Chapter 7 – Complementary Experimental Tools  309

transient. Long-​lived DSBs are highly reactive free ends of DNA, which have the potential

for incorrectly religating to different parts of the DNA sequence through binding to DSBs in

potentially a completely different region of DNA if it is accessible in the nucleus, which could

have highly detrimental effects on the cell. Cellular mechanisms have unsurprisingly evolved

to repair DSBs, but a competing cellular strategy, if repair is insufficient, is simply to destroy

the cell by triggering cell death (in eukaryotes this is through a process of apoptosis, and

prokaryotes have similar complex mechanisms such as the SOS response).

The main issue with radiotherapy is that similar doses of ionizing radiation affect normal

and cancerous cells equally. The main task then in successful radiotherapy is to minimize the

relative dose between normal and cancerous tissue. One way to achieve this is through spe­

cific internal localization of the ionizing radiation source. For example, iodine in the blood

is taken up preferentially by the thyroid gland. Thus, the iodine-​131 radionuclide, a positron

emitter generating gamma rays used in PET scanning, can be used to treat thyroid cancer.

Brachytherapy, also known as internal radiotherapy or sealed source radiotherapy, uses a

sealed ionizing radiation source that is placed inside or next to a localized cancerous tissue

(e.g., a tumor). Intraoperative radiotherapy uses specific surgical techniques to position an

appropriate ionizing radiation source very close to the area requiring treatment, for example,

in intraoperative electron radiation therapy used for a variety of different tissue tumors.

A more common approach, assuming the cancer itself is suitably localized in the body to

a tumor, is to maximize the dose of ionizing radiation to the cancerous tissue relative to the

surrounding normal tissue by using a narrow x-​ray beam centered on the tumor and then at

subsequent x-​ray exposures to use a different relative orientation between the patient and

the x-​ray source such that the beam still passes through the tumor but propagates through a

different region of normal tissue. Thus, this is a means of “focusing” the x-​ray beam by time

sharing its orientation but ensuring it always passes through the tumor. Such treatments

are often carried out over a period of several months, to assist the regrowth of normal

surrounding tissue damaged by the x-​rays.

7.8.9  PLASMA PHYSICS IN BIOMEDICINE

Plasma medicine (not to be confused with blood plasma, which is the collection of essen­

tial electrolytes, proteins, and water in the blood) is the controlled application of physical

plasmas (i.e., specific ionized gases induced by the absorption of strong electromagnetic radi­

ation) to biomedicine. A standard clinical use of such plasmas is the rapid sterilization of

medical implements without the need for bulky and expensive autoclave equipment that rely

on superheated steam to destroy biological, especially microbial, contaminants. Plasmas are

also used to modify the surfaces of artificial biomedical implants to facilitate their successful

uptake in native tissues. In addition, therapeutic uses of plasmas have involved improving

wound healing by localized destruction of pathogenic microbes (i.e., nasty germs that can

cause wound infections).

Worked Case Example 7.3: PET Scanning

A time-​of-​flight positron emission tomography (TOF-​PET) scan was performed on a bio­

logical tissue sample at 2.7 GHz sampling rate using a delayed-​coincidence method, with

the uncertainty in the difference of arrival times being ~10% of the sampling time.

a

Estimate the spatial resolution for localizing the position–​electron annihilation

events in the sample.

b

To explore the sensitivity of the TOF-​PET instrument, the doping of the sample was

lowered to produce a rate of coincident detection from two gamma ray detectors

that was only 1 ± 0.1 MBq and the rate of random signal detection from and single

gamma ray detector was 300,000 ± 80,000 counts per second. What is the signal-​

to-​noise (SNR) in light of the precision of its measurement for true coincident signal

detection?

KEY BIOLOGICAL

APPLICATIONS:

BIOMEDICAL

PHYSICS TOOLS

Multiple health-​care diagnostic

and treatment applications.