Increasing interests using large dose fractions

1
A Generalized Linear-Quadratic Formula for
High-Dose-Rate Brachytherapy and Radiosurgery
Jian Z Wang, PhD,1 Nina A Mayr, MD,1 William TC
Yuh, MD, MSEE2 Department of 1Radiation Medicine
and 2Radiology, The Ohio State University,
Columbus, OH 2007 49th ASTRO Annual Meeting, Los
Angeles, CA
Why?
Findings

• Increasing interests using large dose fractions
• to irradiate human cancer in IGRT,HDR
• brachytherapy and radiosurgery.
• 2. The dose prescription has been guided by
• clinical experience of low doses using the
• Linear-Quadratic (LQ) model. But extension of
  
• the LQ Model to high doses would overestimate
  
• cell killing and lead to significant
  underdosing.
• 3. In this study, we derived a generalized LQ
• formula to bridge the RT regimens from the
• conventional RT (2-Gy fractions) and LDR
• brachytherapy to HDR and radiosurgery.

• A close-form solution of the gLQ model has been
  derived (see Eq. 2 below).
• Both the gLQ and the conventional LQ models were
  used to fit the Hela data (Fig. 1).
• Basd on low-dose data, the gLQ model can well
  reproduce the cell response of high doses,
• but the conventional LQ model overestimates
  the cell killing by one order at 7 Gy.

Fig. 1a, both models could well fit the data in
the full dose range Fig. 1b, models were used to
fit only the low dose data ( 2.5 Gy, similar to
conventional RT) and to predict the response in
high dose region. Legends Exp. data,
gLQ model ----- LQ model
Comments to wang.993_at_osu.edu
Slide 1
How?
How?

• Two types of radiation damage in LQ model
• (a) lethal lesion (rate ?)
• (b) sublethal lesion (?1).
• Sublethal lesions can be
• (a) repaired (?), or
• (b) lethal (?2) with further radiation
  damage.
• LQ model to describe this process (see Eq. 1
• right) with arbitrary dose rate
• I(t)Ii, ti-1lttltti, I1,,N
• 4. Test with in vitro dataset
• Puck and Marcus, J Exp Med 1956103653
• Hela cell irradiation up to 7 Gy

Take home message

• A generalized LQ formula was developed to
  bridge conventional RT to HDR and radiosurgery.
• It can be used to extend current RT experience
  into the high-dose irradiation of human cancer.
• This model can be applied to any dose-delivery
  patterns with variable dose rate, and to guide
• dose fractionation designs of
  hypofractionated IGRT/IMRT, HDR and radiosurgery.