Hall Book Ch 25 (Alternative Radiation Modalities)

Question 1

Particle therapy using beams of neutrons, protons, or heavy ions is sometimes referred to as ( ), that is, therapy with particles that are made of ( ).

Answer 1

hadron therapy, quarks

Question 2

Neutrons are ( ) ionizing. In tissue, they give up their energy to produce ( ).

Answer 2

indirectly, recoil protons, α-particles, and heavier nuclear fragments

Question 3

Biologic properties of neutrons differ from those of x-rays in several ways:

Answer 3

1) reduced OER, 2) little or no repair of sublethal damage or potentially lethal
damage, and 3) less variation of sensitivity through the cell cycle.

Question 4

The rationale for the use of neutrons in radiotherapy has changed over the years. The earlier rationale was the reduced OER to overcome the problem of hypoxic cells. The revised rationale is based on a higher neutron RBE for slowly growing tumors.

A small advantage has been demonstrated in clinical trials for neutrons in the
treatment of salivary gland and prostate tumors and soft tissue sarcomas but
not for most cancer sites tested. The downside of neutrons is the unacceptable
level of normal tissue damage.
A new generation of hospital-based cyclotrons with isocentric mounts and
generating neutrons by the p+ → Be reaction became available, but by then,
enthusiasm for neutrons had waned.
Boron Neutron Capture Therapy
The principle of BNCT is to deliver a drug-containing boron that localizes only
in tumors and then to treat with low-energy thermal neutrons that interact
with boron to produce α-particles.
Boron is a suitable substance because it has a large cross section for thermal
neutrons and emits short-range, densely ionizing α-particles if bombarded by
thermal neutrons. Its chemistry is such that it can be incorporated into a wide
range of compounds.
Many attempts have been made to synthesize boron-containing compounds
that are selectively localized in tumors relative to normal tissues but with
limited success. They fall into two categories:
1. Low-molecular-weight agents that simulate chemical precursors needed
for tumor cell proliferation
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2. High-molecular-weight agents such as monoclonal antibodies and
bispecific antibodies
Thermal neutrons are poorly penetrating in tissue, with a half-value layer of
only 1.5 cm.
Epithermal neutrons are somewhat more penetrating. They are degraded to
thermal neutrons by collisions with hydrogen atoms in tissue. The peak dose
is at 2 to 3 cm, and the high surface dose is avoided.
Results of clinical trials of the efficacy of BNCT are tantalizing but not
definitive.
The concept of BNCT is very attractive, but there are formidable practical
difficulties in making it a treatment modality even for relatively shallow
tumors.