FROM 2D TO 3D: ONLY BENEFITS OR ALSO PITFALS? Primo Strojan - - PowerPoint PPT Presentation

FROM 2D TO 3D:

ONLY BENEFITS OR ALSO PITFALS?

Primož Strojan

Conformity: High-dose volume is shaped to closely “conform” to the designed target volumes & Dose to critical normal tissues is minimal (as much as possible)

“2D” RT:

1.

Targets defined & dose calculated in 2-dimensions

2.

Simple beam arrangements

3.

Beams are not shaped or simple beam-shaping devices are used

• pre-manufactured blocks
• individual shielding blocks

4.

Forward treatment planning

(trial-and-error process: field/beam weights are modified iteratively & manually)

2-D PLANNING

Computer based planning, calculation & visiualization distribution of dose

Pre-manufactured (standard) blocks Custom-made alloy blocks Missing tissue compensators

“Conformal” RT:

1.

Targets defined & dose calculated in 3- dimensions

2.

Multiple beam directions

3.

Beams are shaped (or intensity modulated)

4.

Forward (or inverse*) treatment planning

*Computer-assisted optimization: definition of objectives and constrains determination of optimal beam arrangement & weighting

2-D PLANNING 3-D PLANNING

Anatomical data acquisitation

5 1 2 3 4

2D-RT 3D-RT IMRT

TARGET(S) more conformal dose distribution NORMAL TISSUES as low dose as possible

TU

NT CONFORMITY

TARGET(S) more conformal dose distribution NORMAL TISSUES as low dose as possible

TU

NT CONFORMITY

STEEP DOSE GRADIENTS

“Conformal” RT:

1.

Targets defined / dose calculated in 3- dimensions

2.

Multiple beam directions

3.

Beams are shaped (or intensity modulated)

4.

Forward (or inverse) treatment planning PROBLEMS ?!

• 1. Increased risk of a marginal miss

Tumor is not eradicated

• Accurat identification of target(s) and OAR
• Movements

patient target beams

Identification of target and AORs

CT PET CT + PET

ICRU Report 62, 1999

Movements

TARGET(S) more conformal distribution NORMAL TISSUES as low dose as possible

TU

NT CONFORMITY

+ STEEP DOSE GRADIENTS

TU

NT

Mayer JL. Karger: Basel, 2007. p.8.

Tumor reduction & weight loss

• 2. Less homogenous dose distribution
• Radiobiology – “double – trouble” effect

late-responding tissues, large treatment volumes

• Interpretation & verification of resulted dose

distribution

ICRU reference point, min, max, mean dose Dose distribution Dose Volume Histograms – DVHs to predict NTCP to assess quality of treatment plan

DVHs = 2D presentation of 3D dose distribution

(what % of volume is raised to a defined dose)

DVHs

• quantitative description of dose distribution
• no information on spatia distribution
• all regions of an organ are eaqually important
• as good as is the anatomic information provided

how accurately routine imaging reflect underlying anatomy marked inter-physician differences in image segmentation

• interactions between organs are not considered
• no information on functional status of

nonirradiated organ volume

• 3. Larger total body dose

Risk of radiation-induced malignancies

• increased beam-on time ( MUs 2-3x)

leakage through the collimator

• more fields

volume of NT exposed to lower RT doses 1% 1.75% for IMRT (Hall E & Wuu CS IJROBP 2003;

Hall E. IJROBP, 2006)

• 4. Increase in costs

More labour intensive and expensive

CONCLUSIONS

“3D”:

Targets defined in 3-dimensions using CT More complex beam arrangements and shaping More conformal dose distribution Steep dose gradients Identification of target(s) Patient immobilization and setup

• risk of marginal miss
• homogenous dose distribution
• total body dose
• in costs