Radiobiology 2

Question 1

Irreparable DNA lesions are an issue because…

Answer 1

It leads to sterility in the cell, loss of cell division and viability.
Necessary in tumour cells to succeed in killing the cancer.

Question 2

Partial sterilisation is?

Answer 2

Perhaps providing clinical remission but regrowth could be possible for cells retaining proliferative capacity (survivors).

Question 3

What are acute effects?

Answer 3

Have a time-scale of days or weeks e.g skin reaction

Usually in epithelia or hemopoietic tissues which have a rapid turnover.

Question 4

What are late effects?

Answer 4

Have a time-scale of months or years, usually in endothelial, neuroglial tissues etc.
Risk of late effects is the dose limiting factor.

Question 5

Is the ideal radiotherapy plan possible?

Answer 5

Ideal plan would be 100% uniform prescribed dose going to the PTV and none to surrounding areas of the body.

Impossible because of photon fluence at skin surface. Highest dose deposition at 1-3cm depth, and exponential fall off.

Question 6

What do the serial and parallel models mean?

Answer 6

A serial circuit, one bulb blows then the whole circuit breaks. In a parallel circuit, the circuit can still run if one breaks.
Want to aim for a parallel circuit model e.g lungs or liver.

Spinal cord is a serial circuit model example.

Question 7

What must be done regarding serial structures?

Answer 7

E.g. spinal cord.
Must keep dose to a minimum - be careful about exit beams etc. Must always be kept below the tolerance dose, even if it means compromising the treatment.

Question 8

What is quantec?

Answer 8

A paper by Emami, which has formed the basis of tolerance doses since 1991

Question 9

What is the most common DVH?

Answer 9

Integral (cumulative) DVH.
The fractional volume of an organ receiving a certain dose or greater.
Mainly used for examining the PTV and OAR.