Table 1. Techniques to analyze metastasis progression in the brain.

There are multiple available techniques (columns) for assessing the development and treatment of brain metastasis. In selecting the most appropriate approach, it is essential that the researcher considers different parameters (rows). Each cell in the table provides binary information (yes (white background)/ no (red background)) regarding the compatibility of the technique with the specific parameter. However, for several parameters it is important to consider additional quantitative or qualitative aspects. For example, the row testing “sensitivity” can be divided into the different stages of brain metastasis development. The different stages derive from the number of cells in a given metastasis (see (*) at the bottom). The parameters “technical requirement” and “time requirement” do not respond to binary options but are better defined by semiquantitative considerations of feasibility and resource requirements. Finally, the last row reflects potential limitations for a given technique, such as the need to have the BrM cells engineered with particular reporters or the need to use antibodies. We provide a graded code using “+” signs to suggests what is (+) somehow suitable, (++) suitable, and (+++) optimal. A key reference has been added to each technique to provide an example of its implementation.

	Histology⁵²	FACS²⁵	PCR³⁷	MRI³¹	Bioluminescence^22,53	Two-photon video-microscopy⁵⁴
Techniques to analyze metastasis progression
Compatible with in vivo	No	No	No	Yes⁽⁺⁺⁺⁾ (Non-invasive)	Yes⁽⁺⁺⁺⁾ (Non-invasive)	Yes ⁽⁺⁺⁺⁾ (Partially-invasive)
Sensitivity ^(*)	Single cell→Big	Medium-Big	Medium-Big	Medium-Big	Medium-Big	Single cell→Small
Number of metastases	Yes ⁽⁺⁺⁺⁾	No	No	Yes⁽⁺⁺⁾	Yes ⁽⁺⁾	No (In field-of-view only
Size	Yes ⁽⁺⁺⁺⁾	No	No	Yes⁽⁺⁺⁾	Yes ⁽⁺⁾	Yes ⁽⁺⁺⁺⁾
Distribution	Yes ⁽⁺⁺⁺⁾	No	No	Yes⁽⁺⁺⁺⁾	Yes⁽⁺⁺⁾	No
Growth	Yes ⁽⁺⁺⁺⁾ (cellular pattern)	No	No	Yes⁽⁺⁺⁺⁾ (over time)	Yes⁽⁺⁺⁺⁾ (over time)	Yes⁽⁺⁺⁺⁾ (cell, pattern/over time)
Microenvironment features	Yes ⁽⁺⁺⁺⁾	Yes ⁽⁺⁺⁾	Yes ⁽⁺⁾	Yes ⁽⁺⁺⁾	No	Yes⁽⁺⁺⁺⁾
Technical requirement (Equipment/ expertise)	Low/ Low ⁽⁺⁺⁺⁾	Low/ Low ⁽⁺⁺⁺⁾	Low/ Low ⁽⁺⁺⁺⁾	High/ High	Medium/ Medium ⁽⁺⁺⁾	High/ High
Time requirement	Medium-high	Low ⁽⁺⁺⁺⁾	Low ⁽⁺⁺⁺⁾	Medium	Low ⁽⁺⁺⁺⁾	High
Info about extra-cranial	No	No	No	No	Yes⁽⁺⁺⁺⁾	No
Particular considerations	Antibodies	Antibodies and/ or Engineering BrM cells	Relative quantification	High field strength	Engineering BrM cells	Engineering BrM eel

^(*)

Sensitivity: Small (micrometastasis): 5–50 cells

Medium (micrometastasis): <500 cells

Big (macrometastasis): >1000 ceils

Suitability:

⁽⁺⁾

Somehow suitable

⁽⁺⁺⁾

Suitable

⁽⁺⁺⁺⁾

Optimal