Depth Dose in Radiotherapy (Percentage).pptx

PDD

1. Dose Distribution By- Dr. Shubhangi Thanvi 2nd year resident

2. Topics Introduction Phantoms Depth Dose Distribution Percentage Depth Dose Factors affecting PDD Isocentre

3. Introduction Data on dose distribution are derived from measurements in Phantoms. This data is used in a dose calculation system devised to predict dose distribution in an actual patient. For Low Energy Beams- PDD, TAR, SAR. For High Energy Beams- Tissue phantom ratio and tissue maximum ratio, are used.

4. Phantoms are Tissue Equivalent materials, large enough to provide full scatter conditions for the given beam. Basic dose distribution data are usually measured in water Phantoms which closely approximate the properties of muscle and other soft tissues. Advantage of water phantoms- universally available with reproducible radiation properties. Disadvantage- radiation detectors are affected by water. Solid phantoms are devloped with same atomic number, electron density and mass density. Most important is to have same electron density. PHANTOMS

5. Water Polystylene Polyethylene Paraffin Solid water- epoxy resin based mixture. One such commecially available system known as Alderson Rando Phantom, shaped into a human torso. Examples

6. Depth Dose Distribution As the beam is incident on a patient or a Phantom, the absorbed dose in the patient varies with the depth. This variation depends on- beam energy, depth,field size, distance from source, beam collimation system. An essential step is to calculate depth dose variation along central axis of beam. These quantities are usually derived using small ion chambers as they have - better precision and smaller energy dependence. and are calculated in water Phantoms.

7. Percentage Depth Dose PDD is the quotient, expressed as a percentage, of the absorbed dose at any depth d to the absorbed dose at a reference depth d , along the central axis of the beam. For low energies d is taken zero(surface) For high energies it is taken at peak absorbed dose (D max ).

8. Parameters afffecting PDD 2.Depth 3.Field size 1.Beam Quality or energy 4.Source to surface distance

9. Beam quality or energy- PDD increases with beam energy beyond the depth of maximum dose, this is because higher energy beams have greater penetrating power and thus deliver higher percentage depth dose at a given depth. The beam quality affects the PDD by avergae attenuation coefficient μ, as the μ decreases ,the beam becomes more penetrating so PDD increases.

10. DEPTH Beyond the depth of dose maximum PDD decreases with depth for a particular energy. Factor1- Inverse square law. Factor2- Exponential attenuation of primary photon beam.

11. The initial build up region Before the D max there is an initial build up region of PDD with depth. In low energy rays, dose build up to a maximum is very close to the surface. In high energy rays, point of maximum dose lies deeper into the tissue or phantom. This gives a Skin Sparing Effect , for megavoltage beams such as cobalt-60 and higher energies the surface dose is much smaller than D max. Thus, higher doses can be given to deep seated tumors without exceeding the tolerance of the skin.

12. The surface dose is the contribution of - Photons scattered from the collimator. Photons backscattered from the patient. High energy electrons produced by the photon interactions in air and any shielding structure in the vicinity of the patient. How is the build up region formed- three beams of radiation will be considered for analysis 250 kv xray - low energy beam Cobalt 60 machine - mid energy beam 6 MV linac - high energy beam

13. Build up phenomenon and kerma Kerma represents energy transferred from photons to directly ionizing electron. (kinetic energy released in medium) Kerma is maximum at surface then decreases with depth whereas absorbed dose first increases with depth, approximately upto the range of electrons in medium and there after decreases with depth.

14. Inverse Square Law The radiation intensity will decrease with the inverse square of the distance from the source. Applicable to only point source which spreads its intensity equally in all directions. All measures of exposure will drop off by inverse square law.

15. Exponential Attenuation When a beam of X ray passes through the matter its intensity is reduced. Process of removal of X ray photons either by absorption or scattering is called as Attenuation. Monoenergetic beam will follow the exponential attenuation.

16. Field Size It may be specified either Geometric or Dosimetric. Geometric field size- Projection on a plane perpendicular to the beam axis as seen from the centre of the source. Dosimetric field size- Distance intercepted by a given isodose curve, usually 50% isodose, on a plane perpendicular to the beam axis at a stated distance from source.

17. PDD with Field size As the field size increases the PDD increases with it. This is due to more contribution of the scatter radiation since the volume that can scatter radiation gets larger with field size. This also depends on energy of the beam, with higher energy there in more forward scatter of the photons so increase in PDD will be less pronounced.

18. PDD with SSD SSD- source to surface distance Percentage depth dose increases with SSD as a result of inverse square law. Although the actaul dose rate at a point decreases with increase in distance from the source, the PDD which is a relative dose with respect to a reference point, increases with SSD. Higher SSD is used in daily practice machines to make the source dimension unimportant(as clinically the source not actually a point source). PDD for clinical use are usually measured at a SSD of 80cm- cobalt60 and 100cm- linac.

19. Thus, the PDD for standard SSD must be converted to those applicable to the actual treatment SSD. For this Mayneord F factor is used .

20. Application of PDD PDD is mainly used in calculation of treatment time. Treatment time= prescribed dose/dose at d max . x PDD at treatment depth

21. Isocentre It is a point at which the centre of Gantry, Couch and Collimator coinside. All machines are calliberated according to isocentre. According to isocentre there are two types of treatment 1. SSD type- Isocentre is on skin or surface 2. SAD type- Isocentre is at a depth, mostly at the centre of deep tumor. source to isocentre distance - cobalt is 80cm - linac is 100cm

22. Thank You