Tp (potential doubling time or cell doubling time after cell loss)

Tp is the potential doubling time of tumors [1]. It is the constant repopulation rate, i.e., a constant cell doubling time up to the end of the radiation treatment [2]. It is is the average doubling time of the clonogenic cells in days [3]. For example, in head and neck tumours or cervix cancer, it can be as short as 2 days; therefore one loses up to 1 Gy worth of cell killing when prolonging the course of radiotherapy. Tp it the fastest time in which a tumour can double its volume and depends on cell type and can be of the order of 2 days in fast growing tumours [1]. This "volume doubling time is determined by cell cycle time, growth factor, and rate of cell loss." Tumours such as squamous cel carcinomas have a relatively short volume doubling time; adenocarcinomas have a longer volume doubling time [4]. It can be measured in cell biology experiments (that requires optimal conditions for the tumour and is a worst case scenario) [1].
Patients on treatment should be prioritised according to the volume doubling time. The prolongation of overall treatment time affects treatment outcome or local tumour control in patients with tumours with short Tp [4].
In patients with muscle-invasive transitional cell bladder carcinoma, the clonogen doubling time is about 5–8 days [5].

Bibliographic references:

[1] 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: https://rpop.iaea.org/RPOP/RPoP/Content/Documents/TrainingRadiotherapy/Lectures/RT03_RadBiol2_RT_WEB.ppt [Accessed 12 Jul. 2016].

[2] Fowler, JF. 21 years of Biologically Effective Dose. Br J Radiol. 2010 Jul; 83(991): 554–568. Availavle at: http://dx.doi.org/10.1259%2Fbjr%2F31372149.

[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: http://dx.doi.org/10.1088/0031-9155/48/18/402.
[4] 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: https://www.rcr.ac.uk/sites/default/files/publication/BFCO(08)6_Interruptions.pdf [Accessed 17 Jul. 2016].
[5] Bese NS, Hendry J, Jeremic B. Effects of prolongation of overall treatment time due to unplanned interruptions during radiotherapy of different tumor sites and practical methods for compensation. Int J Radiat Oncol Biol Phys. 2007 Jul 1;68(3):654-61. Available at: http://dx.doi.org/10.1016/j.ijrobp.2007.03.010.