11. Dosimetry


Even though I simplified this drawing, it remains a bit complex. The curved lines show the likelihood of tumor control and complications. The red vertical lines relate the curved lines to each other. The vertical dotted line achieves the best combination of tumor control with fewest complications. That is possible because the red curved line has moved farther to the right, thereby increasing the therapeutic window fourfold. This increase is made possible by pencil beam scanning (PBS).


This chapter on dosimetry shows a lot of anatomical views. This one shows the location of the lymph nodes (in red) and the bladder (in yellow). Protons can treat all of the lymph nodes without irradiating the bladder. X-rays cannot avoid going right across the bladder.





This graph is for high risk prostate cancer, meaning it is also treating the semial vesicles and lymph nodes. It shows how treating from each side avoids potential damage to the bladder, in the midle.







Using Ray Station software, the dosimetrists draw the various organs in different colors, for easy reference. They do this for each patient. These are for me. The lymph nodes are down at the bottom, quite a ways from the prostate (outlined in red) and its margins (pink circle cencentric to the red one). I calculated the entire treatment area to be more than seven inches across.














The following are bonus drawings, to show some of the views the dosimetrists use.







This bonus drawing shows the prostate in the center, outlined in red.






This fused picture of two CT scans from different days shows movement in most of the organs (bladder in yellow, femur heads left and right, rectum on the bottom) but none in the prostate itself (red). That’s what really counts; which they verify each day using the x-rays of my fiducial markers. The location of the rest of the anatomy can vary slightly without affecting the treatment of the prostate.



This iconic proton therapy graph shows energy release. The orange represents protons, which at the end of their journey have a big spike in energy called the Bragg’s Peak and then stop, with no exit dose. The gray area represents traditional single-beam x-rays which have a considerable entry dose and continue past the target (exit dose). This very popular graphic shows an important relationship, but doesn’t reflect the fact that single x-ray beams are rarely used any more.



In reality, numerous Bragg's peaks are combined to form an area called the Spread Out Bragg's Peak (SOBP).





Here the protons are in white and the x-rays in pink.





This graph shows the methodology of two types of radiation, protons (left) and x-rays (right). IMRT involves lower dose x-rays from numerous direction converging to give a full does to the target. This reduces the damage to healthy tissue. Nevertheless, x-rays result in far more secondary cancer than protons, so the exposure must have a negative effect.





Here you can see how the magnets control the scanning for the pencil beams.






This gantry is in the proton therapy center in Prague.







I walked past this poster every day on the way to my treatment room, so I took a photo of it.






You may also be interested in our list of proton centers around the United States, found at:

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