Re: Unresectable subcutaneous mast cell tumour

Kara Gibson — Sat, 28 Jun 2014 07:53:31 GMT

Thank you so much Gerry!

Re: Unresectable subcutaneous mast cell tumour

Gerry Polton — Fri, 27 Jun 2014 15:49:50 GMT

Hi Kara

Personally I would neither test for Ki-67 or c-kit. I would simply start the masitinib therapy. Ki-67 is a marker of proliferation. A high rate of proliferation is a generic marker of badness in (almost) all cancers. if you were unsure about whether this tumour was bad or had a high rate of proliferation this could be a helpful test. however, the elevated mitotic index already tells us everything we need to know about the proilferation of the tumour. And I gather that the CT shows evidence of infiltrative growth so I would consider it to be bad. The c-kit tests that are routinely available I believe are simply immunohistochemical tests of where the c-kit molecule is found to be expressed in the cancerous mast cells. There is some evidence of an association between expression in the wrong places and malignancy. However, once again, we know that this is a malignant tumour from the mitotic index and the infiltrative growth seen on CT, so I can't see how the c-kit test will advance your knowledge of the tumour. It is a common misconception that c-kit immunohistochemistry might somehow predict responsiveness to a tyrosine kinase inhibitor. If in fact you were proposing a pcr analysis of c-kit mutation status I apologise, but I still wounldn't do it. It takes too long, it misses some mutations, and it still only manages to predict response in approximately 67% of cases whereas recent data appears to indicate that an experienced oncologist just looking at a case and deciding it is a bad one accurately predicts response to masitinib in 89% of cases. So, to reiterate, my advice would be just to get on and start treatment.

Best of luck

Gerry