DOI: 10.5061/dryad.mg783s0
title: Data from: Probabilistic interfractional motion carbon ion radiation therapy dose distribution for prostate cancer shows rectum sparing with moderate target coverage degradation

description: 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.
description: PLoSOneDataA compressed folder containing 40 anonymized carbon ion radiotherapy treatment plans; calculated DVH of rectum, CTV, PTV1, and PTV2; and IMXT tableshifts from prostate cancer patients treated Monday, Wednesday, and Friday to 3 Gy per fraction for 21 fractions.