Secondary doses in proton therapy and IMRT

Secondary doses in proton therapy and IMRT

Zacharatou Jarlskog C and Paganetti H: The risk of developing second cancer due toneutron dose in proton therapy as a function of field characteristics, organ, andpatient age. International Journal of Radiation Oncology, Biology, Physics in press

Where are the neutrons coming from inpassive scattered proton therapy ?

Brass aperture Brass apertureOpening (target) Opening (target)

~20% of the beam treats~80% of the beam produces neutr.

~60% of the beam treats~40% of the beam produces neutr.

Neutron yield depends on aperture sizeNeutron dose generatedin the treatment head(typically) decreaseswith increasing treatmentvolume

Relative contribution of external neutrons to thyroidcancer risk as a function of field index

Zacharatou Jarlskog C and Paganetti H: The risk of developing second cancer due toneutron dose in proton therapy as a function of field characteristics, organ, andpatient age. International Journal of Radiation Oncology, Biology, Physics in press

How do neutron equivalent doses inproton therapy compare to doses from

CT imaging ?

Putting it into perspective:protons vs. imaging

Zacharatou Jarlskog et al. (2008). Assessment of organ-specific neutron equivalentdoses in proton therapy using computational whole-body age-dependent voxelphantoms. Phys Med Biol, 53, 693-717

4.00.72.35.8RATIO

5.05.94.9H (stomach) fromchest CT [mSv]

3.813.528.5H (stomach) fromtherapy [mSv]

22.722.431.821.6RATIO

6.95.29.0H (thyroid) fromchest CT [mSv]

155.1166.0195.4H (thyroid) fromtherapy [mSv]

average14-yr old11-yr old4-yr old

What is the risk for developing secondmalignancies ?

Organ equivalent dose after H&N tx

thyroid (circles) lung (squares) liver (triangles)

Zacharatou Jarlskog et al. (2008). Assessment of organ-specific neutron equivalentdoses in proton therapy using computational whole-body age-dependent voxelphantoms. Phys Med Biol, 53, 693-717

Zacharatou Jarlskog C and Paganetti H: The risk of developing second cancer due toneutron dose in proton therapy as a function of field characteristics, organ, andpatient age. International Journal of Radiation Oncology, Biology, Physics in press

Field 1Field 2Field 3

The risk associated with neutron doses inproton therapy is similar to the risk associatedwith scattered photon doses in IMRT or CT

The uncertainties when estimating neutronequivalent doses in proton therapy are bigger

than for scattered IMRT and CT doses

The risk associated with neutron doses inproton therapy is presumably smaller than the

lifetime baseline risk for most fields

The integral dose in proton therapy is roughly afactor of 2-3 lower than with any type of

photon therapy

What is the difference between IMRT, passivescattered proton therapy, and scanned beamproton therapy in terms of out-of-field organ

specific absorbed dose and equivalent dose as afunction of treatment field ?

How does this depend on patient’s age?

Research to be done:

Such studies can help to reduce ouruncertainties in risk models in the long run !