Dose Distributions George Starkschall, Ph.D. Department of Radiation Physics U.T. M.D. Anderson Cancer Center Purpose To familiarize the resident with dose distributions and the factors that affect them Isodose distributions Cannot use PDD tables to display doses to off-axis points Use isodose distributions Show distribution of doses within patient volume 1
Specifications To select an isodose distribution, specify: beam quality therapy machine field size SSD Sample isodose distribution Theratron-780 60 Co machine field size 10 cm 10 cm SSD 80 cm Sample isodose distribution Note: beam profile curved -- less dose at edges of field due to decrease in scattered radiation 2
Sample isodose distribution Note: large penumbra due to extended 60 Co source Definition of field size Area covered by 50% isodose curve at depth of central axis maximum Consequences: smaller area receives substantial dose, such as prescription dose field of specified width will cover area smaller than specified width Normalization single field isodose curves normalized to 100% at d max other normalizations possible, e.g. 100% at isocenter 100% of unmodified field at d max always important to specify normalization of dose distribution 95% means 95% of what? 3
Examples The following isodose distributions characteristic of types of photon beam isodose distributions frequently encountered in clinical radiotherapy: Orthovoltage x-ray beam 200 kvp 2.0 mm Cu HVL SSD=50 cm d max occurs at patient surface rapid fall off with depth -- PDD at 10 cm 35% Orthovoltage x-ray beam sharp beam edge due to small focal spot of incident electron beam significant dose contribution outside geometric edge of beam due to Compton scattered radiation at lower beam energies 4
Cobalt machine SSD 80 cm d max at 0.5 cm increased penetration of photons -- PDD at 10 cm 56% Cobalt machine beam edge not as well defined -- larger geometric penumbra due to extended cobalt source Cobalt machine dose outside geometric beam small due to large forward orientation of scattered radiation 5
6 MV photons characteristic of isodoses from lower energy megavoltage machine d max at 1.5 cm PDD at 10 cm of 67% 6 MV photons geometric penumbra smaller than that of cobalt because effective source size much smaller 20 MV photons characteristic of isodose distribution from higher energy megavoltage accelerator d max at 4.5 cm PDD at 10 cm 80% 6
20 MV photons increased amount of surface dose, probably due to contamination radiation generated in machine head 20 MV photons noticeable for higher energy accelerators higher energy than that generated in low energy accelerator more penetrating Effect of field size Compare 5 cm 5 cm field with 10 cm 10 cm field for 60 Co central axis depth dose larger for larger field size 7
Effect of field size increase in amount of scattered radiation for smaller field very small area over which field is flat Use of wedges Normally isodose curves intersect central axis of beam at right angles Modify angle by use of wedge filter Use of wedges define wedge angle as angle between some specified isodose line (50% isodose) and line perpendicular to beam axis isodose curves here normalized to central axis d max for unmodified field d max 8
Combinations of fields With single radiation field, dose at d max generally exceeds dose at depth in order to deliver desired dose to tumor with single field, necessary to overdose healthy tissue problem overcome by use of multiple radiation fields Parallel-opposed fields most common form of multiple field treatment generally calculate treatment time required to deliver specified dose to midline Central-axis depth doses Compare central axis depth dose for different modalities and different patient diameters Note: lower energy radiations have maximum dose at d max just below the surface higher energy radiations have deeper d max and large flat area for large patient diameters 9
Acute hinge angle hot spot likely align fields so intersection lies past hot spot -- past pointing weight third field less than oblique fields large area over which the dose is relatively uniform 3-field plan 10
Wedge plan Note: thick sides of wedges positioned next to one another wedge angle can be related to hinge angle by wedge angle = 90 ½ ½ hinge angle Dose-volume histogram Plot of fraction of volume of region of interest receiving at least specified dose Dose-volume histogram DVH described by solid line indicates that 40% of volume of region of interest receives at least 40 Gy 11
Applications of DVH Assessing potential success or failure of treatment plan Comparing treatment plans Shortcomings of DVH Does not indicate where problems may occur Does not account for fractionation 12