The impact of dose prescription on treatment volume

Summer Chaudhari1st year medical physics resident

University of Minnesota Medical Center

Advisor: Dr. Patrick Higgins

NCCAAPM chapter meeting – April 24, 2009

Purpose of Study

Previously published literature has stressed the importance of:– prescribing– recording– reporting radiation therapy treatment doses and their treatment volumes clearly and

unambiguously so that they can be easily tracked worldwide.

One of the recommendations of ICRU 50 and its supplement ICRU 62 is to prescribe to a point inside the planning target volume (ICRU reference point) instead of prescribing to the periphery of the PTV (an isodose line).

Our study evaluated the effects on the dose distribution of prescribing to the ICRU reference point versus prescribing to an isodose line.

Introduction:Dose Specification

ICRU 50 states:

“The most significant difference found in the literature appears to be the location where one prescribes and delivers the dose and how one reports the full treatment.”

Introduction:ICRU reference point

The ICRU Reference Point is selected according to the following criteria:

– Must be clinically relevant and representative of dose throughout the PTV

– Should be easy to define in a clear and unambiguous way– The point should be chosen where the dose can be accurately

determined – The point should not lie in a region where there is a steep dose gradient

These recommendations are fulfilled if the ICRU reference point is:– Always at the center (central part) of the PTV– When possible, at the intersection of the beam axis

Methods

Pinnacle treatment planning system Cases:

– Phantom: Drew 2 cm and 8 cm spherical diameter targets on 30 cm3 phantom 4-field box

– Clinical: 4-field prostate 10-field stereotactic body radiotherapy (SBRT) treatment plans

– Different combinations of: Block margin (0, 0.5 and 1.0 cm) Prescriptions (ICRU or scaled)

– 2 mm dose calculation grid

Phantom Plans

Methods

Prescription Criteria:– Prescription 1 - ICRU: 100% dose to isocenter– Prescription 2 - Scaled: Pick an isodose line so

that 95% volume gets 100% dose Our clinic tries to meet the following treatment

planning goals (using ICRU):– 95% target volume gets 95% dose

Methods

Evaluations:

The effect of different block margins (0, 0.5 and 1 cm) on doses and volumes

The effect of different dose prescription (ICRU vs scaled) on normal tissue volumes

Plots: Calculated each plan and plotted dose vs. volume for

comparison

Overview

Plots:– DVH plots for phantom and clinical plans

– Comparison of 2 different prescription methods Prescription 1 - ICRU: 100% dose to point Prescription 2 - Scaled: 95% volume gets 100% dose

– Different scale factors were applied to these plans in order to meet the prescription criteria

– Our clinic goals: 95% planning target volume gets 95% dose

Baseline case: 8 cm sphere8 cm PTV only

Isodose Volume vs Dose

0

50

100

150

200

250

300

50 60 70 80 90 100 110 120

Isodose line (%)

Volu

me

(cm

3)

0 cm Block ICRU

0 cm Block 89% Rx line

0.5 cm Block ICRU

0.5 cm Block 96% Rxline1 cm Block ICRU

1 cm Block 97.5% Rxline95% dose

Solid line: ICRU prescription Dotted line: Scaled prescription

Baseline case: 8 cm sphere

8 cm CaseIsodose Volume vs Dose

0

200

400

600

800

1000

1200

1400

40 50 60 70 80 90 100 110 120

Isodose line (%)

Volu

me

(cm

3)

0 cm Block ICRU

0 cm Block 89% Rx line

0.5 cm Block ICRU

0.5 cm Block 96% Rxline1 cm Block ICRU

1 cm Block 97.5% Rxline

Solid line: ICRU prescription Dotted line: Scaled prescription

Baseline Case: 2 cm sphere

2 cm PTV only

0

0.5

1

1.5

2

2.5

3

3.5

40 50 60 70 80 90 100 110 120 130 140

Isodose line (%)

Volu

me

(cm

3)

0 cm Block ICRU

0 cm Block 76% Rx line

0.5 cm Block ICRU

0.5 cm Block 96.7% Rx line

1 cm Block ICRU

1 cm Block 99% Rx line

95% dose

Solid line: ICRU prescription Dotted line: Scaled prescription

Example 1: ProstateProstate Case - PTV only

0

50

100

150

200

250

300

40 60 80 100 120 140 160

Isodose line (%)

Volu

me

(cm

3)

0 cm Block ICRU

0 cm Block 68% Rx line

0.5 cm Block ICRU

0.5 cm Block 90% Rx line

1 cm Block ICRU

1 cm Block 96.5% Rx line

95% dose

Solid line: ICRU prescription Dotted line: Scaled prescription

Example 2: SBRT –small sized target

SBRT Target PTV onlyIsodose Volume vs Dose

Case 1

05

101520253035404550

40 60 80 100 120 140 160

Percent Isodose

Volu

me

(cm

**3)

0 cm block ICRU

0 cm block 71.8% Rx line

0.5 cm block ICRU

0.5 cm block 81.5% Rx line

1 cm block ICRU

1 cm block 90% Rx line

95% dose

Solid line: ICRU prescription Dotted line: Scaled prescription

Example 3: SBRT –Large sized target

SBRT Target PTV onlyIsodose Volume vs Dose

Case 11

0

20

4060

80

100

120

140160

180

200

40 50 60 70 80 90 100 110 120 130

Percent Isodose

Volu

me

(cm

**3)

0 cm block ICRU

0 cm block 82% Rx line

0.5 cm block ICRU

0.5 cm block 94% Rx line

1 cm block ICRU

1 cm block 98% Rx line

95% dose

Solid line: ICRU prescription Dotted line: Scaled prescription

Results – average 11 patients

Solid line: ICRU prescription Dotted line: Scaled prescription

Average 11 cases

0

20

40

60

80

100

120

140

160

180

200

40 50 60 70 80 90 100 110 120 130

Percent Isodose

Volu

me

(cm

**3)

0 cm block ICRU0 cm block 88.5% Rx line0.5 cm block ICRU0.5 cm block 95.2% Rx line1 cm block ICRU1 cm block 97% Rx line95% dose

Conclusions

Advantages of the ICRU definition of prescription:

– Can meet the dose prescription criteria using adequate block margins

– Can reduce the hot spot doses by as much as 30% as compared to the isodose prescription

– Uniformity in dose reporting can be achieved

Summary

If the ICRU reference point is used as the dose prescription point, the dose to the healthy tissue will be lower as compared to prescribing to an isodose line.

Prescribing to the ICRU reference point will make it more feasible to record the dose in a way that follows the ICRU recommendations on dose prescription, recording and reporting.

Thank you!

Questions?

Example 2: SBRT-small sized target

SBRT Target Case 1Isodose Volume vs Dose

0

100

200

300

400

500

600

40 60 80 100 120 140

Percent Isodose

Volu

me

(cm

**3)

0 cm block ICRU

0 cm block 71.8% Rx line

0.5 cm block ICRU

0.5 cm block 81.5% Rx line

1 cm block ICRU

1 cm block 90% Rx line

Solid line: ICRU prescription Dotted line: Scaled prescription

Example 1: Prostate

Prostate Case

0

500

1000

1500

2000

2500

3000

3500

4000

0 20 40 60 80 100 120 140 160Isodose line (%)

Volu

me

(cm

3)

0 cm Block ICRU0 cm Block 68% Rx line0.5 cm Block ICRU0.5 cm Block 90% Rx line1 cm Block ICRU1 cm Block 96.5% Rx line

Solid line: ICRU prescription Dotted line: Scaled prescription

Example 3: SBRT –Large sized target

SBRT Target Case 11Isodose Volume vs Dose

0

500

1000

1500

2000

2500

3000

0 20 40 60 80 100 120 140

Percent Isodose

Volu

me

(cm

**3)

0 cm block ICRU

0 cm block 82% Rx line

0.5 cm block ICRU

0.5 cm block 94% Rx line

1 cm block ICRU

1 cm block 98% Rx line

Solid line: ICRU prescription Dotted line: Scaled prescription

Example 3: SBRT –Large sized target

SBRT Target Case 11Isodose Volume vs Dose

0

100

200

300

400

500

600

0 20 40 60 80 100 120 140

Percent Isodose

Volu

me

(cm

**3)

0 cm block ICRU

0 cm block 82% Rx line

0.5 cm block ICRU

0.5 cm block 94% Rx line

1 cm block ICRU

1 cm block 98% Rx line

Solid line: ICRU prescription Dotted line: Scaled prescription

Baseline case: 8 cm sphere 8 cm Case

Isodose Volume vs Dose

0

500

1000

1500

2000

2500

3000

3500

40 50 60 70 80 90 100 110 120

Isodose line (%)

Volu

me

(cm

3)

0 cm Block ICRU

0 cm Block 89% Rx line

0.5 cm Block ICRU

0.5 cm Block 96% Rxline1 cm Block ICRU

1 cm Block 97.5% Rxline

Solid line: ICRU prescription Dotted line: Scaled prescription

Baseline case: 2 cm sphere

2 cm CaseIsodose Volume vs Dose

0

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140

Isodose line (%)

Volu

me

(cm

3)

0 cm Block ICRU

0 cm Block 76% Rx line

0.5 cm Block ICRU

0.5 cm Block 96.7% Rxline1 cm Block ICRU

1 cm Block 99% Rx line

Solid line: ICRU prescription Dotted line: Scaled prescription

2 cm CaseIsodose Volume vs Dose

0

100

200

300

400

500

600

700

800

900

1000

0 20 40 60 80 100 120 140

Isodose line (%)

Volu

me

(cm

3)

0 cm Block ICRU

0 cm Block 76% Rx line

0.5 cm Block ICRU

0.5 cm Block 96.7% Rxline1 cm Block ICRU

1 cm Block 99% Rx line

Baseline case: 2 cm sphere

Solid line: ICRU prescription Dotted line: Scaled prescription