Dosimetric evaluation of prostate rotations and their correction by couch rotations

doi:10.1016/j.radonc.2008.03.016

Radiotherapy and Oncology

Volume 88, Issue 1, July 2008, Pages 156-162

https://doi.org/10.1016/j.radonc.2008.03.016 Get rights and content

Abstract

Purpose

To investigate the dosimetric effect of prostate rotations and limited on-line corrections by couch rotations (⩽3°) for prostate, seminal vesicles and organs at risk.

Methods

For 5 patients IMRT plans were made, treating the prostate plus base of the seminal vesicles. Realistic and idealised dose distributions were considered, the latter demonstrating extreme effects for rotations and their corrections. Translation errors were assumed to be corrected on-line.

For each patient 20 treatments with different rotation errors were simulated: 20 systematic errors were generated and 20 times 35 random deviations were superimposed to simulate day-to-day variations. Using a research module of PLATO-RTS treatments with rotation errors, with and without on-line corrections, were simulated.

Results

The largest dosimetric effect of rotation errors and corrections was found for the seminal vesicles with idealised dose distribution: coverage improved from 92.6% (range 89.9–96.0%) to 95.9% (94.7–98.1%). The gain for the idealised prostate was less: 95.9% (94.4–97.0%) to 97.5% (95.5–98.4%). For the femoral heads the dose increase could be as large as 12.2% (6.2–19.3%).

Conclusions

On-line correction of rotations can improve target coverage slightly. For organs at risk at a large distance from the isocentre the result can be a significant increase in dose.

Section snippets

Patient data

CT-scans of 5 consecutive patients with prostate cancer were selected. The prostate, the base of the seminal vesicles, rectum, bladder and femoral heads were contoured according to our clinical protocol. The base of the seminal vesicles was defined as the first 2 cm of the seminal vesicles from the base of the prostate in the sagittal view. The clinical target volume (CTV) contained the prostate and the base of the seminal vesicles. No-margin was added in order to explore the maximum effect of

Results

On average the rotation correction could reduce the systematic rotation error fully in 50% of all rotations around the left–right axis and in 75% of all rotations around the anterior–posterior and cranial–caudal axis. For the combined systematic and random rotations, i.e. for each treatment, these numbers were 35%, 70%, and 62%, respectively.

In the five patients considered we observed similar behaviour in the results, which allows us to discuss them jointly. For clarity the results of a single

Discussion

The dosimetric effects of rotations and consequently the corrections of those rotations turn out to be modest. Averaged over 5 patients, the coverage for the idealised seminal vesicles is improved from 92.6% (range 89.9–96.0%) to 95.9% (range 94.7–98.1%). The gain for the idealised prostate was less: the deteriorated coverage of 95.9% (range 94.4–97.0%) could be improved to 97.5% (range 95.5–98.4%). That this effect is small is due to the more or less spherical shape of the prostate_95 and the

Conclusion

The dosimetric effect of prostate rotations was studied and it is concluded that prostate rotations have no large impact on prostate coverage. The coverage of the seminal vesicles on the other hand can in extreme cases deteriorate more. With on-line correction of rotations the dose coverage of the target generally improves. For organs at risk near the target, the dose distribution could be improved as well as deteriorated. Most importantly, the dose to organs at risk at large distances from the

Acknowledgement

The authors thank Nucletron B.V. for the research module of PLATO-RTS that was used for the simulations.

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Dosimetric impact of interfraction prostate and seminal vesicle volume changes and rotation: A post-hoc analysis of a phase III randomized trial of MRI-guided versus CT-guided stereotactic body radiotherapy
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Interfraction volumetric changes/rotations in the prostate and proximal seminal vesicles (SVs) might compromise target coverage when tight margins are used for prostate stereotactic body radiotherapy (SBRT). We investigated on-board MRI images from MRI-guided SBRT to better understand this.
Twenty consecutive patients treated with MRI-guided prostate SBRT (40 Gy/5 fractions) enrolled on the MRI arm of a phase III randomized trial were included. A 2 mm isotropic margin was used for prostate and proximal SVPTV. Target volume, prostate dimensions, angles of the proximal SV on axial (angle α) and sagittal view (angle θ) were measured on a 0.35 T simulation MRI and five on-board pre-treatment MRIs. Dice coefficient of the targets and target dosimetry were calculated.
All patients experienced an isotropic increase in prostate volume during SBRT (p = 0.0016): 0.1%, 9.0%, 12.1%, 15.1%, and 14.2% (median) at fractions 1–5, respectively, regardless of baseline volume, which was significantly reduced with neoadjuvant ADT (p = 0.0042). There was minimal interfraction rotation of prostate, however, considerable variations in proximal SV angle α (median 21.5°) and angle θ (median 17.6°) were seen. Median V100% was 97.5% and 87.1% for prostate and proximal SV CTV, respectively. V95%≥95% was achieved in 94% of fractions for the prostate and in 59% for proximal SV.
Prostate volume consistently increased during SBRT. Interfraction prostatic rotation was minimal while rotation of the proximal SV was considerable. Prostate dosimetry was favorable, but online adaptive therapy may be indicated occasionally to account for prostatic swelling and in particular proximal SV rotations.
The dosimetric impact of prostate rotations during electromagnetically guided external-beam radiation therapy
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Van Hertin et al (9), in contrast, concluded that prostate rotations around RL axis have minimal impact on prostate coverage given that translations were managed. This conclusion was based on evaluating dosimetric impact of prostate rotations using simulated treatment plans with different rotation errors (9). This disagreement could be attributed to using simulated rotational errors on 5 patients’ CT data vs real rotations measured in 26 patients or the nature of simulated error.
To study the impact of daily rotations and translations of the prostate on dosimetric coverage during radiation therapy (RT).
Real-time tracking data for 26 patients were obtained during RT. Intensity modulated radiation therapy plans meeting RTOG 0126 dosimetric criteria were created with 0-, 2-, 3-, and 5-mm planning target volume (PTV) margins. Daily translations and rotations were used to reconstruct prostate delivered dose from the planned dose. D₉₅ and V₇₉ were computed from the delivered dose to evaluate target coverage and the adequacy of PTV margins. Prostate equivalent rotation is a new metric introduced in this study to quantify prostate rotations by accounting for prostate shape and length of rotational lever arm.
Large variations in prostate delivered dose were seen among patients. Adequate target coverage was met in 39%, 65%, and 84% of the patients for plans with 2-, 3-, and 5-mm PTV margins, respectively. Although no correlations between prostate delivered dose and daily rotations were seen, the data showed a clear correlation with prostate equivalent rotation.
Prostate rotations during RT could cause significant underdosing even if daily translations were managed. These rotations should be managed with rotational tolerances based on prostate equivalent rotations.
Electromagnetic detection and real-time DMLC adaptation to target rotation during radiotherapy
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As with translational motion, rotational motion can severely compromise target dose coverage and normal tissue sparing if it is not accounted for. Rotation may cause part of the target volume to move out of the treatment field and result in underdose (13, 14). Recently, Li et al. (15) found that the dosimetric discrepancies caused by prostate rotation were more significant than those caused by translational intrafractional motion.
Intrafraction rotation of more than 45° and 25° has been observed for lung and prostate tumors, respectively. Such rotation is not routinely adapted to during current radiotherapy, which may compromise tumor dose coverage. The aim of the study was to investigate the geometric and dosimetric performance of an electromagnetically guided real-time dynamic multileaf collimator (DMLC) tracking system to adapt to intrafractional tumor rotation.
Target rotation was provided by changing the treatment couch angle. The target rotation was measured by a research Calypso system integrated with a real-time DMLC tracking system employed on a Varian linac. The geometric beam-target rotational alignment difference was measured using electronic portal images. The dosimetric accuracy was quantified using a two-dimensional ion chamber array. For each beam, the following five delivery modes were tested: 1) nonrotated target (reference); 2) fixed rotated target with tracking; 3) fixed rotated target without tracking; 4) actively rotating target with tracking; and 5) actively rotating target without tracking. Dosimetric performance of the latter four modes was measured and compared to the reference dose distribution using a 3 mm/3% γ-test.
Geometrically, the beam-target rotational alignment difference was 0.3° ± 0.6° for fixed rotation and 0.3° ± 1.3° for active rotation. Dosimetrically, the average failure rate for the γ-test for a fixed rotated target was 11% with tracking and 36% without tracking. The average failure rate for an actively rotating target was 9% with tracking and 35% without tracking.
For the first time, real-time target rotation has been accurately detected and adapted to during radiation delivery via DMLC tracking. The beam-target rotational alignment difference was mostly within 1°. Dose distributions to fixed and actively rotating targets with DMLC tracking were significantly superior to those without tracking.
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1016 CBCT datasets were analyzed for this report. The mean of the rotations were not meaningfully different from zero. 18.1% of the fractions had rotations with a magnitude ⩾2°, 5.0% had rotations ⩾3° and 0.9% had rotations ⩾4°. For the 2° rotational simulation, the gEUD values of the PTV and critical structures changed by less than 2%. For the 4° simulation, parallel type normal structures had minor changes (<2%), but serial type normal structures and the PTV had changes of 10% and 5%, respectively.
The majority of rotational errors observed were less than 1°. A rotational error of 2° produced negligible changes in the gEUD to critical structures or target volumes. Rotational errors ⩾4° produced undesirable results, therefore, at a minimum, errors >2° should be corrected.
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To quantify the daily rotation of the prostate during a radiotherapy course using stereoscopic kilovoltage (kV) x-ray imaging and intraprostatic fiducials for localization and positioning correction. From 2005 to 2009, radio-opaque fiducial markers were inserted into 38 patients via perineum into the prostate. The ExacTrac/Novalis Body X-ray 6-day image acquisition system (ET/NB; BrainLab AG, Feldkirchen, Germany) was used to determine and correct the target position. During the first period in 10 patients we recorded all rotation errors but used only Y (table) for correction. For the next 28 patients we used for correction all rotational coordinates, i.e., in addition Z (superior-inferior [SI] or roll) and X (left-right [LR] or tilt/pitch) according to the fiducial marker position by use of the Robotic Tilt Module and Varian Exact Couch. Rotation correction was applied above a threshold of 1° displacement. The systematic and random errors were specified. Overall, 993 software-assisted rotational corrections were performed. The interfraction rotation errors of the prostate as assessed from the radiodense surrogate markers around the three axes Y, Z, and X were on average 0.09, −0.52, and −0.01° with standard deviations of 2.01, 2.30, and 3.95°, respectively. The systematic uncertainty per patient for prostate rotation was estimated with 2.30, 1.56, and 4.13° and the mean random components with 1.81, 2.02, and 3.09°. The largest rotational errors occurred around the X-axis (pitch), but without preferring a certain orientation. Although the error around Z (roll) can be compensated on average by a transformation with 4 coordinates, a significant error around X remains and advocates the full correction with 6 coordinates. Rotational errors as assessed via daily stereoscopic online imaging are significant and dominate around X. Rotation possibly degrades the dosimetric coverage of the target volume and may require suitable strategies for correction.
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DosimetryDosimetric evaluation of prostate rotations and their correction by couch rotations

Abstract

Purpose

Methods

Results

Conclusions

Section snippets

Patient data

Results

Discussion

Conclusion

Acknowledgement

Int J Radiat Oncol Biol Phys

Radiother Oncol

Radiother Oncol

Radiother Oncol

Radiother Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Radiother Oncol

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Int J Radiat Oncol Biol Phys

Dosimetry
Dosimetric evaluation of prostate rotations and their correction by couch rotations