Superficial X-rays

Question 1

Name some of the key differences between a superficial treatment X-ray tube and a diagnostic X-ray tube.

Answer 1

The target is non-rotating.
The anode is earthed, meaning that the tube is water cooled (opposed to oil cooled).
Since image quality is not a concern, small focal spot is not required. This is the reason that the target does not rotate.

Note that orthovoltage units are held at a higher potential and therefore have their anode at a high potential and are oil cooled.

Question 2

What is the heel effect?

Answer 2

It is the differential self-absorption which in the target due to the angle between the target and the incident x-ray’s spatial distribution.

Question 3

What are typical anode angles for superficial x-ray units?

Answer 3

Between 26 to 30 degrees.

Question 4

What is wanted from the filtration within the tube?

Answer 4

To remove the low energy x-rays which will only contribute to skin dose and are not therapeutically useful.

Question 5

How is filtration material selection decided?

Answer 5

It is based on the mechanical stability (rigidity).

Minimal reduction in the overall intensity is desired.

Minial production of characteristic x-rays (k-edge especially).

Question 6

What filters are used for superficial and orthovoltage equipment?

Answer 6

Medium Z materials:
Al and Cu with 0.5-3mm and 1-4 mm thickness, respectively.

Al is used for SXT units.

Orthovoltage units use a combination of materials, known as Thoreus. This material is composed of Su + Cu + Al. The higher Z materials are placed up field since these will reduce the soft X-rays. However, due to Z^3 dependence on photoelectric effect, these higher Z materials will generate more characteristic X-rays. Therefore, by placing the Al the most down field enables the reduction of the contribution from the characteristic X-rays.

Question 7

What are typical SSDs for SXT and orthovoltage units?

Answer 7

SXT: 15 cm and 25 cm
Orthovoltage: 30 cm and 50 cm.

Question 8

Why do orthovoltage applicators have an end plate?

Answer 8

Orthovoltage units are at a high enough potential that there is a small but noticable build-up effect. Therefore, the end plate will generate scatter which helps to remove the build-up effect.

Question 9

How do SXT and orthovoltage units modulate doses delivered?

Answer 9

The SXT units use a timer.

Ortho uses MU.

Question 10

What is the primary source of superficial X-ray beam data? And what does it contain?

Answer 10

BJR supplement 25 (1996).
This document contains beam qualities, PDDs, backscatter factors, and tissue-air ratios. These quantities are useful to have on record since they are difficult to measure (especially the backscatter factors).

Question 11

Describe the characteristics of a superficial beam’s PDD.

Answer 11

• There is little or no buildup.
• D_{max} is close to the surface.
• There is a very steep dose falloff.

Note that PDDs can be measured with an IC and radiochromic film, but radiographic film is unsuitable due to the Z-high components over-responding.

Question 12

How is beam quality defined?

Answer 12

HVL (specified per material)

Question 13

What parameters need to be considered to accurately measure a HVL for SXT and orthovoltage units?

Answer 13

Corrections for water-to-air and mass energy absorption coefficient ratios.
Corrections for backscatter factors.
Good, robust geometry with narrow beams and otherwise scatter free conditions.

Question 14

How can energy spread be sampled using HVLs?

Answer 14

The homogeneity factor is defined as the ratio of the first to second HVLs.

Note thata a homogeneity factor of one will correspond to a monoenergetic beam, and less than one to a polyenergetic beam.

Question 15

How is backscatter defined?

Answer 15

It is the ratio of water collision kerma at the surface of the patient on the central axis to that at the same point without the patient.

Question 16

Upon which two quantities is the backscatter factor dependent?

Answer 16

The field size.

The HVL.

Question 17

How is the superficial X-ray COP segmented in terms of energy, and give indications of the approximate corresponding voltages.

Answer 17

• Very low (8 - 50 kVp)
• Low (50 - 160 kVp)
• Medium (160 - 300 kVp)

Question 18

The COP gives chamber correction factors, why are these needed?

Answer 18

The chamber specific correction factors account for the presense of the chamber in the path of the beam, and account for the requirement that the very low and medium energy calibrations require a dose to water and not in air.

Question 19

Give the equation required to calculate dose for a medium energy beam. Using the in-phantom protocol.

Answer 19

D = M * N * k_{ch} * mu_{en}/rho @ 2cm depth

where M is the temperature and pressure corrected raw measurement. k_{ch} is the chamber correction factor. mu_{en}/rho is the ratio of mass energy absorption coefficients for air to water for a given field size.

Question 20

Give the equation required to calculate dose for a low energy beam (the same as for medium energy in air).

Answer 20

D = M * N_{k} * B_{w} * mu_{en}/rho @ depth of 0 cm,

where M is the temperature and pressure corrected raw measurement, N_{k} is the air kerma to water kerma calibration determined by the NPL, B_{w} is the backscatter correction factor, mu_{en}/rho is the ratio of mass energy absorption coefficients.

Question 21

Why use low energy x-rays at all?

Answer 21

• Shallow high dose penumbra. This allows for small margins and hance small and tightly conformed treatment areas.
• Is easily shielded.
• Treatment units are cheaper than electron treatment units.

Question 22

Why are the isocontours curved for SXT cross-field profiles?

Answer 22

This is due to the ISL, due to the short SSDs, and due to the obliquity of filtration.

Question 23

How is the heel effect compensated for?

Answer 23

Optmising of the target angle, with the aim being to give the maximum beam uniformity and intensity across the beam. This is achieved by combining the angle of maximum X-ray production with the heel effect.

Question 24

What are the clinical advantages, and considerations when considering superficial X-rays as a treatment?

Answer 24

• To treat superficial lesions.
• Small fields, and therefore small dimenions.
• It is worth noting that due to the low energy spectra considered, a substantial dose increase is observed when considering higher Z materials (due to photo-electric effect).

Question 25

What is the stand-off correction and how is it accounted for?

Answer 25

These treatments assume the surface is at the end of the applicator. If this is not the case then an ISL correction is required. This is the standoff correction.

It is defined as:
SOC = FSD / (FSD + d)^2
where FSD is the focal spot to surface distance and d is the difference in distance from the end of the applicator to the surface.

Question 26

What parameters can affect the clinical beam output?

Answer 26

• Applicator dimensions.
• Cutouts.
• ISL.
• Internal and external shielding.

Question 27

What are example beam qualities for SXT and orthovoltage units?

Answer 27

SXT: 1-8 mm Al.
Ortho: 0.5-4 mm Cu.

Question 28

What is the benefit of a cutout?

Answer 28

All superficial treatment areas are treated with cutouts. The purpose of these is to conform to the target. They also help to avoid geometric misses (immobilisation devices are not used).

Question 29

How to measure backscatter?

Answer 29

Use LiB TLD placed on a phantom’s surface, which flip the gantry by 180 degrees and measure dose without phantom. Take ratio of these doses.

Question 29

How to measure backscatter?

Answer 29

Use LiB TLD placed on a phantom’s surface, which flip the gantry by 180 degrees and measure dose without phantom. Take ratio of these doses.

Question 30

Describe how a backscatter changes with beam diameter.

Answer 30

The BSF will increase initially since the amount of phantom scatter will increase. However, this quantity will plateau due to attenuation of scattered photons.

Furthermore, BSF will increasae with increasing HVL as range of scatter increases. Will also take longer to plateau.

Question 31

Describe how BSF varies with HVL?

Answer 31

Initially, the BSF will increase due to the increasing contribution of compton scattered photons. However, after a peak value, the BSF will decrease due to the photons being increasing forward peaked.

Question 32

How is the backscatter factor ratio defined?

Answer 32

BSFR = BSF with cutout / BSF with open field

Question 32

How is the backscatter factor ratio defined?

Answer 32

BSFR = BSF with cutout / BSF with open field

Question 33

How does an internal shield impact on the dose upfield from the shield?

Answer 33

Upstream the dose will decrase due to the lack of back scatter from deeper within the patient.

However, just upstream from the shield there is a spike in dose due to a small amount of backscatter from the shield, and the generation of photo-electrons. Therefore, a thin piece of water-equvialent material is used to attenuate this contribution.

Question 34

How is this internal shielding dose measured?

Answer 34

Use Al in place of the typical gold.

Place varying amounts of mylar to simulate tissue upstream from the shield. Use a parallel plate detector to sample the dose.

It is typical that the dose will spike just upstram from the shield. However, this dose will decrease with increasing distance from the shield (due to lack of back scatter from further downstream). However, if a sufficient amount of mylar is positioned, then the full backscatter conditions will be measured.