Vx

Vx is the percent volume of the organ or target receiving ≥x Gy [1,3], or the volume covered by the isodose of x% [2].

Bibliographic references:
[1] Lu JY, Lin Z, Zheng J, et al. Dosimetric evaluation of a simple planning method for improving intensity-modulated radiotherapy for stage III lung cancer. Sci Rep. 2016 Mar 24;6:23543. Available at: https://dx.doi.org/10.1038/srep23543.
[2] Zhang T, Liang ZW, Han J, et al. Double-arc volumetric modulated therapy improves dose distribution compared to static gantry IMRT and 3D conformal radiotherapy for adjuvant therapy of gastric cancer. Radiat Oncol. 2015 May 19;10:114. Available at: https://dx.doi.org/10.1186/s13014-015-0420-x.

[3] Huang EX, Bradley JD, El Naqa I, et al. Modeling the risk of radiation-induced acute esophagitis for combined Washington University and RTOG trial 93-11 lung cancer patients. Int J Radiat Oncol Biol Phys. 2012 Apr 1;82(5):1674-9. Available at: https://dx.doi.org/10.1016%2Fj.ijrobp.2011.02.052.

MATHEWS OPEN ACCESS JOURNALS: Reirradiation of Skin Tumors

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Tpot (potential doubling time without cell loss)

It describes the proliferation rate of the tissues, which can be measured from biopsies by flow cytometry after administration in situ to the patient of a deoxyribonucleic acid thymidine tracer [2]. "This is difficult to determine in vivo." It is "the time which the cell population of tumor doubles if there is no cell loss" [3]. Tp (potential doubling time or cell doubling time after cell loss) is a little shorter than Tpot (measured before the tumour has received any cytotoxic treatment) [1].

Pretreatment Tpot measured in human tumor biopsies [2].
Tumor	Median (days)	Range (days)
Larynx	4	2-19
Tongue	4-6	2-16
Mouth, cheek	3.4	2-15
Esophagus	5	2.5-20
Cervix	5	3-20
Rectum	5	3-18
Prostate	42	15-70
Breast	14	3-70

Bibliographic references:

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

[2] Fowler JF. The radiobiology of prostate cancer including new aspects of fractionated radiotherapy. Acta Oncol. 2005;44(3):265-76. Available at: http://dx.doi.org/10.1080/02841860410002824.
[3] 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].

K value or K factor

It is “the BED [biologically effective dose] equivalent of repopulation (in units of Gy/day)”, “the BED-equivalent repopulation factor” [1]. It is the "increase in dose required daily to obtain same biologic effect: recommended extra dose to add to total radiation dose for each day of prolongation, which can be given by increasing daily dose of remaining treatment". "Prolongation of overall treatment time due to unplanned interruptions during radiotherapy causes deleterious effect of accelerated repopulation of tumor clonogens. For the cancers of the head and neck, even a 1-day interruption resulted in a decrease in the local control rate by 1.4%" [9]. A “K value of 0.9 Gy/day represents the BED required each day (after Tdelay has been passed) to offset repopulation” [2]. It is “the biological dose per day required to compensate for on-going tumour cell repopulation. K is the BED equivalent of 1 days’ worth of repopulation” [3], “a factor used to determine the amount of radiation ‘wasted’ due to ongoing tumour repopulation” [4]. “If chemotherapy is able to completely inhibit repopulation then K=0 and would not be required” [1].
For head and neck treatments, it is “recommend K=0.1 Gy/day and Tdelay=0 in cases where the final overall times are less than 28 days” [1]. However, after this time, the K value is 0.5-0.7 Gy/day [9].

“There is at present very little data relating to K and Tdelay factors for other tumour types” [1].
“For cervix tumours, the time factors are probably around half those for head and neck tumours, i.e., 0.5 Gy/day” [1,5]. “For breast tumours, the K factors are likely to be around 0.3 Gy/day while for prostate tumours they are likely to be in the region 0.1–0.3 Gy/day, or even lower [1,6-8]”. “Little is known about the corresponding values of Tdelay” [1]. For transitional cell bladder carcinoma, K=0.36 Gy/day for a Tdelay=5-6 weeks [9]. For non-small cell lung cancer and a Tdaley>20 days, K=0.45 Gy/day [10,11]; but, for cases without mediastinal involvement, K=0.2 Gy/day [10].

Bibliographic references:

[1] Stamatakos, G. (2012). 2012 5th International Advanced Research Workshop on In Silico Oncology and Cancer Investigation (IARWISOCI). Piscataway, NJ: IEEE. Available at: http://www.5th-iarwisoci.iccs.ntua.gr/data/_uploaded/file/PROCEEDINGS_2012_5th_IARWISOCI_OPEN-ACCESS_VERSION_2012_DEC_05.pdf [Accessed 19 Jul. 2016].

[2] Dale RG, Hendry JH, Jones B, et al. Practical methods for compensating for missed treatment days in radiotherapy, with particularreference to head and neck schedules. Clin Oncol (R Coll Radiol). 2002 Oct;14(5):382-93. Available at: http://dx.doi.org/10.1053/clon.2002.0111.

[3] Dale RG, Jones B, Sinclair JA. Dose-equivalents of tumour repopulation during radiotherapy: the potential for confusion. Br J Radiol. 2000;73(872):892–894. Available at: http://dx.doi.org/10.1259/bjr.73.872.11026867.

[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] Hendry JH, Bentzen SM, Dale RG, et al. A modelled comparison of the effect of using different ways to compensate for missed treatment days in radiotherapy. Clin Oncol. 1996;8(5):297–307. Available at: http://dx.doi.org/10.1016/s0936-6555(05)80715-0.

[6] Haustermans K, Fowler J, Geboes K, et al. Relationship between potential doubling time (Tpot), labelling index and duration of DNA synthesis in 60 oesophageal and 35 breast tumours: is it worthwhile to measure Tpot? Radiother Oncol. 1998;46(2):157–167. Available at: http://dx.doi.org/10.1016/s0167-8140(97)00164-3.

[7] Trott KR, Kummermehr J. What is known about tumour proliferation rates to choose between accelerated fractionation or hyperfractionation? Radiother Oncol. 1985;3(1):1–9. Available at: http://dx.doi.org/10.1016/s0167-8140(85)80002-5.

[8] King CR. What is the Tpot for prostate cancer? Radiobiological implications of the equivalent outcome with 125I or 103Pd. Int J Radiat Oncol Biol Phys. 2000;47(5):1165–1167. Available at: http://dx.doi.org/10.1016/s0360-3016(00)00543-5.
[9] 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.
[10] Koukourakis M, Hlouverakis G, Kosma L, et al. The impact of overall treatment time on the results of radiotherapy for nonsmall cell lung carcinoma. Int J Radiat Oncol Biol Phys. 1996 Jan 15;34(2):315-22. Available at: http://dx.doi.org/10.1016/0360-3016(95)02102-7.
[11] Chen M, Jiang GL, Fu XL, et al. The impact of overall treatment time on outcomes in radiation therapy for non-small cell lung cancer. Lung Cancer. 2000 Apr;28(1):11-9. Available at: http://dx.doi.org/10.1016/S0169-5002(99)00113-0.