BRT HDR della prostata: analisi RNM multiparametrica. Dose painting

Transcript

1 BRT HDR della prostata: analisi RNM multiparametrica Dose painting

2 CONTENUTI 1. Introduzione 2. Impianti e sorgenti 3. BRT ad alto rateo di dose (HDR) 4. BRT HDR ginecologica 5. BRT HDR della prostata 6. Cenni ad altre applicazioni di BRT HDR 7. Il calcolo della dose 8. La pianificazione di un trattamento

3 CENNI AD ALTRE APPLICAZIONI DI BRT HDR

12 CENNI AD ALTRE APPLICAZIONI DI BRT HDR Accelerated partial breast irradiation (APBI) Prescrizione e frazionamento: 3.4 Gy per 10 fz, 2 fz al giorno intervallate di almeno 6h

15 CENNI AD ALTRE APPLICAZIONI DI BRT HDR: APBI

16 CENNI AD ALTRE APPLICAZIONI DI BRT HDR Indici di qualità del trattamento DHI V V = Dose Homogeneity Index V100 COIN = PTV PTV PTV ref V ref Conformal Index

18 Dose calculation The formalism of dose calculation in brachytherapy is defined in AAPM TG-43 publication and in its subsequent update.

19 Dose calculation Equation for dose calculation: (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) r X ϑ

20 Dose calculation: air kerma strength (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) S K : Air kerma strength: is the air kerma rate on the trasversal axis at a reference distance from the source, measured in free-air geometry Units: 1 U = 1 cgy cm 2 h -1

21 Dose calculation: air kerma strength 1. AK strength and reference air kerma rate are numerically identical, they are related by equation: 2. AK strength and apparent activity (A app ) of the source are related by equation: S K = A app Γ AKR where Γ AKR is the air kerma rate constant [µgy m 2 GBq -1 h -1 ]

22 Dose calculation: air kerma strength Once the physical source characteristics are known, one out of the three physical quantities is sufficient to easily determinate the other two. S K A app

23 Dose calculation: dose rate constant (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) Λ : Dose rate constant: converts the value of S K, measured in air, to the value of dose rate at the reference point P(r 0,ϑ 0 ), measured in water Units: cgy h -1 U -1 which reduces to cm -2

24 Dose calculation: dose to a reference point (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) D& ( ) r 0,ϑ 0 The first part of the equation returns the dose rate in water at the reference point P (r 0,ϑ 0 ) P X r 0 = 1cm ϑ 0 = 90

25 Dose calculation: constants Λ and Γ AKR depend on both the radionuclide and source model, and are influenced by both the source internal design and the experimental methodology used to measure S K

26 Dose calculation: dose to all points (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) Starting from the dose rate value in P(r 0,ϑ 0 ), second part of the equation returns the dose rate value in water in each other point Q(r,ϑ) of the space P X (r 0, ϑ 0 ) Q X (r, ϑ)

27 Dose calculation: geometry factor (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) G r G r0 (, ϑ) (, ϑ ) : 0 Geometry Factor: provides an effective inverse square law correction, accounting for the active length of the source

28 Dose calculation: geometry factor G r, ϑ r 2 and when r>>l: ( )

29 Dose calculation: radial dose function (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) g(r) : Radial Dose Function: This function describes the relative variation of the dose in water along the transverse axis of the source. It accounts for absorption and scatter along this axis.

30 Dose calculation: radial dose function Ir-192 data

31 Dose calculation: anisotropy function (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) F ( r,ϑ): Anisotropy Function: This function describes the relative variations in dose at points away from the transverse axis. It corrects for oblique filtration through the source active material (auto-absorption) and the source wall.

32 Dose calculation: anisotropy function Ir-192 data

33 Dose calculation: Data for Nucletron Ir-192 source

34 Dose calculation: important approximations (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) Equation is defined for a HOMOGENEOUS and INFINITE WATER medium. In some specific cases, these approximations are not negligible.

35 QA for dose calculation (, ϑ) (, ϑ ) ( ϑ) G r D& r, = S Λ g( r) K G r 0 0 F ( r, ϑ) It s the only one changing with time and at each source replacement. Certified by the vendor, it requires QA at least at each source new installation.

37 LA PIANIFICAZIONE DI UN TRATTAMENTO Partendo dalle immagini CT della regione comprendente l impianto, la pianificazione di un trattamento può essere schematizzata in 4 passaggi fondamentali

38 LA PIANIFICAZIONE DI UN TRATTAMENTO 1.Ricostruzione del catetere/dei cateteri

39 Catheter / applicator reconstruction Aim of the catheter / applicator reconstruction procedure is to describe to the system the shapes of the treatment catheters which have been introduced into the patient, and which can be seen on the tomographic images (e.g., CT or MR) or on any cross-section of the image data set (Multi- Planar Reconstruction images MPR). This procedure is performed by placing strategic points on the images along the lengths of the catheters.

40 Catheter / applicator reconstruction If a catheter is straight, two points are sufficient to describe it, one at each end of the catheter. If a catheter is curved, additional points around the curve are required to describe it. The closer the reconstruction points, the more accurately the curve is described.

41 Multi-planar Reconstruction (MPR) Images By using Multi-Planar Reconstruction (MPR) images, catheter reconstruction points can be added, inserted, moved and deleted on each of the transversal, sagittal, coronal or oblique planes. Before placing catheter reconstruction points, the view should be rotated so that the complete catheter is (as much as possible) in one plane.

42 Extra Coordinate System (ECS) The MPR images can be adjusted for catheter reconstruction by rotation and displacement of the axes of an extra coordinate system (ECS) By doing this you can find the planes cutting the complete catheter or a large part of the catheter for placing catheter reconstruction points

43 Catheter reconstruction with MPR and ECS Multi-Planar Reconstruction (MPR) Z x-z plane y z x x

44 Catheter reconstruction with MPR and ECS Extra Coordinate System (ECS) z x -z plane y z x x

45 Catheter reconstruction with MPR and ECS Extra Coordinate System (ECS) z x -z plane y z x x Now you are able to place the reconstruction points along the entire applicator on the x -z plane image

46 Catheter reconstruction with MPR and ECS Oblique MPR/ECS views are very powerful tools: they can provide the best view of the catheter in which a very accurate reconstruction can easily be done.

47 Manual catheter reconstruction Definition of the shape of the catheter / applicator is not sufficient for having a complete description of it to be used for treatment planning. At least one reference point has to be defined and its distance from the brachytherapy unit has to be given with great accuracy. 1) first source dwell position (i.e., the most distal) and 2) its distance to the brachytherapy unit indexer are chosen as references.

48 Manual catheter reconstruction It is like having a treasure map Start Path Treasure? If you want to find with great accuracy the treasure, knowing the path it s not sufficient; it is as well necessary to be known the number of steps to be taken to reach it.

49 Manual catheter reconstruction: 1 st dwell position 1) FIRST SOURCE DWELL POSITION Reconstructed by using localization markers Deduced starting from the tip of the catheter / applicator (without localization markers)

50 Catheter reconstruction with localization markers To simulate the possible dwell positions, localization markers are used in order to: Easily distinguish them on the CT/MRI slices Accurately depict the possible dwell positions Coded such that the user can determine which markers correspond to a given catheter or applicator The main factor determining the accuracy in the reconstructed localization is the distance between CT/MRI consecutive image slices

51 Catheter reconstruction with localization markers first dwell position, the most distal point that might be reached by the center of the source (reference point) RECONSTRUCTION POINT L P Localization marker in tungsten inside the catheter

52 Catheter reconstruction without localization markers First source dwell position is reconstructed starting from the catheter tip. Knowing the distance from the catheter tip to the center of the first possible dwell position, the tip can be digitized and the distance entered as a negative offset. No localization marker inside the catheter RECONSTRUCTION Virtual POINT + first OFFSET dwell position L P

53 Catheter reconstruction without localization markers L P : catheter tip length Offset = L P + L C + L s L C : catheter safety margin; clearence between the source tip and inner end of the catheter L S : length source center-outer source margin Virtual first dwell position L P

54 Manual catheter reconstruction: indexer length 2) DISTANCE OF THE FIRST SOURCE DWELL POSITION TO THE BRACHYTHERAPY UNIT INDEXER

55 Manual catheter reconstruction After having defined the way to lacalize the first dwell position, its important to define the distance of that reference point from the brachytherapy unit. Reference distance Indexer ring first dwell position, center of the source Indexer length: In most of the cases this is a standard value which is depending on the type of the afterloader and the transfer tubes between the indexer and the applicator

56 Applicator / catheters QA At their commissioning, always perform QA of the applicator / catheters in combination with their transfer tubes and their simulation markers.

57 Applicator / catheters QA

58 LA PIANIFICAZIONE DI UN TRATTAMENTO 2. Impostazione delle possibili posizioni di stazionamento per la sorgente all interno del catetere/dei cateteri

59 LA PIANIFICAZIONE DI UN TRATTAMENTO 3. Definizione dei punti di prescrizione della dose

60 Prescription points Patient points: The points are linked to the patient coordinate system. Applicator points: The points are linked to the applicator (extra) coordinate system (ECS). Dose points can be calculated based on: the axis of the applicator co-ordinate system. the shape of the catheter (related to dwell pos.) the shape of the target (random distribution) the Paris system: basal dose points. the distance at lowest dose. manual automatic

61 Dose points: catheter dose points Distributed at a fixed distance from the catheter d Catheter dose points can be created in space around an applicator in a way that they will represent the shape of the target volume, even if the target volume is not or can not be defined directly.

62 Dose points: target dose points Distributed randomly over all the target volume Target Number of dose points must be not less than the number of selected source dwell positions

63 Dose points: basal dose points Geometrical methods of the Paris dosimetry system Triangles Squares Lines Basal dose points are important to limit the dose inside the catheter implant

64 LA PIANIFICAZIONE DI UN TRATTAMENTO 4. Ottimizzazione della dose a punti di prescrizione

65 LA PIANIFICAZIONE DI UN TRATTAMENTO 3./4. Definizione dei punti di prescrizione della dose e ottimizzazione della dose L algoritmo di ottimizzazione della dose si basa sul metodo dei minimi quadrati. Esso determina il tempo di stazionamento della sorgente per ogni posizione impostata, al fine di ottimizzare la dose ai punti di prescrizione t 1 t 2 t 3 t 4 t 5

66 Optimization: definition When treating with a stepping source afterloading unit, both the dwell positions and the dwell times can be adjusted to improve the dose distribution by keeping the prescription isodose as close as possible to the dose points and the doses as homogeneous as possible through the implant. This procedure of adapting the dose distribution is called dose optimization.

67 Optimization: goals Homogeneous dose distribution inside the implant Coverage of the target with minimal required dose High dose gradient outside the target (implant) with resulting sparing of critical organs

68 Optimization: possible methods Manual dwell weights/times Graphical optimization Points optimization

69 Optimization: manual dwell weights / times Using this method, optimization is done by manual change of the dwell weights or times Because the weight is a relative number, the highest weight value in the whole set is always rescaled to 1. However, absolute times are not normalized, so the user has to use this method carefully!

70 Optimization: graphical Tool for moving isodose lines in order to optimize the dose distribution. Adjustable behaviour: from Global to Local correction Global optimization Before optimization Local optimization

71 Optimization: points based The algorithm for points based optimization works on dose points located at a given distance from each dwell position and is based on the minimisation of a mathematical objective function iterative calculation

72 Optimization: points based The function to be minimized consists of the squares of the difference ε i of the prescribed dose D prescr,i at each dwell position i and the actual dose D calc,i at that dwell position. : dose contribution from dwell position i to point j

73 LA PIANIFICAZIONE DI UN TRATTAMENTO 3./4. Definizione dei punti di prescrizione della dose e ottimizzazione della dose peso relativo posizione sorgente

74 Optimization: points based A parameter called Dwell Time Gradient Restriction (DTGR) was introduced to restrict the differences between successive dwell weights. The DTGR allows dwell weights to be restrained in their fluctuations.

75 Optimization: points based Increasing DTGR Decreasing dwell times fluctuations and negative values

76 Il brachy-team Grazie per la vostra attenzione