A Prospective Comparison of the Effects of Interfractional Variations on Proton Therapy and Intensity Modulated Radiation Therapy for Prostate Cancer

Abstract

Purpose To quantify and compare the impact of interfractional setup and anatomy variations on proton therapy (PT) and intensity modulated radiotherapy (IMRT) for prostate cancer. Methods and Materials Twenty patients with low- or intermediate-risk prostate cancer randomized to receive passive-scattering PT (n=10) and IMRT (n=10) were selected. For both modalities, clinical treatment plans included 50.4 Gy(RBE) to prostate and proximal seminal vesicles, and prostate-only boost to 79.2 Gy(RBE) in 1.8 Gy(RBE) per fraction. Implanted fiducials were used for prostate localization, and endorectal balloons for immobilization. Patients in PT and IMRT arms received weekly CT and cone-beam CT (CBCT) scans, respectively. The planned dose was re-calculated on each weekly image, scaled and mapped onto the planning CT using deformable registration. The resulting accumulated dose distribution over the entire treatment course was compared with the planned dose using dose-volume histogram (DVH) and gamma analysis. Results The target conformity index remained acceptable after accumulation. The largest decrease in the average prostate D98 was 2.2 and 0.7 Gy for PT and IMRT, respectively. On average, the mean dose to bladder increased by 3.26±7.51 Gy and 1.97±6.84 Gy for PT and IMRT, respectively. These values were 0.74±2.37 and 0.56±1.90 for rectum. Differences between changes in DVH indices were not statistically significant between the two modalities. All volume indices remained within the protocol tolerances after accumulation. The average pass rate in gamma analysis, assuming tolerances of 3 mm and 3%, for PT/IMRT for CTV, bladder, rectum and whole patient were 100/100, 92.6/99, 99.2/100 and 97.2/99.4, respectively. Conclusion The difference in target coverage and OAR dose deviations for PT and IMRT was not statistically significant under the guidelines of this protocol.

Proton therapy- the modality of choice for future radiation therapy management of Prostate Cancer?

“…11 studies demonstrate plan robustness is acceptable for prostate proton plans [35,36,39,[41][42][43][44][45][46][47][48], as summarised in Table 2. Soukup et al reported that the sensitivities of Intensity Modulated-PR (IMPR) and IMRT plans to organ motion are similar if a rectal gas water-equivalent density overwrite on the original planning CT is applied [36].…”

“…Plan robustness with respect to inter and intrafraction motion was discussed in 7 studies. Wang et al and Moteabbed et al stated that PR plans were generally robust to interfractional variations, but rectal gas was the leading cause of target coverage reduction [41,48], while Thornqvist et al [42] found the prostate robust to interfraction motion with fiducial-based positioning. Seminal vesicle variation was a concern, and also reported elsewhere [37].…”

Three Discipline Collaborative Radiation Therapy (3DCRT) Special Debate: I would treat prostate cancer with proton therapy

“…In the case of the prostate, the Bragg peak might represent an advantage in unidirectional sparing predefined OARs, but the central location of the prostate and the lateral incidence of the proton beams implies for the beams the need to be degraded to reach a homogeneous dose distribution while failing in their conformality when compared to photons . Goddard showed that even when using hypofractionation to treat prostate cancer, VMAT is still superior to IMPT in terms of target conformity and OARs sparing, while interfractions and intrafractions organs motion still represent a problem in proton delivery . It is interesting that in their zeal of finding usefulness for protons, investigators now point out that while the high‐dose areas in OARs are not any better with protons, maybe the low‐dose areas (where protons presumably offer some advantage) are the ones instrumental to injury!…”

“…Inter‐ and intra‐fractional organ motion poses similar challenges to both proton therapy and IMRT: Moteabbed et al found no statistically significant differences in DVH indices between passive‐scattering proton therapy and IMRT.…”

Dosimetric Impact of the Positional Imaging Frequency for Hypofractionated Prostate Radiotherapy – A Voxel-by-Voxel Analysis

“…While the anatomic changes during the course of therapy have been widely studied, the consequences of these changes for the applied radiation doses remain incompletely understood ( 10 – 12 ). The majority of previous publications was based on weekly CT scans and rigid co-registration algorithms and demonstrated significant dosimetric variability for normofractionated IMRT and proton radiotherapy ( 13 – 15 ). So far, very few data are available comparing planned and applied doses for prostate radiotherapy based on daily imaging and deformable imaging registration, and no information is available for hypofractionated treatment concepts ( 16 , 17 ).…”