Tattoo
Tattoo
[Megan is in the panel. Megan points at her chest.] Megan: I just have one tattoo - it's six dots on my chest, done by my oncologist.
Megan: I need them for aligning the laser sights on a flesh-searing relativistic particle cannon,
Megan: So it will only kill the parts of me
[Dramatic zoom, the panel background is black, with white text.] Megan: That are holding me back.
[The panel is larger, revealing who they're talking to.] Megan: But your barbed wire bicep tattoo is pretty hardcore, too! Cueball: No, it's OK.  I'll just go put a shirt on.

An oncologist is a doctor who specializes in the treatment of cancer . This comic is certainly related to Randall's wife's breast cancer . Megan in this comic has a tattoo for the alignment lasers of the radiotherapy machine which will fire a beam of radiation with sufficient intensity to kill the cells in the targeted area. A common such machine is a linear accelerator or "Linac" which accelerates electrons to very high speed, these can then either be used to generate high energy X-rays to treat the patient, or the electron beam itself can be used (both are types of radiation; the electrons being beta radiation and x-rays being EM [electromagnetic] radiation). Commonly when radiotherapy is used as part of breast cancer treatment some combination of both is prescribed. In order to allow healthy tissue to recover better, rather than deliver all the radiation in one go, the treatment is delivered a little bit each day over the course of about a month. It is therefore vital that the radiation can be delivered to the correct target area day after day, and this is done by lining up the alignment lasers of the linac with the skin markers - that is Megan's tattoo dots. It may not be considered a "traditional" tattoo (because she says it was done by her oncologist and not in a tattoo parlor).
In the last frame, it is mentioned that Cueball has a barbed wire bicep tattoo, which is common in the US as a tattoo that people get when they want to seem tough, even if they aren't tough already.
The joke in the comic is that Cueball got this barbed wire tattoo to look tough, but it pales in comparison to the tattoo from (or for) the cancer removal or treatment. It is kind of funny because Cueball has his whole shirt off just to show a biceps tattoo.
The title text references gamma ray therapy after describing electron linear accelerator-based treatment systems; however, technically gamma ray therapy only refers to radionuclide (i.e., Cobalt-60) based radiation therapy systems.  In regards to a 90-second session killing a horse, typical dose rates of modern radiation therapy systems are of the order of several gray per minute for the field sizes used, for example, in the treatment of breast cancer .  It is feasible that a single 90-second delivery of radiation could deliver over 10 Gy in a single instance to the specific areas of the body that could be fatal, such as neuropathy or radiation induced liver disease.
[Megan is in the panel. Megan points at her chest.] Megan: I just have one tattoo - it's six dots on my chest, done by my oncologist.
Megan: I need them for aligning the laser sights on a flesh-searing relativistic particle cannon,
Megan: So it will only kill the parts of me
[Dramatic zoom, the panel background is black, with white text.] Megan: That are holding me back.
[The panel is larger, revealing who they're talking to.] Megan: But your barbed wire bicep tattoo is pretty hardcore, too! Cueball: No, it's OK.  I'll just go put a shirt on.
