blast morphology and cytochemistry [5], whereas the
World Health Organization (WHO) classification
scheme also incorporates immunophenotyping and cytogenetics
[36].
Clinically and biologically, ALL and lymphoblastic
lymphoma are considered a single entity and the terms
are often used interchangeably [58]. However, the term
“lymphoma” is preferred when the bulk of the disease
is in the lymph nodes or soft tissues, whereas “leukemia”
should be used when the bulk of the disease is
in the bone marrow and blood [21, 53]. Approximately
80% of patients with ALL have enlarged lymph nodes,
most likely due to involvement with the leukemic process
[21].
In this chapter we present diagnostic criteria for
ALL and its subtypes and discuss the importance of cytogenetic
and molecular abnormalities for diagnosis,
classification, and determining clinical management.
8.2 Morphology
Lymphoblasts in patients with ALL tend to be heterogeneous
in size and shape. Unlike the recent WHO classification,
which takes cytogenetic and immunologic features
into account, the FAB classification of ALL emphasizes
the presence of subgroups of precursor lymphoblasts:
L1, which is more common in children than in
adults (85% vs. 30%) and L2, which is more common
in adults than in children (60% vs. 15%). The FAB
and WHO classifications both recognize the more mature
subtypes of B-cells as Burkitt L3 cells [5, 80].
L1 precursor lymphoblasts are small with scant cytoplasm,
fine chromatin, and indistinct nucleoli (> 90% of
total blasts) (Fig. 8.1). L2 precursor lymphoblasts, on the
other hand, are typically medium-to-large cells with
high nucleus-to-cytoplasm ratios, prominent nucleoli,
and irregular or folded nuclear membrane outlines
(Fig. 8.1). Morphologic heterogeneity is almost always
seen in L2 and, to a lesser degree, L1 precursor lymphoblasts.
Occasional cells with vacuoles can be seen in L2type
precursor lymphoblasts, especially after relapse or
therapy [64]. Although the reproducibility of classifying
L1 and L2 precursor lymphoblasts is poor, distinguishing
L1 from L2 morphology remains useful for diagnosis
and for its descriptive value. Several studies suggest
that patients with the L1 cell type have better response
to therapy, with better disease-free survival than patients
with the L2 cell morphology [2, 5, 44, 56, 77].
L3 (Burkitt) blasts have distinct morphology, with
medium-sized and more uniformly rounded nuclei
and finely clumped chromatin. The diagnostic feature
of this cell subtype is a deeply basophilic and vacuolated
cytoplasm (Fig. 8.2). The vacuoles in L3-type cells contain
lipids and stain positively with oil-red O stain. Nucleoli
are seen but are not dominant [5]. The cells of
Burkitt leukemia have a very high rate of turnover (proliferation
and apoptosis). This phenomenon manifests
morphologically as the starry-sky appearance frequently
seen in bone marrow biopsy specimens or tissue
sections (Fig. 8.1), and biochemically with extremely
high levels of lactate dehydrogenase [7, 72].
In addition to morphology, ALL is classified according
to the B-cell or T-cell status. B-cell precursor ALL
accounts for about 85% of ALL cases, with T-cell ALL
accounting for about 15%. Although T-cell ALL lymphoblasts
occasionally demonstrate conspicuous folded or
cerebriform nuclei, T-cell precursor lymphoblasts cannot
be reliably distinguished from B-cell lymphoblasts
based on morphology alone [80]; immunophenotyping
is always needed for confirmation.
8.3 Cytochemistry and Immunophenotyping
The key diagnostic cytochemical feature of ALL is the
lack of myeloperoxidase (MPO) activity and negativity
for nonspecific esterase (NSE) [5, 71]. The functional
MPO test using cytochemistry remains the gold standard
for assessing MPO activity, but laboratories are increasingly
using the chloroacetate esterase stain and immunostain,
especially for detection by flow cytometry
[62]. To distinguish ALL from increased peripheral
blood or bone marrow blasts, fewer than 3% of blasts
should express MPO activity [5]. However, it is not unusual
to detect slightly greater than 3% MPO-positive
blasts in patients with chronic myeloid leukemia
(CML) in lymphoid blast crisis, with overwhelming
lymphoid surface markers. Most likely these few
MPO-positive blasts reflect the active chronic cell population
that coexists along with the lymphoid blasts. Sudan
black B (SBB) can also be used to confirm the presence
of MPO granules in these cells [71]. However, some
cases of ALL exhibit fine SBB-positive granules rather
than large, dark positive granules. Periodic acid-Schiff
(PAS) staining is also positive in ALL lymphoblasts,
showing a large, globular pattern. This PAS pattern is