Data from: Probabilistic interfractional motion carbon ion radiation therapy dose distribution for prostate cancer shows rectum sparing with moderate target coverage degradation

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.