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Multi-Fraction Radiation Treatment
Radiation Delivery
Once-a-day dosing schemes are often used in the treatment of cancer. On each treatment day, a total dose of approximately 2 Gy is delivered to the tumor. By irradiating the tumor from several different directions, treatment plans can be designed that deliver a large dose to the tumor and much smaller amounts to nearby normal tissues. This preferential targeting of the tumor mass can be an effective way to eradicate harmful tumor cells while simultaneously sparing normal tissue function.

As illustrated in Figure 1,  daily doses are often given over a 4-6 week period. The tumor is usually not treated over the weekend.  Figure 1 shows a 6 week treatment where the 2 Gy fractions are delivered on weekdays. The total treatment dose is (2 Gy) x (30 fractions) = 60 Gy.  In so-called step-and-shoot intensity modulated radiation therapy (IMRT), the 2 Gy fraction is typically delivered in 15 to 45 minutes.  Clinicians custom design the size and timing of the fractions for each a patient and type of tumor.  The biologically optimal way to deliver a patient-specific radiation treatment is a very active area of research at many hospitals and universities, including the School of Health Sciences at Purdue.


Figure 1. Multi-fraction radiation treatment
(click on figure to enlarge)multi-fraction radiation therapy (example 1)


In general, biological systems are sensitive to temporal fluctuations in the instantaneous dose rate.  However, the sensitivity of a biological system to changing dose rates depends on the endpoint.  For many endpoints such as DNA damage repair and reproductive cell death, very rapid fluctuations in the instantaneous dose rate cannot be detected by relevant biochemical processes, and temporal fluctuations in a radiation field that occur over a few seconds or minutes can be treated using an average (effective) dose rate.

Table 1 lists some average (effective) dose rates important in external beam radiation therapy (EBRT). Effective dose rates from about 3 to 30 Gy/h are appropriate when modeling the biological effects of a dose fraction.  For daily dosing schemes, the compounded biological effects of a series of doses can often be well approximated by the formula S(D) = [S(d)]n.  Here, S(D) is the fraction of the tumor cells that survive a treatment that delivers total dose D using n fractions of size d (i.e., D = nd).


Table 1.  Effective dose rates (Gy/h) common to IMRT and
other forms of EBRT.

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Last updated: 10 June, 2011
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