Dose Constraints

Question 1

What is Collins law as it pertains to the max length of f/u needed for pediatric tumors?

Answer 1

Defines period of risk for recurrence (age at Dx + 9 mos [gestational period]). If tumor was present in utero, then age at Dx + 9 mos determines rate of growth for it to become clinically evident. Residual Dz should become evident in same timeframe. (Sure U, Clin Neurol Neurosurg 1997)

Question 2

What factors predict for greater decline in IQ after CSI?

Answer 2

Factors for decline in IQ after CSI:

Age <7 yrs (most important)

Higher dose (36 Gy vs. 23.4 Gy)

Higher IQ at baseline

Female sex

(Ris MD et al., JCO 2001)

Question 3

For how long can the pt’s IQ decline after CSI?

Answer 3

>5 yrs. Hoppe-Hirsch et al. reviewed 120 MB pts treated with CSI to 36 Gy. At 5 yrs, 58% had an IQ >80. At 10 yrs, only 1% had an IQ >80. (Childs Nerv Syst 1990)

Question 4

What are some important factors influencing IQ scores/neurotoxicity after RT?

Answer 4

Age at Tx with RT (most important), volume and dose of RT, and sex (female > male)

Question 5

What is the dose constraint to the cochlea?

Answer 5

V30 <50% is the dose constraint to the cochlea (max is 35 Gy with chemo).

Question 6

What is the most common hormone deficiency after RT to the brain? What is the dose threshold?

Answer 6

GH. The threshold dose for GH deficiency is ∼10 Gy.

Question 7

What is the annual IQ drop after full PF boost in MB pts younger and older than 7 yrs? What structure is most important?

Answer 7

IQ drop of 5 points/yr if <7 yo and 1 point/yr if >7 yo. The dose to the supratentorial brain(temporal lobes) is most important.