sec.3 practical application 临床处方剂量的计算. what we should calculate? time or mu...

Sec.3 Practical application

临床处方剂量的计算

What we should calculate?

Time or mu required to delivery proper dose for tumor

Dose distribution at the region of tumor

What kind of Parameters we should consider in dose

calculation?

Field size and shape

Depth

Beam quality

Calibration of output : SSD technique or SAD technique:

Radiotherapy institution vary in their treatment techniques and

calibration practices.

处方剂量 (prescription dose)

为达到一定的靶区（或肿瘤）剂量（ DT）所需要的使用射野的射野中心轴上最大剂量点处的剂量，即处方剂量 Dm。

Rember :prescription dose is not air dose

The main contents

Accelerator calculations1. SSD technique

2. Isocentric technique

Cobalt-60 calculations Irregular fields Other practical methods of calculating

depth dose distribution

SSD technique Percent depth dose is a suitable quantity for calculations

involving SSD techniques.

Machines are usually calibrated to deliver 1rad(10-2Gy) per monitor unit(MU) at the reference field size 10×10cm and a source-to-calibration point distance of SCD.

Assuming that the Sc factors relate to collimator field sizes

defined at the SAD, the monitor units necessary to deliver a certain tumor dose(TD) at depth d for a field size r at the surface at any SSD are given by the following.

SSD technique

SSDfactorrSrSPDD

TDMU

pcC

SSD technique

SSD technique

2

0

tSSD

SCDSSDfactor

SSD

SADrrc

Where rc is the collimator field size, given by

It must be remembered that, whereas the field size for the SC factor is defined at the SAD, the Sp factors relate to the field irradiating the patient.

Example 1. A 4-MV linear accelerator is calibrated to give

1rad per MU in phantom at a reference depth of

maximum dose of 1cm, 100cmSSD, and

10×10cm field size. Determine the MU values to

deliver 200rads(cGy) to a patient at 100cm

SSD, 10cm depth, and 15×15cm field size,

given SC(15×15)=1.020, Sp(15×15)=1.010,

PDD(10,15×15,100)=65.1%

Answer:

2981010.1020.1651.0

200

SSDfactorSSPDD

DTMU

PC

Example 2. Determine the MU for the treatment conditions

given in Example 1 except that the treatment SSD is 120cm, given SC(12.5×12.5)

=1.010 Answer:

697.01120

1100

5.12120

10015(100cm) SADat projected size Field

2

SSDfactor

442697.0010.1010.1667.0

200

MU

Isocentric technique TMR is the quantity of choice for dosimetric calculations involving

isocentric techniques.

Since the unit is calibrated to give 1rad/MU at the reference depth t0,

calibration distance SCD and for the reference field (10cm×10cm),

then the monitor units necessary to deliver isocentric dose (ID) at

depth d are give by the following:

2

factor

,

SAD

SCDSAD

SADfactorrSrSrdTMR

IDMU

dpcCd

Example 3. A tumor dose of 200 rads is to be delivered at the isocentric which

is located at a depth of 8cm, given 4-MV x-ray beam, field size at

the isocentric=6×6cm, Sc(6×6)=0.970, Sp(6×6)=0.990, machine

calibrated at SCD=100cm, TMR(8,6×6)=0.787. Since the

calibration point is at the SAD, SAD factor=1.

Answer:

2651990.0970.0787.0

200

MU

Example 4. Calculate MU values for the case in Example 3, if the unit is

calibrated nonisocentrically, i.e. source to calibration point

distance=101cm.

Answer:

26002.199.0970.0787.0

200

020.1100

1012

MU

SADfactor

Cobalt-60 calculation The above calculation system is sufficiently general that it can be

applied to any radiation generator including 60Co.

In the latter case, the machine can be calibrated either in air or in phantom provided the following information is available:1. Dose rateD0(t0,r0,f0) in phantom at depth t0 of maximum dose for a reference field r0 and standard SSD f0; 2. Sc factor; 3. Sp factor; 4.PDD and TMR values.

If universal depth dose data for 60Co are used, then the Sp factor and TMR can be obtained by using the following equations:

d

d

d

p

rBSF

rdTARrdTMR

rBSF

rBSFrS

,,

0

Example 5. A tumor dose of 200 rads is to be delivered at 8cm depth,

using 15×15cm field size, 100cm SSD, and penumbra

trimmers up. The unit is calibrated to give 130rads/min in

phantom at 0.5cm depth for a 10×10cm field with trimmers up

and SSD=80cm. Determine the time of irradiation, given

SC(12×12)=1.012, Sp(15 ×15)=1.014, PDD(8,

15×15,100)=68.7%.

Answer: field size projected at SAD (80cm)

min40.3642.0014.1012.1687.0130

200

15151212100,1515,880,1010,5.0

642.05.0100

5.080

1212100

8015

2

SSDfactorSSPDDD

TDTIME

SSDfactor

cmcm

pC

Dose calculation in irregular fields－－－ Clarkson’s method

Any field other than the rectangular, square, or circular field may be

termed irregular.

Irregularly shaped fields are encountered in radiotherapy when

radiation sensitive structures are shielded from the primary beam or

when the field extends beyond the irregularly shaped patient body

contour.

Examples of such fields are the mantle and inverted Y fields employed

for the treatment of Hodgkin’s disease.


Since the basic data are available usually for rectangular fields,

methods are required to use these data for general cases of irregularly

shape fields. One such method, originally proposed by Clarkson and

later developed by Cunningham, has proved to be the most general in

its application.

Clarkson’s method is based on the principle that the scattered

component of the depth dose, which depends on the field size and

shape, can be calculated separately from the primary component

which is independent of the field size and shape. A special

quantity,SAR, is used to calculate the scattered dose.


Assume this field cross section to be at depth d and perpendicular to the beam axis. Let Q be the point of calculation in the plane of the field cross section.

Radii are drawn from Q to divide the field into elementary sectors. Each sector is characterized by its radius and can be considered as part of a circular field of that radius.

If we suppose the sector angle is 10º, then the scatter contribution from this sector will be 10º/360º=1/36 of that contributed by a circular field of that radius and centered at Q.


Thus, the scatter contribution from all the sectors can be calculated and summated by considering each sector to be a part of its own circle whose scatter-air ratio is already known and tabulated.

Using an SAR table for circular fields, the SAR values for the sectors are calculated and then summed to give the average SAR for the irregular field at point Q.

For sectors passing through a blocked area,the net SAR is determined by subtracting the scatter contribution by the blocked part of sector.


For example,

The computed average SAR is converted to average TAR by the equation:

QBQCQA

BCQAQC

SARSARSAR

SARSARSAR

net

SARTARTAR 0


BSF can be calculated by Clarkson’s method. This involves determining average TAR at the depth dm on the axis, using the field contour or radii projected at the depth dm.

BSFdf

dfTARPDD

eTAR

m

dd m

1

02

Other practical methods

Irregular fields Point off-axis Point outside the field Point under the block

Irregular fields Clarkson’s techniques is a general method of

calculating depth dose distribution in an irregularly shaped field but it is not practical for routine manual calculations.

Even when computerized, it is time consuming since a considerable amount of input data is required by the computer program.

However, with the exception of mantle, inverted Y, and a few other complex fields, reasonably accurate calculations can be made for most blocked fields using an approximate method .

Irregular fields Approximate rectangles may be drawn containing the point

of calculation so as to include most of the irradiated area surrounding the point and exclude only those areas that are remote to the point.

In so doing, a blocked area may be included in the rectangle provided this area is small and is remotely located relative to that point.

The rectangle thus formed may be called the effective field while the unblocked field, defined by the collimator, may be called collimator field.

It is important to remember that, whereas the SC factors are relatedto the collimator field, the PDD,TMR, or SP

corresponds to the effective field.

Point off-axis

It is possible to calculate depth dose distributions at any point within the field or outside the field using Clarkson’s technique. However, it is not practical for manual calculations.

Day has proposed a particularly simple calculation method for rectangular fields. In this method, PDD can be calculated at any point within the medium using the central axis data.

43214

1DDDDdD

PDDBSFDPDDDD

Q

fsmd

4

1

4

1

4

1

4321

4

1

4

1

4

14

1

ii

iii

fs

fsQ

m

QQ

iiifsQQ

Q

fsmd

PDD

PDDBSFDBSF

DK

D

DdPDD

PDDBSFDKdD

DDDDdD

PDDBSFDPDDDD

4

14

1

iiifsQQ PDDBSFDKdD

4321

4

1

4

1i

4

1

4

1

4

1

PDDPDDPDDPDDPDD

PDD

PDDBSFDBSF

DK

D

DdPDD

Q

ii

iifs

fsQ

m

QQ

Suppose the field size is 15×15cm. a=10, b=5, c=10,d=5, assume KQ=0.98

and SSD=80cm. PDDP=58.4%

PDDQ=55.8%

KQ=1,PDDQ=56.9%

This off-axis decrease in dose is due to the reduced scatter at Q

compared to P. Similarly, it can be shown that the magnitude of the

reduction in scatter depends on the distance of Q from P as well as

depth.

Point outside the field

中大

中中大大

中大

PDDPDDPDD

baBSF

PDDBSFPDDBSFPDD

DDD

Q

Q

Q

2

1

2

2

1

Suppose

it is required to determine PDD at Q (relative to Dm

at P) outside a 15×10cm field at a distance of 5cm

from the field border. a=15, b=10,c=5. Suppose Q is

at the center of the middle rectangle of dimensions

2c×b. Then the dose DQ at 10cm depth is given by

the above equation.

PDDQ=2.1%

Think about ：

Point outside field

Point under block

sec.3 practical application 临床处方剂量的计算. what we should calculate? time or mu...

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