How to avoid ulcers in s.c. injected tumors ?

Recently, we have injected s.c. B16F10 tumor cells in BL/6 mice. At around 12-15th day some animals showed ulcerations in tumors , with a big hole in the middle (it looks like a volcano). We have also observed this problem in MC38 and EG7 tumors. This did not happen to us so often until now. Any suggestion or personal experience that helps us to resolve this issue will be really appreciated. Thanks in advance.

Neoplasms

Ulcer

Mice

Rosa Ana Lacalle

Centro Nacional de Biotecnología

Thank you Michaelia for your suggestions, we usually inject 0,2x106B16F10 cells and we end the experiment at day 20. Perhaps we should change the initial tumor cells dose and finish the experiment before .