For running DICOM_Phantom.ipynb in Google colab where I was working, you must do the following things:

1. In your content folder, create a new folder called TPS.
2. Then, drop in the the CT DICOM file attached.
3. Then, create a new folder to save your phantom as a DICOM series, I call it phantom_tryx, where x is the number of tries that I have going.

Once runtime is disconnected, these local files are discarded.