Purpose: This observational study investigates the influence of interfractional motion on clinical target volume (CTV) coverage, planning target volume (PTV) margins, and rectum tissue sparing in carbon ion radiation therapy (CIRT). It reports dose coverage to target structures and organs at risk in the presence of interfractional motion, investigates rectal tissue sparing, and provides recommendations for further lowering the rate of toxicity. We also propose probabilistic DVH for consideration in treatment planning to represent probable dose to the clinic’s patient population. Methods: At Gunma University Hospital intensity-modulated x-ray therapy (IMXT, aka IMRT) prostate cancer patients are positioned on a table which is shifted twice based on cone-beam computed tomography (CBCT) to align bones and then align prostate tissue to isocenter. These shifts thereby contain interfractional motion. 1306 such tableshifts from 85 patients were collected. Normal probability distributions were fit to the difference between bone-matching and prostate-matching CBCT-to-planning CT tableshifts (i.e. interfractional motion). Between 2011 and 2016 CIRT prostate patients were treated with PTV1 and PTV2 margins as follows: PTV1 extends the prostate contour by 10/10, 5/10, 6/6 mm in the right/left, posterior/anterior, and superior/inferior directions, respectively, and the proximal seminal vesicles contour by 5 mm superiorly and inferiorly, 3 mm right and left. PTV2 reduces PTV1 posteriorly along a straight line to exclude the rectum and reduces the superior and inferior margins by 6 mm. From those treated with these margins, 40 patients’ beam data were selected to create probable interfractional motion: The previously fit normal probability distributions were randomly sampled 2000 times per patient and beams shifted to simulate this motion. These shifted dose distributions were scaled down proportionately in magnitude and summed to obtain probable blurred dose distributions. Results: Probable dose to rectum is substantially less than planned for doses higher than 10 Gy(RBE). Absolute DVH show that mean clinical target volumes are about 138-670 cm3 smaller for a given probable dose than planned doses higher than 57 Gy(RBE) after accounting for standard error. Cumulative DVH show mean CTV fraction receiving a given probable dose is less than the mean fraction receiving the corresponding planned dose for doses larger than 52 Gy(RBE), up to 19% less at 57.4 Gy(RBE). Our PTV1 margins generally cover 95% of interfractional motion but seminal vesicles and inferior prostate receive less dose than planned due to insufficient PTV2 margins. Conclusion: Assuming rigidly shifting interfractional motion around the prostate region and neglecting minor changes in soft tissue stopping power, interfractional motion resulted in underdosing or tissue sparing in all cases. Given our low rates of relapse and recurrence, it appears less curative dose is needed than previously thought or else the target may be smaller than previously thought. In-room CT may be useful to lower dose, shrink target margins, conduct PET auto-activation dose verification studies and account for interfractional motion.