Using Bolus

[radoncmonkey]

I'm too afraid to ask such a dumb question to my attendings or co-residents, so I'll cop out and ask my anonymous friends on SDN here..be nice =)

I don't feel I have a good understanding of when to use bolus and how much to use. I understand what bolus does, but when I'm on the spot in a sim and the therapist says "how much bolus do you want?" I never know if it should be 0.5 or 1cm.

When using IMRT, do you ever need to use bolus? Except maybe with say an N+ groin in anal or something, I don't see other uses..

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[RadOncDoc21]

Since you stated you know what bolus does, I'll bypass the physics lecture.

For the most part, we use 0.5 cm for photons (6 MV). In my experience (pgy-3), the only time it gets tricky is for electrons... in that case you have to calculate the depth and use the E/3 or E/4 for whichever isodose line you're prescribing to. I'll leave the rest to my peers to guide you to the correct explanation in more detail.

 

[ShirleyT]

well i can give you my two cents,although it may not be worth much.

I usually use bolus when I want to decrease the skin-sparing effect of photons. Like in postmastectomy chest wall cases, i add bolus as needed to get a brisk skin reaction, 3mm or 5mm. For IMRT, you often don't need bolus unless you want to get good dose to the skin as you mentioned. I still bolus the scar (3mm) for postop head and neck cases, although this may not be necessary since the skin reactions are brisk with imrt between the mask and the tangential beams with IMRT. If you are doing something like total scalp IMRT, i use 5mm bolus and our physics team says 5mm bolus combined with imrt will get full dose to the skin and 5mm bolus is much easier to work with than 1 cm bolus....

For electrons, if you want the skin to get close to full dose, you need to add bolus unless it is 16-20 MeV electrons since these electrons get close to full dose at the surface. Then I usually use 1 cm. Sometimes I will use bolus in electrons to act as a "compensator" to protect underlying structures. For example, in post-op parotid cases, I will add bolus (1-2) cm over the temporal lobe to decrease dose to the brain, but still cover the lymph nodes below the brain level. In this case, the bolus absorbs the excess exit dose rather than the brain (good).