Wednesday, 13 July 2016

Tk (kick-off time or onset time) or Tdelay or Tdel

Tk, Tdelay [1], or Tdel [2] "represents any delay in the start of tumour cell repopulation in response to radiation treatment after radiation treatment has started." Repopulation of tumour cells due to radiation treatment starts after a passed lag period of Tk treatment days [3]. The repopulation time of tumour cells appears to vary during radiotherapy; at the commencement it may be slow (e.g., due to hypoxia); however, a certain time after the first fraction of radiotherapy (kick-off time), repopulation accelerates [4]. Tk is "the apparent starting time of rapid compensatory repopulation in tumor or tissue after the start of treatment, when it is assumed that there are just two rates of cell proliferation during radiotherapy: zero from start to Tk, then constant doubling each Tp days [cell doubling time] until end of treatment at T days [overall treatment time]" [5].
Tk could be between 21 and 32 days in human head and neck tumors [6,7] and non-small-cell lung cancer, and unlikely may be shorter, but may be longer in prostate cancer [8]. Fowler et al. [9] considered that prostate cancer would have a TK value up to 10 times the TK for head and neck tumors, approximately between 210 and 300 days, and Leborgne et al. [10] considered a Tfor prostate cancer of 52 days. Values for adenocarcinoma of the anus have not been defined [11].
Bibliographic references:
[1] The timely delivery of radical radiotherapy: standards and guidelines for the management of unscheduled treatment interruptions. (2008). 3rd ed. London, United Kingdom: Board of Faculty of Clinical Oncology, The Royal College of Radiologists. Available at: [Accessed 17 Jul. 2016].
[2] Wyatt RM, Beddoe AH, Dale RG. The effects of delays in radiotherapy treatment on tumour control. Phys Med Biol. 2003 Jan 21;48(2):139-55. Available at:
[3] Tomé WA, Fowler JF. On the inclusion of proliferation in tumour control probability calculations for inhomogeneously irradiated tumours. Phys Med Biol. 2003 Sep 21;48(18):N261-8. Available at:
[4] IAEA Training Material on Radiation Protection in Radiotherapy - Radiation Protection in Radiotherapy - Part 3 - Biological Effects - Lecture 2: High Doses in Radiation Therapy. (2013). [Lectures/Slides]. Vienna, Austria: International Atomic Energy Agency. Available at: [Accessed 12 Jul. 2016].
[5] Fowler, J. (2006). Part I: Basic Concepts in Treatment Planning, 1. Practical Time-Dose Evaluations, or How to Stop Worrying and Learn to Love Linear Quadratics. In: S. Levitt, J. Purdy, C. Perez and S. Vijayakumar, ed., Technical Basis of Radiation Therapy, Practical Clinical Applications, 4th ed. Springer-Verlag Berlin Heidelberg, pp.3-31.
[6] Brenner, DJ.  Accelerated repopulation during radiotherapy.  Quantitative evidence for delayed onset. Radiat Oncol Invest. 1993;1(3):167–72. Available at:
[7] Roberts, SA, Hendry, JH.  Time factors in larynx tumor radiotherapy: lag times and intertumor heterogeneity in clinical datasets from four centers. Int J Radiat Oncol Biol Phys. 1999;45(5):1247–57. Available at:
[8] Fowler JF. Development of radiobiology for oncology—a personal view. Phys Med Biol. 2006 Jul 7;51(13):R263-86. Available at:
[9] Fowler, JF, Ritter, MA, Fenwick , JD, Chappell, RJ. How low is the alpha/beta ratio for prostate cancer? In regard to Wang et al., IJROBP 2003;55:194-203, Int J Radiat Oncol Biol Phys. 2003;57(2):593–5. Available at:
[10] Leborgne, F, Fowler, J, Leborgne, JH, Mezzera, J. Later outcomes and alpha/beta estimate from hypofractionated conformal threediomensional radiotherapy versus standard fractionation for localized prostate cancer. Int J Radiat Oncol Biol Phys. 2012;82(3):1200–7. Available at:
[11] Joon DL, Chao MW, Ngan SY, Joon ML, Guiney MJ. Primary adenocarcinoma of the anus: a retrospective analysis. Int J Radiat Oncol Biol Phys. 1999 Dec 1;45(5):1199-205. Available at:

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