Radiotherapy application

The Radiotherapy application contains some utilities for the simulation of a teletherapy linear accelerator and dose calculations in the patient. Many of the utilities can also be useful in the simulation of brachytherapy treatments.

Geometrical modules

Two geometrical modules are defined related to Radiotherapy: JAWS and MLC (multileaf collimators). They can be defined in the usual geometry text file (see chapter on Geometry) with the format described below.

JAWS module

This module serves to describe the jaws of a radiotherapy accelerator. The jaws are described by their dimensions and its rotation is calculated with the parameters of its projection in into a plane

The following parameters have to be provided

:MODULE JAWS

Module name

Module type, giving the direction of the jaws. It has to be X or Y.

- End leaf type It must be STRAIGHT, ROUND, ROUND_DISP. ROUND_DISP means that the centre of the circle that defines the profile is not at the line in the middle of the leaf.

X half dimension

Y half dimension

Z half dimension

- If the end leaf type is ROUND or ROUND_DISP, end leaf radius

- If the end leaf type is ROUND_DISP, distance between leaf top and radius centre. If the end leaf type is ROUND, the radius center is at the middle of the leaf, so this parameter is not needed.

- If the end leaf type is ROUND or ROUND_DISP, half value layer . This half value layer should correspond to the distance at which only half of the gammas with the spectrum generated by the accelerator traverse the material of the leaf. It is used to move the leafs so that the line from the focus to the aperture position in the isocenter traverses a distance of the rounded leaf end equal to the half value layer.

Z position of the focus

Radius of rotation of aperture movement (usually the same as the distance between the focus and the volume centre)

Z of the position where the centre of the volume would be if the field were 0 (no rotation)

Z position of the isocenter, where the projections are calculated

Field X1 or Y1 projection

Field X2 or Y2 projection

Material

Parent volume name

MLC module

Introduction

A MLC (Multi Leave Collimator) is a collimator system composed of a set of pairs of leaves a few millimeters thick that serves to conform a hole in the field of view of an irradiation treatment beam. These systems are used to shape irregulars volumes.

This module can describe a high variety of MLC models, whether focused to a point in the Z axis or to an off-axis point. Any shape of leaf profile can be described by enumerating the list of 2-dimensional points that describe it, and two endleaf types are supported: rounded or straight. The endleaf straight implicates a circular leaf shape, as in the Siemens PRIMUS MLC. Each leaf cross profile is described by its projection onto a reference plane. The endleaf position is described by its projection at the isocenter plane.

Geometrical module definition and variables list

The MLC module description has to start with the two words (words have to be separated by blank spaces, and follow the order described here, but there is no constraint on the number of lines they may occupy):

:MODULE MLC

Then the following parameters have to be filled

- Module name

- Module type . The type of MLC has to be FOCUSED, what means that all leaves profiles in the cross place are focused to a point. Type UNFOCUSED will be supported in future releases.

- Leaves orientation . Orientation of the leaves, i.e. the axis of movement, can be x, y, X or Y

- End leaf type . It must be STRAIGHT, ROUND, ROUND_DISP. ROUND_DISP means that the centre of the circle that defines the profile is not at the line in the middle of the leaf.

- Leaf half dimension in the inline plane. It is X direction if the orientation is X and Y if it is Y. If the endleaf is curved this length varies, and it is defined as the maximum extent. If the endleaf is straight, and therefore it has a circular shape, the length also varies, and it is defined as the arc length of the upper part of the leaf (the side closest to the source).

- If the end leaf type is ROUND or ROUND_DISP, end leaf radius

- If the end leaf type is ROUND_DISP, distance between leaf top and radius centre. If the end leaf type is ROUND, the radius center is at the middle of the leaf, so this parameter is not needed.

- If the end leaf type is ROUND or ROUND_DISP, half value layer . This half value layer should correspond to the distance at which only half of the gammas with the spectrum generated by the accelerator traverse the material of the leaf. It is used to move the leafs so that the line from the focus to the aperture position in the isocenter traverses a distance of the rounded leaf end equal to the half value layer.

- Z of plane where leaf gap is defined . The Z position is defined with respect to the Z position of the top of the leaves.

- Z position of the focus

- Cross position of the focus . It the leaves orientation is X the cross position is in the Y axis and viceversa.

- Z of position of the isocenter . The XY plane at this Z is used to calculate the cross positions that define the leaf profiles as well as the positions that the define the leaf apertures.

- Z of position of the top of the leaves. This position will be used as the 0 reference to place the leaves. In fact, it is used as the Z=0 position for defining the leaf profiles; this means that it will only be the top of the leaves if the leaf profiles have been defined so that the minimum Z is 0. By "top" we refer to the position closer to the source; usually the linac extends along the positive Z direction, so the top refers to the position with minimum Z.

- Gap between leaf in the cross direction . Separation between leaves defined at Z_GAP.

- Cross leaf starting point . Cross position of the leaf situated at the most negative position. It is defined at the same Z plane as the leaf gap.

- Number of different types of leaf cross profiles.

For each leaf cross profile type the following data has to be defined:

  • - Leaf type. It will be a LEAF or BLOCK, but this options is not implemented yet, so it must be LEAF

  • - Number of 2-dimensional points that define the cross profile

  • For each point the Z and cross coordinates must be defined

    - Z coordinate

    - Cross coordinate

Number of leaf pairs

For each leaf pair the following data has to be defined:

  • Leaf type number 1, 2, 3, ...; following the order of leaf types defined

  • - Cross projection of the left part (negative coordinate) of the leaf on isocenter plane

    The projection is taken as that of the leaf profile point that gives an smaller (more negative) coordinate.

  • - Opening distance of negative leaf, projected on the isocenter plane In case of rounded end leafs, the point projected is not the end of the leaf, but a line is traced from the source towards the isocenter plane traversing the leaf, so that the distance traversed is the same as the half value layer given above.

  • - Opening distance of positive leaf, projected on the isocenter plane

Finally the material of the leaves have to be defined

material

Mother volume name

Module MLC example

On the next lines we include a simplified geometrical description, that you can use as example to build your own. On the figure 1 and 2 we can see the corresponding representation dimensions.


  :MODULE MLC                 // MLC // Realistic_CASE 
  myMLC_x                     // Module_name   
  FOCUSED X STRAIGHT          // Type, Orientation, End_leaf_type
  -5  2.5 200                 // Z_focus, Cross_Leave_Focus Z_isocenter
  100                         // Z_top
  297.024                     // Leaf_length
  10.                         // Half_Value_Layer
  5                           // N Leaves Cross Profiles
  LEAF 14                    // N Leaves Cross Points and type 
  -2                   -5.0/3 // Z  and C coordinate
  1                    -5.0/3
  1                    -5.0/3 
  75.8                 -5.1/3
  75.8                  5.0/3
  75.8-1                5.0/3
  75.8-1               18.7/3
  22.24+25+0.05        18.7/3
  20.24+25+0.05        18.7/3-0.65
  22.24-0.05           18.7/3-0.65
  20.24-0.05           18.7/3
  1                    18.7/3
  1                     5.0/3
  -2                    5.0/3
  LEAF 32                     // N  Leaves Cross Points and type
  -2                    -2/3  // Z  and C coordinate
  1                     -2/3
  2                  -2.78/2
  6                  -2.78/2
  6+1                -2.78/2+1 
  6+2                -2.78/2
  20.24              -2.78/2
  22.24              -2.78/2-0.65
  20.24+25           -2.78/2-0.65
  22.24+25           -2.78/2
  22.24+25+6         -2.78/2
  22.24+25+7         -2.78/2+1
  22.24+25+8         -2.78/2
  75.8-1             -2.78/2
  75.8-1                -1/2 
  75.8                  -1/2
  75.8                   4/5
  75.8-1                 6/5
  75.8-1              2.78/2
  75.8-6              2.78/2
  75.8-7              2.78/2-1
  75.8-8              2.78/2
  22.24+25+0.05       2.78/2
  20.24+25+0.05       2.78/2-0.65
  22.24-0.05          2.78/2-0.65
  20.24-0.05          2.78/2
  20.24-0.05-6        2.78/2
  20.24-0.05-7        2.78/2-1
  20.24-0.05-8        2.78/2
  2                   2.78/2
  1                   2.78/2-1/2
  -2                  2.78/2-2/3
  LEAF 32                       // N  Leaves Cross Points and type
  -2                    -1/2    // Z  and C coordinate
  1                     -1/2
  2                  -2.78/2
  20.24-6-2          -2.78/2
  20.24-6-1          -2.78/2+1 
  20.24-6            -2.78/2
  20.24             -2.78/2
  22.24             -2.78/2-0.65
  20.24+25          -2.78/2-0.65
  22.24+25          -2.78/2
  75.8-8            -2.78/2
  75.8-7            -2.78/2+1
  75.8-6            -2.78/2
  75.8-1            -2.78/2
  75.8-1               -6/5 
  75.8                 -4/5
  75.8                  1/2
  75.8-1                1/2
  75.8-1             2.78/2
  22.24+25+8         2.78/2
  22.24+25+7         2.78/2-1
  22.24+25+6         2.78/2
  22.24+25+0.05      2.78/2
  20.24+25+0.05      2.78/2-0.65
  22.24-0.05         2.78/2-0.65
  20.24-0.05         2.78/2
  8                  2.78/2
  7                  2.78/2-1
  6                  2.78/2
  2                  2.78/2
  1                  2.78/2-1/2
  -2                 2.78/2-2/3
  LEAF 15                     // N  Leaves Cross Points and type 
  -2                -5.0/3    // Z  and C coordinate
  1                 -5.0/3
  1                -18.7/3 
  20.24            -18.7/3
  22.24            -18.7/3-0.65
  20.24+25         -18.7/3-0.65
  22.24+25         -18.7/3
  75.8-1           -18.7/3
  75.8-1           -18.7/3 
  75.8-1            -5.0/3
  75.8              -5.0/3
  75.8               5.0/3
  1                  5.0/3
  1                  5.0/3
  -2                 5.0/3
  7                         // Total Leaf Number
  1    -1.0     1           // Leaf_type, Left and Right at isocenter  
  3    -1.0     2           
  2    -1.0     3   
  3    -1.0     4   
  2    -1.0     5   
  3    -1.0     6  
  4   -10.0    29 
  G4_W                     // Material

Figure 1. Cross leaves representation.

Figure 2. Inline leaves representation for endleaf type ROUND.

If we change the END_LEAF_TYPE for STRAIGHT type, the variable RDIM represents a new value named END_LEAF_FOCUS. If we set it to -10 mm, the leaf profile changes as it is described in the figure 3.

Figure 3. Inline leaves representation for endleaf type STRAIGHT.

The example uses four leaf cross profiles types, the data written are relative to the Z ref (Z_TOP) plane. In the figure 4 we can see a representation of each profile referred to the Z_TOP plane, and the result after the corresponding projection in the final geometry representation.

Figure 4. Cross leaves types representation relative to the Z_TOP plane for four leaf cross profiles and its representation in the final geometry.