Dosimetric consequences of intrafraction prostate motion

PURPOSE: To analyze characteristics of intrafraction prostate motion, monitored using the Calypso system, and investigate dosimetric consequences of the motion for different clinical target volume (CTV) to planning target volume (PTV) margins. METHODS AND MATERIALS: Motion characteristics were analyzed for 1,267 tracking sessions and 35 patients. Using prostate-PTV margins of 0, 1, 2, 3, and 5 mm, dose metrics for the prostate gland, bladder, and rectum were evaluated for scenarios including patient population, individual patients showing the greatest motion during the course of treatment, and the individual session with the largest overall movement. Composite dose distributions incorporating motion blurring were calculated by convolving static intensity-modulated radiotherapy plans with corresponding motion probability functions. RESULTS: For prostate-PTV margins of 2 mm or greater, intrafraction motion did not compromise prostate dose coverage for either the patient population or individual patients. For the patient showing the largest overall movement, the prostate equivalent uniform dose was reduced by only 17.4 cGy (0.23%), and the minimum prostate dose remained greater than 95% of the nominal dose. For margins less than 2 mm, the prostate dose-volume histogram in the same patient was slightly compromised, and the equivalent uniform dose was reduced by 38.5 cGy (0.51%). Sparing of the bladder and rectum was improved substantially by reducing margins. CONCLUSIONS: Although significant motion can be observed during individual fractions, the dosimetric consequences are insignificant during a typical course of radiotherapy (30-40 fractions) with CTV-PTV margins of 2 mm or greater provided that the Calypso system is applied for pretreatment localization. Further reduction of the margin is possible if intrafraction realignment is performed.     

Influence of intrafraction motion on margins for prostate radiotherapy

PURPOSE: To assess the impact of intrafraction intervention on margins for prostate radiotherapy. METHODS AND MATERIALS: Eleven supine prostate patients with three implanted transponders were studied. The relative transponder positions were monitored for 8 min and combined with previously measured data on prostate position relative to skin marks. Margins were determined for situations of (1) skin-based positioning, and (2) pretreatment transponder positioning. Intratreatment intervention was simulated assuming conditions of (1) continuous tracking, and (2) a 3-mm threshold for position correction. RESULTS: For skin-based setup without and with inclusion of intrafraction motion, prostate treatments would have required average margins of 8.0, 7.3, and 10.0 mm and 8.2, 10.2, and 12.5 mm, about the left-right, anterior-posterior, and cranial-caudal directions, respectively. Positioning by prostate markers at the start of the treatment fraction reduced these values to 1.8, 5.8, and 7.1 mm, respectively. Interbeam adjustment further reduced margins to an average of 1.4, 2.3, and 1.8 mm. Intrabeam adjustment yielded margins of 1.3, 1.5, and 1.5 mm, respectively. CONCLUSION: Significant reductions in margins might be achieved by repositioning the patient before each beam, either radiographically or electromagnetically. However, 2 of the 11 patients would have benefited from continuous target tracking and threshold-based intervention.     

Three-dimensional intrafractional movement of prostate measured during real-time tumor-tracking radiotherapy in supine and prone treatment positions

To quantify three-dimensional (3D) movement of the prostate gland with the patient in the supine and prone positions and to analyze the movement frequency for each treatment position.

The real-time tumor-tracking radiotherapy (RTRT) system was developed to identify the 3D position of a 2-mm gold marker implanted in the prostate 30 times/s using two sets of fluoroscopic images. The linear accelerator was triggered to irradiate the tumor only when the gold marker was located within the region of the planned coordinates relative to the isocenter. Ten patients with prostate cancer treated with RTRT were the subjects of this study. The coordinates of the gold marker were recorded every 0.033 s during RTRT in the supine treatment position for 2 min. The patient was then moved to the prone position, and the marker was tracked for 2 min to acquire data regarding movement in this position. Measurements were taken 5 times for each patient (once a week); a total of 50 sets for the 10 patients was analyzed. The raw data from the RTRT system were filtered to reduce system noise, and the amplitude of movement was then calculated. The discrete Fourier transform of the unfiltered data was performed for the frequency analysis of prostate movement.

No apparent difference in movement was found among individuals. The amplitude of 3D movement was 0.1–2.7 mm in the supine and 0.4–24 mm in the prone positions. The amplitude in the supine position was statistically smaller in all directions than that in the prone position (p < 0.0001). The amplitude in the craniocaudal and AP directions was larger than in the left-right direction in the prone position (p < 0.0001). No characteristic movement frequency was detected in the supine position. The respiratory frequency was detected for all patients regarding movement in the craniocaudal and AP directions in the prone position. The results of the frequency analysis suggest that prostate movement is affected by the respiratory cycle and is influenced by bowel movement in the prone position.

The results of this study have confirmed that internal organ motion is less frequent in the supine position than in the prone position in the treatment of prostate cancer. RTRT would be useful in reducing uncertainty due to the effects of the respiratory cycle, especially in the prone position.     

A randomized comparison of interfraction and intrafraction prostate motion with and without abdominal compression

BACKGROUND AND PURPOSE: To quantify inter- and intrafraction prostate motion in a standard VacLok (VL) immobilization device or in the BodyFix (BF) system incorporating a compression element which may reduce abdominal movement. MATERIALS AND METHODS: Thirty-two patients were randomly assigned to VL or BF. Interfraction prostate motion >3 mm was corrected pre-treatment. EPIs were taken daily at the start and end of the first and last treatment beams. Interfraction and intrafraction prostate motion were measured for centre of mass (COM) and individual markers. RESULTS: There were no significant differences in interfraction (p0.002) or intrafraction (p0.16) prostate motion with or without abdominal compression. Median intrafraction motion was slightly smaller than interfraction motion in the AP (7.0 mm vs. 7.6 mm) and SI direction (3.2 mm vs. 4.7 mm). The final image captured the maximal intrafraction displacement in only 40% of fractions. Our PTV incorporated >95% of total prostate motion. CONCLUSIONS: Intrafraction motion became the major source of error during radiotherapy after online correction of interfraction prostate motion. The addition of 120 mbar abdominal compression to custom pelvic immobilization influenced neither interfraction nor intrafraction prostate motion.     

Planning target margin calculations for prostate radiotherapy based on intrafraction and interfraction motion using four localization methods

PURPOSE: To determine planning target volume (PTV) margins for prostate radiotherapy based on the internal margin (IM) (intrafractional motion) and the setup margin (SM) (interfractional motion) for four daily localization methods: skin marks (tattoo), pelvic bony anatomy (bone), intraprostatic gold seeds using a 5-mm action threshold, and using no threshold. METHODS AND MATERIALS: Forty prostate cancer patients were treated with external radiotherapy according to an online localization protocol using four intraprostatic gold seeds and electronic portal images (EPIs). Daily localization and treatment EPIs were obtained. These data allowed inter- and intrafractional analysis of prostate motion. The SM for the four daily localization methods and the IM were determined. RESULTS: A total of 1532 fractions were analyzed. Tattoo localization requires a SM of 6.8 mm left-right (LR), 7.2 mm inferior-superior (IS), and 9.8 mm anterior-posterior (AP). Bone localization requires 3.1, 8.9, and 10.7 mm, respectively. The 5-mm threshold localization requires 4.0, 3.9, and 3.7 mm. No threshold localization requires 3.4, 3.2, and 3.2 mm. The intrafractional prostate motion requires an IM of 2.4 mm LR, 3.4 mm IS and AP. The PTV margin using the 5-mm threshold, including interobserver uncertainty, IM, and SM, is 4.8 mm LR, 5.4 mm IS, and 5.2 mm AP. CONCLUSIONS: Localization based on EPI with implanted gold seeds allows a large PTV margin reduction when compared with tattoo localization. Except for the LR direction, bony anatomy localization does not decrease the margins compared with tattoo localization. Intrafractional prostate motion is a limiting factor on margin reduction.     

Observations on real-time prostate gland motion using electromagnetic tracking

PURPOSE: To quantify and describe the real-time movement of the prostate gland in a large data set of patients treated with radiotherapy. METHODS AND MATERIALS: The Calypso four-dimensional localization system was used for target localization in 17 patients, with electromagnetic markers implanted in the prostate of each patient. We analyzed a total of 550 continuous tracking sessions. The fraction of time that the prostate was displaced by >3, >5, >7, and >10 mm was calculated for each session and patient. The frequencies of displacements after initial patient positioning were analyzed over time. RESULTS: Averaged over all patients, the prostate was displaced >3 and >5 mm for 13.6% and 3.3% of the total treatment time, respectively. For individual patients, the corresponding maximal values were 36.2% and 10.9%. For individual fractions, the corresponding maximal values were 98.7% and 98.6%. Displacements >3 mm were observed at 5 min after initial alignment in about one-eighth of the observations, and increased to one-quarter by 10 min. For individual patients, the maximal value of the displacements >3 mm at 5 and 10 min after initial positioning was 43% and 75%, respectively. CONCLUSION: On average, the prostate was displaced by >3 mm and >5 mm approximately 14% and 3% of the time, respectively. For individual patients, these values were up to three times greater. After the initial positioning, the likelihood of displacement of the prostate gland increased with elapsed time. This highlights the importance of initiating treatment shortly after initially positioning the patient.     

The impact of stool and gas volume on intrafraction prostate motion in patients undergoing radiotherapy with daily endorectal balloon

Purpose

The aim of this study was to quantify the impact of rectal stool/gas volumes on intrafraction prostate motion for patients undergoing prostate radiotherapy with daily endorectal balloon (ERB).

Methods

Total and anterior stool/gas rectal volumes were quantified in 30 patients treated with daily ERB. Real-time intrafraction prostate motion from 494 treatment sessions, at most 6 min in length, was evaluated using Calypso® tracking system.

Results

The deviation of prostate intrafraction motion distribution was a function of stool/gas volume, especially when stool/gas is located in the anterior part of the rectum. Compared to patients with small anterior stool/gas volumes (<10 cm³), those with large volume (10–60 cm³) had a twofold increase in 3D prostate motion and interquartile data range within the 6th minute of treatment time. The 10% of the overall CBCT session where large anterior rectal volumes were observed demonstrated larger percentage of time at displacement greater than our proposed internal margin 3 mm.

Conclusion

Volume and location of stool/gas can directly impact the ERB’s intrafraction immobilization ability. Although our patient preparation protocol and the 100 cm³ daily ERB effectively stabilized prostate motion for 90% of the fractions, a larger-sized ERB may improve prostate fixation for patients with greater and/or variable daily rectal volume.     

Evaluation of intrafraction prostate motion tracking using the Clarity Autoscan system for safety margin validation

BackgroundThe purpose of the study was to monitor intrafraction prostate motion in real-time using transperineal 4D ultrasound (TPUS) and analyze trajectories to validate clinical safety margins.Methods401 trajectories of US monitoring sessions were retrospectively evaluated for 14 patients treated for prostate cancer. The Elekta Clarity Autoscan system was used for intrafraction monitoring along the 3 directions: superior-inferior (SI), left-right (LR) and anterior-posterior (AP).ResultsThe intrafraction monitoring resulted in a mean prostate displacement of (-0.06 ± 0.49) mm, (-0.09 ± 0.61) mm and (-0.01 ± 0.78) mm in the SI, LR and AP directions, respectively. Even though large deviations up to 8 mm were detected, the frequency of occurrence was less than 0.1%. The prostate moved within ±2 mm in 99%, 98.1%, and 96.6% of the treatment time in the SI, LR and AP directions, respectively. During 100 s of monitoring, the median displacement increased from 0.2 mm to 0.8 mm and the maximum displacements increased from 5.2 mm to 7.8 mm. The majority of displacement values (99%) were within the clinical safety margins which ensures a good target coverage.ConclusionsThe largest variation of intrafraction prostate displacement was observed along the AP direction. Throughout most of the treatment time, the prostate moved within a few millimeters. The extent of prostate displacement increased for longer monitoring times. During most of the tracking time, the prostate position was within the clinically safety margins.     

前列腺癌大分割精确放疗分次治疗间和分次治疗内位置变动分析

目的 了解前列腺癌精确大分割放疗时分次间和分次内前列腺靶区位移情况。方法 对 2013—2016年间28例接受5 Gy9次放疗的前列腺癌患者,定位前2周B超引导下经直肠穿刺前列腺内植入纯金标记3颗,仰卧位体膜固定充盈膀胱并直肠内插置直肠扩张球囊充气60 ml后CT定位,Pinnacle系统制定放疗计划。23例患者Synergy加速器治疗,每次疗前CBCT校位,扫描图像与计划图像行骨配准记录摆位误差,然后通过前列腺内金标位置配准记录前列腺位移误差,两次之差为分次间位移。5例患者Novalis加速器治疗,通过前列腺内金标配准,疗中ExacTrac系统实时跟踪金标位置变化,观察前列腺分次内位移。结果 23例患者每次疗前均测量位移共计207次,左右、上下、前后位移平均值分别为(0.05±0.10)、(0.20±0.22)、(0.19±0.18) cm;3个方向>0.3 cm位移分别为1、52、49次,>0.5 cm位移分别为1、29、16次。5例患者每次疗时监测测量金标位置移动5次共计225次,左右、上下、前后位移平均值分别为(0.61±0.50)、(0.68±0.69)、(0.70±0.67) mm,各方向>3 mm移动分别为0、1、1次。结论 前列腺癌精确大分割放疗时分次间位移远远大于分次内位移,分次间位移必须校正后才能放疗。分次内靶区位移尽管变化较小,但仍有必要监测分次内靶区位移,以防患者体位变动造成靶区脱靶照射。直肠内球囊插入对前列腺位置具有固定作用。     

Dosimetric consequences of intrafraction prostate motion in scanned ion beam radiotherapy

Background and purpose

Scanned-beam interplay with the intrafraction target motion may result in dose deterioration in particle therapy. The magnitude of this effect and the possibilities to mitigate it were investigated for carbon ion prostate treatments.

Methods and materials

For 12 prostate cases, 9 carbon ion treatment plans were prepared using 3 scanned-beam settings (spot sizes of 6, 7 and 9 mm and, respectively, raster pitches of 2, 2 and 3 mm) for 3 planning margins (3, 6 and 9 mm). Plans were recomputed in presence of 5 intrafraction prostate motion scenarios with and without intra-beam motion compensation.

Results

For 6 mm margin and 7 mm spot, the median (max) CTV D_95% change was −0.2 (−2.6) pp (percentual points) with pure drift motion, −3.8 (−6.0) pp and −2.8 (−3.1) pp in transient motion scenarios and −4.8 (−7.7) pp and −1.8 (−5.7) pp in mixed motion scenarios. No particular raster setting brought distinct advantage, while planning margin expansion showed statistically significant effects for drift-dominated scenarios. Intra-beam motion compensation yielded improved CTV coverage.

Conclusion

Intrafraction prostate motion can lead to marked target coverage deterioration, dependent on individual motion patterns, which can be only partially avoided through planning-time countermeasures. Among possible delivery-time countermeasures, intra-beam motion compensation is capable of improving target coverage.     

Spinal cord tolerance in the age of spinal radiosurgery: lessons from preclinical studies

Clinical implementation of spinal radiosurgery has increased rapidly in recent years, but little is known regarding human spinal cord tolerance to single-fraction irradiation. In contrast, preclinical studies in single-fraction spinal cord tolerance have been ongoing since the 1970s. The influences of field length, dose rate, inhomogeneous dose distributions, and reirradiation have all been investigated. This review summarizes literature regarding single-fraction spinal cord tolerance in preclinical models with an emphasis on practical clinical significance. The outcomes of studies that incorporate uniform irradiation are surprisingly consistent among multiple small- and large-animal models. Extensive investigation of inhomogeneous dose distributions in the rat has demonstrated a significant dose-volume effect while preliminary results from one pig study are contradictory. Preclinical spinal cord dose-volume studies indicate that dose distribution is more critical than the volume irradiated suggesting that neither dose-volume histogram analysis nor absolute volume constraints are effective in predicting complications. Reirradiation data are sparse, but results from guinea pig, rat, and pig studies are consistent with the hypothesis that the spinal cord possesses a large capacity for repair. The mechanisms behind the phenomena observed in spinal cord studies are not readily explained and the ability of dose response models to predict outcomes is variable underscoring the need for further investigation. Animal studies provide insight into the phenomena and mechanisms of radiosensitivity but the true significance of animal studies can only be discovered through clinical trials.     

Intrafractional prostate motion during online image guided intensity-modulated radiotherapy for prostate cancer

Introduction

Intrafractional motion consists of two components: (1) the movement between the on-line repositioning procedure and the treatment start and (2) the movement during the treatment delivery. The goal of this study is to estimate this intrafractional movement of the prostate during prostate cancer radiotherapy.

Material and methods

Twenty-seven patients with prostate cancer and implanted fiducials underwent a marker match procedure before a five-field IMRT treatment. For all fields, in-treatment images were obtained and then processed to enable automatic marker detection. Combining the subsequent projection images, five positions of each marker were determined using the shortest path approach. The residual set-up error (RSE) after kV-MV based prostate localization, the prostate position as a function of time during a radiotherapy session and the required margins to account for intrafractional motion were determined.

Results

The mean RSE and standard deviation in the antero-posterior, cranio-caudal and left-right direction were 2.3 ± 1.5 mm, 0.2 ± 1.1 mm and −0.1 ± 1.1 mm, respectively. Almost all motions occurred in the posterior direction before the first treatment beam as the percentage of excursions >5 mm was reduced significantly when the RSE was not accounted for. The required margins for intrafractional motion increased with prolongation of the treatment. Application of a repositioning protocol after every beam could decrease the 1 cm margin from CTV to PTV by 2 mm.

Conclusions

The RSE is the main contributor to intrafractional motion. This RSE after on-line prostate localization and patient repositioning in the posterior direction emphasizes the need to speed up the marker match procedure. Also, a prostate IMRT treatment should be administered as fast as possible, to ensure that the pre-treatment repositioning efforts are not erased by intrafractional prostate motion. This warrants an optimized workflow with the use of faster treatment techniques.     

Intrafraction motion of the prostate during external-beam radiation therapy: analysis of 427 patients with implanted fiducial markers

PURPOSE: To analyze the intrafraction motion of the prostate during external-beam radiation therapy of patients with prostate cancer. METHODS AND MATERIALS: Between August 2001-December 2005, 427 patients with Stage T3Nx/0Mx/0 prostate carcinoma received intensity-modulated radiation therapy treatment combined with position verification with fiducial gold markers. For a total of 11,426 treatment fractions (average, 27 per patient), portal images were taken of the first segment of all five beams. The irradiation time of the technique varied between 5-7 min. From these data, the location of gold markers could be established within every treatment beam under the assumption of minimal marker movement. RESULTS: In 66% of treatment fractions, a motion outside a range of 2 mm was observed, with 28% outside a range of 3 mm. The intrafraction marker movements showed that motion directions were often reversed. However, the effect was small. Even with perfect online position-correction at the start of irradiation, intrafraction motion caused position uncertainty, but systematic errors (Sigma) were limited to <0.6 mm, and random errors (sigma) to <0.9 mm. This would result in a lower limit of 2 mm for margins, in the absence of any other uncertainties. CONCLUSIONS: Intrafraction motion of the prostate occurs frequently during external-beam irradiation on a time scale of 5-7 min. Margins of 2 mm account for these intrafraction motions. However, larger margins are required in practice to accommodate other uncertainties in the treatment.