A study to quantify the effectiveness of daily endorectal balloon for prostate intrafraction motion management

Determination of internal target volume with Cine-MRI sequence for prostate MRI-Guided radiotherapy

BACKGROUND

In prostate radiotherapy, the intrafractional target motion negatively affects treatment accuracy. Generating internal target volume (ITV) using four-dimensional (4D) images may resolve the issue of intrafractional target motion induced by bladder filling and bowel movement. However, no 4D imaging techniques suitable for the prostate are currently available in clinical practice.

PURPOSE

This study aimed to determine the ITV based on cine magnetic resonance imaging (MRI) sequence for intrafractional target motion management in prostate MRI-guided radiotherapy.

MATERIALS AND METHODS

A reference ITV was generated in simulation process. Then, the reference ITV was adapted with cine MRI sequence before online planning in each fraction. Finally, the reference ITV was updated with the cine MRI sequence acquired during beam delivery after each fraction. Cine MRI sequences and positioning three-dimensional (3D) MRI from 35 patients were retrospectively collected. Clinical target volume (CTV) coverage was computed according to the two-dimensional contour of CTV and ITV on cine MRI images. Relative target size was calculated as the ratio of the volume of ITV and CTV. Isotropic planning target volume (PTV; 5 mm margin) and anisotropic PTV (3 mm margin in the posterior direction and 5 mm margin in other directions) were generated for comparison.

RESULTS

The CTV coverage rate of the proposed ITV had a mean value of 98.61% ± 0.51%, whereas the CTV coverage rates of the isotropic and anisotropic PTVs were 97.43% ± 0.41% and 96.58% ± 0.73%, respectively. The proposed ITV had a relative target size of 1.79 ± 0.17, whereas the anisotropic and isotropic PTVs had relative target sizes of 1.92 ± 0.12 and 2.21 ± 0.19, respectively. For both the CTV coverage rate and target relative size, significant differences were observed between the proposed ITV and the other two PTVs (p < 0.05).

CONCLUSION

The ITV achieved higher CTV coverage with smaller size than conventional isotropic and anisotropic PTVs, indicating that it can effectively deal with the intrafractional movement of the prostate.

Monitoring Intrafraction Motion of the Prostate During Radiation Therapy: Suggested Practice Points From a Focused Review

PURPOSE

External beam radiation therapy to the prostate is typically delivered after verification of prostatic position with image guidance. Prostate motion can occur during the delivery of each radiation treatment between the time of localization imaging and completion of treatment. The objective of this work is to review the literature on intrafraction motion (IFM) of the prostate during radiation therapy and offer clinical recommendations on management.

METHODS AND MATERIALS

A comprehensive literature review was conducted on prostate motion during prostate cancer radiation therapy. Information was organized around 3 key clinical questions, followed by an evidence-based recommendation.

RESULTS

IFM of the prostate during radiation therapy is typically ≤3 mm and is unlikely to compromise prostate dosimetry to a clinically meaningful degree for men treated in a relatively short treatment duration with planning target volume (PTV) margins of ≥3 to 5 mm. IFM of 5 mm or more has been observed in up to ∼10% of treatment fractions, with limited dosimetric effect related to the infrequency of occurrence and longer fractionation of therapy. IFM can be monitored in continuous or discontinuous fashion with a variety of imaging platforms. Correction of IFM may have the greatest value when tighter PTV margins are desired (such as with stereotactic body radiation therapy or intraprostatic nodule boosting), ultrahypofractionated courses, or when treatment time exceeds several minutes.

CONCLUSIONS

This focused review summarizes literature and provides practical recommendations regarding IFM in the treatment of prostate cancer with external beam radiation therapy.

Reducing the margin in prostate radiotherapy: optimizing radiotherapy with a general-purpose linear accelerator using an in-house position monitoring system

Purpose

To study the feasibility of optimizing the Clinical Target Volume to Planning Target Volume (CTV-PTV) margin in prostate radiotherapy(RT) with a general-purpose linear accelerator using an in-house developed position monitoring system, SeedTracker.

Methods

A cohort of 30 patients having definitive prostate radiotherapy treated within an ethics-approved prospective trial was considered for this study. The intrafraction prostate motion and the position deviations were measured using SeedTracker system during each treatment fraction. Using this data the CTV-PTV margin required to cover 90% of the patients with a minimum of 95% of the prescription dose to CTV was calculated using van Herk's formula. The margin calculations were performed for treatment scenarios both with and without applying the position corrections for observed position deviations. The feasibility of margin reduction with real-time monitoring was studied by assessing the delivered dose that incorporates the actual target position during treatment delivery and comparing it with the planned dose. This assessment was performed for plans generated with reduced CTV-PTV margin in the range of 7mm-3mm.

Results

With real-time monitoring and position corrections applied the margin of 2.0mm, 2.1mm and 2.1mm in LR, AP and SI directions were required to meet the criteria of 90% population to receive 95% of the dose prescription to CTV. Without position corrections applied for observed position deviations a margin of 3.1mm, 4.0mm and 3.0mm was required in LR, AP and SI directions to meet the same criteria. A mean ± SD reduction of 0.5 ± 1.8% and 3 ± 7% of V60 for the rectum and bladder can be achieved for every 1mm reduction of PTV margin. With position corrections applied, the CTV D99 can be delivered within -0.2 ± 0.3 Gy of the planned dose for plans with a 3mm margin. Without applying corrections for position deviations the CTV D99 was reduced by a maximum of 1.1 ± 1.1 Gy for the 3mm margin plan and there was a statistically significant difference between planned and delivered dose for 3mm and 4mm margin plans.

Conclusion

This study demonstrates the feasibility of reducing the margin in prostate radiotherapy with SeedTracker system without compromising the dose delivery accuracy to CTV while reducing dose to critical structures.

Dose evaluation of inter- and intra-fraction prostate motion in extremely hypofractionated intensity-modulated proton therapy for prostate cancer

Inter- and intra-fractional prostate motion can deteriorate the dose distribution in extremely hypofractionated intensity-modulated proton therapy. We used verification CTs and prostate motion data calculated from 1024 intra-fractional prostate motion records to develop a voxel-wise based 4-dimensional method, which had a time resolution of 1 s, to assess the dose impact of prostate motion. An example of 100 fractional simulations revealed that motion had minimal impact on planning dose, the accumulated dose in 95 % of the scenarios fulfilled the clinical goals for target coverage (D95 > 37.5 Gy). This method can serve as a complementary measure in clinical setting to guarantee plan quality.

Clinical effects of rectal retractor application in prostate cancer radiotherapy

Background: Radiation-induced rectal toxicities remain as a major risk during prostate radiotherapy. One approach to the reduction of rectal radiation dose is to physically increase the distance between the rectal wall and prostate. Therefore, the aim of this study was to evaluate whether the application of the rectal retractor (RR) can reduce rectal dose and toxicity in prostate cancer 3-dimensional conformal radiotherapy (3D-CRT). Methods: Overall, 36 patients with localized prostate cancer were randomized into the 2 groups, 18 patients with RR in-place and 18 without RR. All patients underwent planning computed tomography (CT). Patients were treated with 70 Gy in 35 fractions of 3D-CRT. In the RR group, RR was used during cone-down 20 treatment fractions. Acute and late gastrointestinal (GI) toxicities were assessed using EORTC/RTOG scoring system weekly during radiotherapy, 3, and 12 months after treatment. Device-related events were recorded according to CTCAE version 4.0. Patient characteristics, cancer differences, and dosimetric data for the RR and non-RR groups were compared using a Man-Whitney U test for continuous variables, and Fisher exact test for categorical data. The EORTC/RTOG scores for the 2 groups were compared using Fisher exact test. A P value <0.05 was considered statistically significant. Results: A RR significantly reduced mean dose (Dmean) to the rectum as well as rectal volume receiving 50% to 95% (V50-95%) of prescribed dose. The absolute reduction of rectal Dmean was 10.3 Gy. There was no statistically significant difference in acute GI toxicity between groups during treatment or at 3 months. At 12 months, 2 patients in the RR group and 9 in the control group experienced late grade ≥ 1 GI toxicity (p=0.027). No patients in the RR group reported late grade ≥ 2 GI toxicity, whereas 3 patients in the control group experienced late grade 2 GI toxicity. In the RR group, 6 patients reported grade 1 rectal discomfort and pain according to CTCAE version 4.0. Conclusion: The application of the RR showed a significant rectum sparing effect, resulting in substantially reducing late GI toxicity.

Reproducibility and accuracy of a target motion mitigation technique for dose-escalated prostate stereotactic body radiotherapy

BACKGROUND AND PURPOSE

To quantitate the accuracy, reproducibility and prostate motion mitigation efficacy rendered by a target immobilization method used in an intermediate-risk prostate cancer dose-escalated 5×9Gy SBRT study.

MATERIAL AND METHODS

An air-inflated (150 cm³) endorectal balloon and Foley catheter with three electromagnetic beacon transponders (EBT) were used to mitigate and track intra-fractional target motion. A 2 mm margin was used for PTV expansion, reduced to 0 mm at the interface with critical OARs. EBT-detected ≥ 2 mm/5 s motions mandated treatment interruption and target realignment prior to completion of planned dose delivery. Geometrical uncertainties were measured with an in-house ESAPI script.

RESULTS

Quantitative data were obtained in 886 sessions from 189 patients. Mean PTV dose was 45.8 ± 0.4 Gy (D95 = 40.5 ± 0.4 Gy). A mean of 3.7 ± 1.7 CBCTs were acquired to reach reference position. Mean treatment time was 19.5 ± 12 min, 14.1 ± 11 and 5.4 ± 5.9 min for preparation and treatment delivery, respectively. Target motion of 0, 1-2 and >2 mm/10 min were observed in 59%, 30% and 11% of sessions, respectively. Temporary beam-on hold occurred in 7.4% of sessions, while in 6% a new reference CBCT was required to correct deviations. Hence, all sessions were completed with application of the planned dose. Treatment preparation time > 15 min was significantly associated with the need of a second reference CBCT. Overall systematic and random geometrical errors were in the order of 1 mm.

CONCLUSION

The prostate immobilization technique explored here affords excellent accuracy and reproducibility, enabling normal tissue dose sculpting with tight PTV margins.

Spatial and dosimetric evaluation of residual distortions of prostate and seminal vesicle bed after image-guided definitive and postoperative radiotherapy of prostate cancer with endorectal balloon

Purpose

To quantify daily residual deviations from the planned geometry after image‐guided prostate radiotherapy with endorectal balloon and to evaluate their effect on the delivered dose distribution.

Methods

Daily kV‐CBCT imaging was used for online setup‐correction in six degrees of freedom (6‐dof) for 24 patients receiving definitive (12 RT_def patients) or postoperative (12 RT_postop patients) radiotherapy with endorectal balloon (overall 739 CBCTs). Residual deviations were evaluated using several spatial and dosimetric variables, including: (a) posterior Hausdorff distance HD^post (=maximum distance between planned and daily CTV contour), (b) point P_worst with largest HD^post over all fractions, (c) equivalent uniform dose using a cell survival model (EUD_SF) and the generalized EUD concept (gEUD_a with parameter a = −7 and a = −20). EUD values were determined for planned (EUDSFplan), daily (EUDSFind), and delivered dose distributions (EUDSFaccum) for plans with 6 mm (=clinical plans) and 2 mm CTV‐to‐PTV margin. Time series analyses of interfractional spatial and dosimetric deviations were conducted.

Results

Large HD^post values ≥ 12.5 mm (≥15 mm) were observed in 20/739 (5/739) fractions distributed across 7 (3) patients. Points P_worst were predominantly located at the posterior CTV boundary in the seminal vesicle region (16/24 patients, 6/7 patients with HD^post ≥ 12.5 mm). Time series analyses revealed a stationary white noise characteristic of HD^post and relative dose at P_worst. The EUD_SF difference between planned and accumulated dose distributions was < 5.4% for all 6‐mm plans. Evaluating 2‐mm plans, EUD_SF deteriorated by < 10% (<5%) in 75% (58.5%) of the patients. EUDSFaccum was well described by the median value of the EUDSFind distribution. PTV margin calculation at P_worst yielded 8.8 mm.

Conclusions

Accumulated dose distributions in prostate radiotherapy with endorectal balloon are forgiving of considerable residual distortions after 6‐dof patient setup if they are observed in a minority of fractions and the median value of EUDSFind determined per fraction stays within 95% of prescribed dose. Common PTV margin calculations are overly conservative because after online correction of translational and rotational errors only residual deformations need to be included. These results provide guidelines regarding online navigation, margin optimization, and treatment adaptation strategies.

Evaluation of an Open Source Registration Package for Automatic Contour Propagation in Online Adaptive Intensity-Modulated Proton Therapy of Prostate Cancer

Objective: Our goal was to investigate the performance of an open source deformable image registration package, elastix, for fast and robust contour propagation in the context of online-adaptive intensity-modulated proton therapy (IMPT) for prostate cancer.

Methods: A planning and 7–10 repeat CT scans were available of 18 prostate cancer patients. Automatic contour propagation of repeat CT scans was performed using elastix and compared with manual delineations in terms of geometric accuracy and runtime. Dosimetric accuracy was quantified by generating IMPT plans using the propagated contours expanded with a 2 mm (prostate) and 3.5 mm margin (seminal vesicles and lymph nodes) and calculating dosimetric coverage based on the manual delineation. A coverage of V_95% ≥ 98% (at least 98% of the target volumes receive at least 95% of the prescribed dose) was considered clinically acceptable.

Results: Contour propagation runtime varied between 3 and 30 s for different registration settings. For the fastest setting, 83 in 93 (89.2%), 73 in 93 (78.5%), and 91 in 93 (97.9%) registrations yielded clinically acceptable dosimetric coverage of the prostate, seminal vesicles, and lymph nodes, respectively. For the prostate, seminal vesicles, and lymph nodes the Dice Similarity Coefficient (DSC) was 0.87 ± 0.05, 0.63 ± 0.18, and 0.89 ± 0.03 and the mean surface distance (MSD) was 1.4 ± 0.5 mm, 2.0 ± 1.2 mm, and 1.5 ± 0.4 mm, respectively.

Conclusion: With a dosimetric success rate of 78.5–97.9%, this software may facilitate online adaptive IMPT of prostate cancer using a fast, free and open implementation.

Definitive, intensity modulated tomotherapy with a simultaneous integrated boost for prostate cancer patients - Long term data on toxicity and biochemical control

Aim

To report long-term data regarding biochemical control and late toxicity of simultaneous integrated boost intensity modulated radiotherapy (SIB-IMRT) with tomotherapy in patients with localized prostate cancer.

Background

Dose escalation improves cancer control after curative intended radiation therapy (RT) to patients with localized prostate cancer, without increasing toxicity, if IMRT is used.

Materials and methods

In this retrospective analysis, we evaluated long-term toxicity and biochemical control of the first 40 patients with intermediate risk prostate cancer receiving SIB-IMRT. Primary target volume (PTV) 1 including the prostate and proximal third of the seminal vesicles with safety margins was treated with 70 Gy in 35 fractions. PTV 2 containing the prostate with smaller safety margins was treated as SIB to a total dose of 76 Gy with 2.17 Gy per fraction. Toxicity was evaluated using an adapted CTCAE-Score (Version 3).

Results

Median follow-up of living patients was 66 (20-78) months. No late genitourinary toxicity higher than grade 2 has been reported. Grade 2 genitourinary toxicity rates decreased from 58% at the end of the treatment to 10% at 60 months. Late gastrointestinal (GI) toxicity was also moderate, though the prescribed PTV Dose of 76 Gy was accepted at the anterior rectal wall. 74% of patients reported any GI toxicity during follow up and no toxicity rates higher than grade 2 were observed. Grade 2 side effects were reported by 13% of the patients at 60 months. 5-year freedom from biochemical failure was 95% at our last follow up.

Conclusion

SIB-IMRT using daily MV-CT guidance showed excellent long-term biochemical control and low toxicity rates.

Penile bulb sparing in prostate cancer radiotherapy : Dose analysis of an in-house MRI system to improve contouring

OBJECTIVE

This study aimed to assess the reduction in dose to the penile bulb (PB) achieved by MRI-based contouring following drinking and endorectal balloon (ERB) instructions.

PATIENTS AND METHODS

A total of 17 prostate cancer patients were treated with intensity-modulated radiation therapy (IMRT) and interstitial brachytherapy (IBT). CT and MRI datasets were acquired back-to-back based on a 65 cm³ air-filled ERB and drinking instructions. After rigid co-registration of the imaging data, the CT-based planning target volume (PTV) used for treatment planning was retrospectively compared to an MRI-based adaptive PTV and the dose to the PB was determined in each case. The adapted PTV encompassed a caudally cropped CT-based PTV which was defined on the basis of the MRI-based prostate contour plus an additional 5 mm safety margin.

RESULTS

In the seven-field IMRT treatment plans, the MRI-based adapted PTV achieved mean (D_mean) and maximum (D_max) doses to the PB which were significantly lower (by 7.6 Gy and 10.9 Gy, respectively; p <0.05) than those of the CT-contoured PTV. For 6 patients, the estimated PB D_max (seven-field IMRT and IBT) for the adapted PTV was <70 Gy, whereas only 1 patient fulfilled this criterium with the CT-based PTV.

CONCLUSION

MRI-based contouring and seven-field IMRT-based treatment planning achieved dose sparing to the PB. Whereas the comparison of MRI and CT contouring only relates to external beam radiotherapy (EBRT) sparing, considering EBRT and IBT shows the improvement in PB sparing for the total treatment.

Rectal radiation dose-reduction techniques in prostate cancer: a focus on the rectal spacer

Prostate cancer is the most common cancer in men. External beam radiotherapy by a variety of methods is a standard treatment option with excellent disease control. However, acute and late rectal side effects remain a limiting concern in intensification of therapy in higher-risk patients and in efforts to reduce treatment burden in others. A number of techniques have emerged that allow for high-radiation dose delivery to the prostate with reduced risk of rectal toxicity, including image-guided intensity-modulated radiation therapy, endorectal balloons and various forms of rectal spacers. Image-guided radiation therapy, either intensity-modulated radiation therapy or stereotactic ablative radiation therapy, in conjunction with a rectal spacer, is an efficacious means to reduce acute and long-term rectal toxicity.

Magnetic Resonance Imaging Guidance Mitigates the Effects of Intrafraction Prostate Motion During Stereotactic Body Radiotherapy for Prostate Cancer

The accurate delivery of stereotactic body radiotherapy (SBRT) for definitive prostate cancer treatment is aided by intrafraction image guidance. The common methods for intrafraction imaging require the invasive placement of fiducial markers or electromagnetic transponders. Recently, a magnetic resonance imaging (MRI)-guided tri-cobalt-60 head radiotherapy system has become available for treatment, which can utilize real-time cine MRI to non-invasively track prostate motion. We report on a clinical vignette using this technique to deliver SBRT for the definitive treatment of intermediate-risk prostate cancer. The incorporation of an MRI-guided radiotherapy system and the implementation of real-time adaptive dose delivery accounting for intrafraction anatomic motion may improve outcomes using this technique.

Volumetric-based image guidance is superior to marker-based alignments for stereotactic body radiotherapy of prostate cancer

Purposes

The aim of this study was to evaluate a dual marker‐based and soft‐tissue based image guidance for inter‐fractional corrections in stereotactic body radiotherapy (SBRT) of prostate cancer.

Methods/Materials

We reviewed 18 patients treated with SBRT for prostate cancer. An endorectal balloon was inserted at simulation and each treatment. Planning margins were 3 mm/0 mm posteriorly. Prior to each treatment, a dual image guidance protocol was applied to align three makers using stereoscopic x ray images and then to the soft tissue using kilo‐voltage cone beam CT (kV‐CBCT). After treatment, prostate (CTV), rectal wall, and bladder were delineated on each kV‐CBCT, and delivered dose was recalculated. Dosimetric endpoints were analyzed, including V_{36.25 Gy} for prostate, and D_{0.03 cc} for bladder and rectal wall.

Results

Following initial marker alignment, additional translational shifts were applied to 22 of 84 fractions after kV‐CBCT. Among the 22 fractions, ten fractions exceeded 3 mm shifts in any direction, including one in the left‐right direction, four in the superior‐inferior direction, and five in the anterior‐posterior direction. With and without the additional kV‐CBCT shifts, the average V_{36.25 Gy} of the prostate for the 22 fractions was 97.6 ± 2.6% with the kV x ray image alone, and was 98.1 ± 2.4% after applying the additional kV‐CBCT shifts. The improvement was borderline statistical significance using Wilcoxon signed‐rank test (P = 0.007). D_{0.03 cc} was 45.8 ± 6.3 Gy vs. 45.1 ± 4.9 Gy for the rectal wall; and 49.5 ± 8.6 Gy vs. 49.3 ± 7.9 Gy for the bladder before and after applying kV‐CBCT shifts.

Conclusions

Marker‐based alignment alone is not sufficient. Additional adjustments are needed for some patients based kV‐CBCT.

Rectal balloon use limits vaginal displacement, rectal dose, and rectal toxicity in patients receiving IMRT for postoperative gynecological malignancies

Pelvic radiotherapy for gynecologic malignancies traditionally used a 4-field box technique. Later trials have shown the feasibility of using intensity-modulated radiotherapy (IMRT) instead. But vaginal movement between fractions is concerning when using IMRT due to greater conformality of the isodose curves to the target and the resulting possibility of missing the target while the vagina is displaced. In this study, we showed that the use of a rectal balloon during treatment can decrease vaginal displacement, limit rectal dose, and limit acute and late toxicities. Little is known regarding the use of a rectal balloon (RB) in treating patients with IMRT in the posthysterectomy setting. We hypothesize that the use of an RB during treatment can limit rectal dose and acute and long-term toxicities, as well as decrease vaginal cuff displacement between fractions. We performed a retrospective review of patients with gynecological malignancies who received postoperative IMRT with the use of an RB from January 1, 2012 to January 1, 2015. Rectal dose constraint was examined as per Radiation Therapy Oncology Group (RTOG) 1203 and 0418. Daily cone beam computed tomography (CT) was performed, and the average (avg) displacement, avg magnitude, and avg magnitude of vector were calculated. Toxicity was reported according to RTOG acute radiation morbidity scoring criteria. Acute toxicity was defined as less than 90 days from the end of radiation treatment. Late toxicity was defined as at least 90 days after completing radiation. Twenty-eight patients with postoperative IMRT with the use of an RB were examined and 23 treatment plans were reviewed. The avg rectal V40 was 39.3% ± 9.0%. V30 was65.1% ± 10.0%. V50 was 0%. Separate cone beam computed tomography (CBCT) images (n = 663) were reviewed. The avg displacement was as follows: superior 0.4 + 2.99 mm, left 0.23 ± 4.97 mm, and anterior 0.16 ± 5.18 mm. The avg magnitude of displacement was superior/inferior 2.22 ± 2.04 mm, laterally 3.41 ± 3.62 mm, and anterior/posterior 3.86 ± 3.45 mm. The avg vector magnitude was 6.60 ± 4.14 mm. For acute gastrointestinal (GI) toxicities, 50% experienced grade 1 toxicities and 18% grade 2 GI toxicities. For acute genitourinary (GU) toxicities, 21% had grade 1 and 18% had grade 2 toxicities. For late GU toxicities, 7% had grade 1 and 4% had grade 2 toxicities. RB for gynecological patients receiving IMRT in the postoperative setting can limit V40 rectal dose and vaginal displacement. Although V30 constraints were not met, patients had limited acute and late toxicities. Further studies are needed to validate these findings.

Dosimetric impacts of endorectal balloon in CyberKnife stereotactic body radiation therapy (SBRT) for early-stage prostate cancer

PURPOSE

In SBRT for prostate cancer, higher fractional dose to the rectum is a major toxicity concern due to using smaller PTV margin and hypofractionation. We investigate the dosimetric impact on rectum using endorectal balloon (ERB) in prostate SBRT.

MATERIALS AND METHODS

Twenty prostate cancer patients were included in a retrospective study, ten with ERB and 10 without ERB. Optimized SBRT plans were generated on CyberKnife MultiPlan for 5 × 7.25 Gy to PTV under RTOG-0938 protocol for early-stage prostate cancer. For the rectum and the anterior half rectum, mean dose and percentage of volumes receiving 50%, 80%, 90%, and 100% prescription dose were compared.

RESULTS

Using ERB, mean dose to the rectum was 62 cGy (P = 0.001) lower per fraction, and 50 cGy (P = 0.024) lower per fraction for the anterior half rectum. The average V_50% , V_80% , V_90% , and V_100% were lower by 9.9% (P = 0.001), 5.3% (P = 0.0002), 3.4% (P = 0.0002), and 1.2% (P = 0.005) for the rectum, and lower by 10.4% (P = 0.009), 8.3% (P = 0.0004), 5.4% (P = 0.0003), and 2.1% (P = 0.003) for the anterior half rectum.

CONCLUSIONS

Significant reductions of dose to the rectum using ERB were observed. This may lead to improvement of the rectal toxicity profiles in prostate SBRT.

Validation of rectal sparing throughout the course of proton therapy treatment in prostate cancer patients treated with SpaceOAR®

The purpose of this study was to investigate the consistency of rectal sparing using multiple periodic quality assurance computerized tomography imaging scans (QACT) obtained during the course of proton therapy for patients with prostate cancer treated with a hydrogel spacer. Forty-one low- and intermediate-risk prostate cancer patients treated with image-guided proton therapy with rectal spacer hydrogel were analyzed. To assess the reproducibility of rectal sparing with the hydrogel spacer, three to four QACTs were performed for each patient on day 1 and during weeks 1, 3, and 5 of treatment. The treatment plan was calculated on the QACT and the rectum V90%, V75%, V65%, V50%, and V40% were evaluated. For the retrospective analysis, we evaluated each QACT and compared it to the corresponding treatment planning CT (TPCT), to determine the average change in rectum DVH points. We were also interested in how many patients exceeded an upper rectum V90% threshold on a QACT. Finally, we were interested in a correlation between rectum volume and V90%. On each QACT, if the rectum V90% exceeded the upper threshold of 6%, the attending physician was notified and the patient was typically prescribed additional stool softeners or laxatives and reminded of dietary compliance. In all cases of the rectum V90% exceeding the threshold, the patient had increased gas and/or stool, compared to the TPCT. On average, the rectum V90% calculated on the QACT was 0.81% higher than that calculated on the TPCT. The average increase in V75%, V65%, V50%, and V40% on the QACT was 1.38%, 1.59%, 1.87%, and 2.17%, respectively. The rectum V90% was within ± 1% of the treatment planning dose in 71.2% of the QACTs, and within ± 5% in 93.2% of the QACTs. The 6% threshold for rectum V90% was exceeded in 7 out of 144 QACTs (4.8%), identified in 5 of the 41 patients. We evaluated the average rectum V90% across all QACTs for each of these patients, and it was found that the rectum V90% never exceeded 6%. 53% of the QACTs had a rectum volume within 5 cm3 of the TPCT volume, 68% were within 10 cm3. We found that patients who exceeded the threshold on one or more QACTs had a lower TPCT rectal volume than the overall average. By extrapolating patient anatomy from three to four QACT scans, we have shown that the use of hydrogel in conjunction with our patient diet program and use of stool softeners is effective in achieving consistent rectal sparing in patients undergoing proton therapy.

Prostate bed target interfractional motion using RTOG consensus definitions and daily CT on rails : Does target motion differ between superior and inferior portions of the clinical target volume?

PURPOSE

Using high-quality CT-on-rails imaging, the daily motion of the prostate bed clinical target volume (PB-CTV) based on consensus Radiation Therapy Oncology Group (RTOG) definitions (instead of surgical clips/fiducials) was studied. It was assessed whether PB motion in the superior portion of PB-CTV (SUP-CTV) differed from the inferior PB-CTV (INF-CTV).

PATIENTS AND METHODS

Eight pT2-3bN0-1M0 patients underwent postprostatectomy intensity-modulated radiotherapy, totaling 300 fractions. INF-CTV and SUP-CTV were defined as PB-CTV located inferior and superior to the superior border of the pubic symphysis, respectively. Daily pretreatment CT-on-rails images were compared to the planning CT in the left-right (LR), superoinferior (SI), and anteroposterior (AP) directions. Two parameters were defined: "total PB-CTV motion" represented total shifts from skin tattoos to RTOG-defined anatomic areas; "PB-CTV target motion" (performed for both SUP-CTV and INF-CTV) represented shifts from bone to RTOG-defined anatomic areas (i. e., subtracting shifts from skin tattoos to bone).

RESULTS

Mean (± standard deviation, SD) total PB-CTV motion was -1.5 (± 6.0), 1.3 (± 4.5), and 3.7 (± 5.7) mm in LR, SI, and AP directions, respectively. Mean (± SD) PB-CTV target motion was 0.2 (±1.4), 0.3 (±2.4), and 0 (±3.1) mm in the LR, SI, and AP directions, respectively. Mean (± SD) INF-CTV target motion was 0.1 (± 2.8), 0.5 (± 2.2), and 0.2 (± 2.5) mm, and SUP-CTV target motion was 0.3 (± 1.8), 0.5 (± 2.3), and 0 (± 5.0) mm in LR, SI, and AP directions, respectively. No statistically significant differences between INF-CTV and SUP-CTV motion were present in any direction.

CONCLUSION

There are no statistically apparent motion differences between SUP-CTV and INF-CTV. Current uniform planning target volume (PTV) margins are adequate to cover both portions of the CTV.

Target margins in radiotherapy of prostate cancer

We reviewed the literature on the use of margins in radiotherapy of patients with prostate cancer, focusing on different options for image guidance (IG) and technical issues. The search in PubMed database was limited to include studies that involved external beam radiotherapy of the intact prostate. Post-prostatectomy studies, brachytherapy and particle therapy were excluded. Each article was characterized according to the IG strategy used: positioning on external marks using room lasers, bone anatomy and soft tissue match, usage of fiducial markers, electromagnetic tracking and adapted delivery. A lack of uniformity in margin selection among institutions was evident from the review. In general, introduction of pre- and in-treatment IG was associated with smaller planning target volume (PTV) margins, but there was a lack of definitive experimental/clinical studies providing robust information on selection of exact PTV values. In addition, there is a lack of comparative research regarding the cost-benefit ratio of the different strategies: insertion of fiducial markers or electromagnetic transponders facilitates prostate gland localization but at a price of invasive procedure; frequent pre-treatment imaging increases patient in-room time, dose and labour; online plan adaptation should improve radiation delivery accuracy but requires fast and precise computation. Finally, optimal protocols for quality assurance procedures need to be established.

Determining intrafractional prostate motion using four dimensional ultrasound system

Background

In prostate radiotherapy, it is essential that the prostate position is within the planned volume during the treatment delivery. The aim of this study is to investigate whether intrafractional motion of the prostate is of clinical consequence, using a novel 4D autoscan ultrasound probe.

Methods

Ten prostate patients were ultrasound (US) scanned at the time of CT imaging and once a week during their course of radiotherapy treatment in an ethics-approved study, using the transperineal Clarity autoscan system (Clarity®, Elekta Inc., Stockholm, Sweden). At each US scanning session (fraction) the prostate was monitored for 2 to 2.5 min, a typical beam-on time to deliver a RapidArc® radiotherapy fraction. The patients were instructed to remain motionless in supine position throughout the US scans. They were also requested to comply with a bladder-filling protocol. In total, 51 monitoring curves were acquired. Data of the prostate motion in three orthogonal directions were analyzed. Finally, the BMI value was calculated to investigate correlation between BMI and the extent of prostate displacement.

Results

The patients were cooperative, despite extra time for applying the TPUS scan. The mean (±1SD) of the maximal intrafractional displacements were [mm]; I(+)/S: (0.2 ± 0.9); L(+)/R: (−0.2 ± 0.8); and A(+)/P: (−0.2 ± 1.1), respectively. The largest displacement was 2.8 mm in the posterior direction. The percentage of fractions with displacements larger than 2.0 mm was 4 %, 2 %, and 10 % in the IS, LR, and AP directions, respectively. The mean of the maximal intrafractional Euclidean distance (3D vector) was 0.9 ± 0.6 mm. For 12 % of the fractions the maximal 3D vector displacements were larger than 2.0 mm. At only two fractions (4 %) displacements larger than 3.0 mm were observed. There was no correlation between BMI and the extent of the prostate displacement.

Conclusions

The prostate intrafractional displacement is of no clinically consequence for treatment times in the order of 2 – 2.5 min, which is typical for a RapidArc radiotherapy fraction. However, prostate motion should be considered for longer treatment times eg if applying conventional or IMRT radiotherapy.

SBRT for prostate cancer: Challenges and features from a physicist prospective

Emerging data are showing the safety and the efficacy of Stereotactic Body Radiation Therapy (SBRT) in prostate cancer management. In this context, the medical physicists are regularly involved to review the appropriateness of the adopted technology and to proactively study new solutions. From the physics point of view there are two major challenges in prostate SBRT: (1) mitigation of geometrical uncertainty and (2) generation of highly conformal dose distributions that maximally spare the OARs. Geometrical uncertainties have to be limited as much as possible in order to avoid the use of large PTV margins. Furthermore, advanced planning and delivery techniques are needed to generate maximally conformal dose distributions. In this non-systematic review the technology and the physics aspects of SBRT for prostate cancer were analyzed. In details, the aims were: (i) to describe the rationale of reducing the number of fractions (i.e. increasing the dose per fraction), (ii) to analyze the features to be accounted for performing an extreme hypo-fractionation scheme (>6-7Gy), and (iii) to describe technological solutions for treating in a safe way. The analysis of outcomes, toxicities, and other clinical aspects are not object of the present evaluation.

Rectal toxicity after intensity modulated radiotherapy for prostate cancer: which rectal dose volume constraints should we use?

BACKGROUND

To define rectal dose volume constraints (DVC) to prevent ⩾grade2 late rectal toxicity (LRT) after intensity modulated radiotherapy (IMRT) for prostate cancer (PC).

MATERIAL AND METHODS

Six hundred thirty-seven PC patients were treated with primary (prostate median dose: 78Gy) or postoperative (prostatic bed median dose: 74Gy (adjuvant)-76Gy (salvage)) IMRTwhile restricting the rectal dose to 76Gy, 72Gy and 74Gy respectively. The impact of patient characteristics and rectal volume parameters on ⩾grade2 LRT was determined. DVC were defined to estimate the 5% and 10% risk of developing ⩾grade2 LRT.

RESULTS

The 5-year probability of being free from ⩾grade2 LRT, non-rectal blood loss and persisting symptoms is 88.8% (95% CI: 85.8-91.1%), 93.4% (95% CI: 91.0-95.1%) and 94.3% (95% CI: 92.0-95.9%) respectively. There was no correlation with patient characteristics. All volume parameters, except rectal volume receiving ⩾70Gy (R70), were significantly correlated with ⩾grade2 LRT. To avoid 10% and 5% risk of ⩾grade2 LRT following DVC were derived: R40, R50, R60 and R65 <64-35%, 52-22%, 38-14% and 5% respectively.

CONCLUSION

Applying existing rectal volume constraints resulted in a 5-year estimated risk of developing late ⩾grade2 LRT of 11.2%. New rectal DVC for primary and postoperative IMRT planning of PC patients are proposed. A prospective evaluation is needed.

Comparison of prostate proton treatment planning technique, interfraction robustness, and analysis of single-field treatment feasibility

BACKGROUND

This study compares target coverage robustness among proton therapy plans for prostate cancer patients treated with 2 laterally opposed fields delivered daily or, alternatively, every other day as single lateral fields, using uniform scanning (US), single-field uniform dose (SFUD), pencil beam scanning (PBS) optimized for uniform target coverage only, SFUD PBS optimized for target coverage and organs at risk (OAR) sparing (SFUD-opt), and intensity modulated proton therapy (IMPT).

METHODS AND MATERIALS

Ten prostate cancer patients treated with proton therapy underwent weekly verification computed tomographic (CT) scans. US, SFUD, SFUD-opt, and IMPT treatment plans were created and recalculated on weekly verification scans evaluating 2-field daily and single-field target coverage and OAR constraints.

RESULTS

The average (±standard deviation) planning target volume conformity index for US, SFUD, SFUD-opt, and IMPT clinical plans was 0.53 ± 0.06, 0.78 ± 0.05, 0.78 ± 0.04, and 0.78 ± 0.03, respectively. The average 2-field internal target volume (ITV) coverage was significantly higher for both US and SFUD when individually compared with SFUD-opt and IMPT. There was no significant difference between US and SFUD ITV coverage when comparing 2-field daily versus single-field daily delivery. The average single-field coverage was greatest using US and SFUD with 99% of the ITV being covered by 96.8% ± 0.9% and 96.7% ± 1.3%, respectively, compared with 95.5% ± 0.7% for SFUD-opt. There were no significant differences among the 4 plans regarding OAR dose constraints assessed.

CONCLUSIONS

Pencil beam scanning techniques are more conformal than US and, when optimized only for uniform target coverage from each field, can be equally as robust relative to anatomic interfraction variations for prostate cancer patients treated with a single field per day technique. The SFUD-opt and IMPT involve highly modulated pencil beam spots and may be less robust to daily interfraction anatomic variations.

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 cm(3)), those with large volume (10-60 cm(3)) 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 cm(3) 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.

Effect of endorectal balloon positioning errors on target deformation and dosimetric quality during prostate SBRT

An inflatable endorectal balloon (ERB) is often used during stereotactic body radiation therapy (SBRT) for treatment of prostate cancer in order to reduce both intrafraction motion of the target and risk of rectal toxicity. However, the ERB can exert significant force on the prostate, and this work assessed the impact of ERB position errors on deformation of the prostate and treatment dose metrics. Seventy-one cone-beam computed tomography (CBCT) image datasets of nine patients with clinical stage T1cN0M0 prostate cancer were studied. An ERB (Flexi-Cuff, EZ-EM, Westbury, NY) inflated with 60 cm(3) of air was used during simulation and treatment, and daily kilovoltage (kV) CBCT imaging was performed to localize the prostate. The shape of the ERB in each CBCT was analyzed to determine errors in position, size, and shape. A deformable registration algorithm was used to track the dose received by (and deformation of) the prostate, and dosimetric values such as D95, PTV coverage, and Dice coefficient for the prostate were calculated. The average balloon position error was 0.5 cm in the inferior direction, with errors ranging from 2 cm inferiorly to 1 cm superiorly. The prostate was deformed primarily in the AP direction, and tilted primarily in the anterior-posterior/superior-inferior plane. A significant correlation was seen between errors in depth of ERB insertion (DOI) and mean voxel-wise deformation, prostate tilt, Dice coefficient, and planning-to-treatment prostate inter-surface distance (p < 0.001). Dosimetrically, DOI is negatively correlated with prostate D95 and PTV coverage (p < 0.001). For the model of ERB studied, error in ERB position can cause deformations in the prostate that negatively affect treatment, and this additional aspect of setup error should be considered when ERBs are used for prostate SBRT. Before treatment, the ERB position should be verified, and the ERB should be adjusted if the error is observed to exceed tolerable values.

Real-time prostate motion assessment: image-guidance and the temporal dependence of intra-fraction motion

Background

The rapid adoption of image-guidance in prostate intensity-modulated radiotherapy (IMRT) results in longer treatment times, which may result in larger intrafraction motion, thereby negating the advantage of image-guidance. This study aims to qualify and quantify the contribution of image-guidance to the temporal dependence of intrafraction motion during prostate IMRT.

Methods

One-hundred and forty-three patients who underwent conventional IMRT (n=67) or intensity-modulated arc therapy (IMAT/RapidArc, n=76) for localized prostate cancer were evaluated. Intrafraction motion assessment was based on continuous RL (lateral), SI (longitudinal), and AP (vertical) positional detection of electromagnetic transponders at 10 Hz. Daily motion amplitudes were reported as session mean, median, and root-mean-square (RMS) displacements. Temporal effect was evaluated by categorizing treatment sessions into 4 different classes: IMRT_c (transponder only localization), IMRT_cc (transponder + CBCT localization), IMAT_c (transponder only localization), or IMAT_cc (transponder + CBCT localization).

Results

Mean/median session times were 4.15/3.99 min (IMAT_c), 12.74/12.19 min (IMAT_cc), 5.99/5.77 min (IMRT_c), and 12.98/12.39 min (IMRT_cc), with significant pair-wise difference (p<0.0001) between all category combinations except for IMRT_cc vs. IMAT_cc (p>0.05). Median intrafraction motion difference between CBCT and non-CBCT categories strongly correlated with time for RMS (t-value=17.29; p<0.0001), SI (t-value=−4.25; p<0.0001), and AP (t-value=2.76; p<0.0066), with a weak correlation for RL (t-value=1.67; p=0.0971). Treatment time reduction with non-CBCT treatment categories showed reductions in the observed intrafraction motion: systematic error (Σ)<0.6 mm and random error (σ)<1.2 mm compared with ≤0.8 mm and <1.6 mm, respectively, for CBCT-involved treatment categories.

Conclusions

For treatment durations >4-6 minutes, and without any intrafraction motion mitigation protocol in place, patient repositioning is recommended, with at least the acquisition of the lateral component of an orthogonal image pair in the absence of volumetric imaging.

Immobilization considerations for proton radiation therapy

Proton therapy is rapidly developing as a mainstream modality for external beam radiation therapy. This development is largely due to the ability of protons to deposit much of their energy in a region known as the Bragg peak, minimizing the number of treatment fields and hence integral dose delivered to the patient. Immobilization in radiation therapy is a key component in the treatment process allowing for precise delivery of dose to the target volume and this is certainly true in proton therapy. In proton therapy immobilization needs to not only immobilize the patient, placing them in a stable and reproducible position for each treatment, but its impact on the depth dose distribution and range uncertainty must also be considered. The impact of immobilization on range is not a primary factor in X-ray radiation therapy, but it is a governing factor in proton therapy. This contribution describes the immobilization considerations in proton therapy which have been developed at Loma Linda over twenty plus years of clinical operation as a hospital based proton center.

Endorectal balloons in the post prostatectomy setting: do gains in stability lead to more predictable dosimetry?

PURPOSE

To perform a comparative study assessing potential benefits of endorectal-balloons (ERB) in post-prostatectomy patients.

METHOD AND MATERIALS

Ten retrospective post-prostatectomy patients treated without ERB and ten prospective patients treated with the ERB in situ were recruited. All patients received IMRT and IGRT using kilovoltage cone-beam computed tomography (kVCBCT). kVCBCT datasets were registered to the planning dataset, recontoured and the original plan recalculated on the kVCBCTs to recreate anatomical conditions during treatment. The imaging, structure and dose data were imported into in-house software for the assessment of geometric variation and cumulative equivalent uniform dose (EUD) in the two groups.

RESULTS

The difference in location (ΔCOV) for the bladder between planning and each CBCT was similar for each group. The range of mean ΔCOV for the rectum was 0.15-0.58 cm and 0.15-0.59 cm for the non-ERB and ERB groups. For superior-CTV and inferior-CTV the difference between planned and delivered D95% (mean ± SD) for the non-ERB group was 2.1 ± 6.0 Gy and -0.04 ± 0.20 Gy. While for the ERB group the difference in D95% was 8.7 ± 12.6 Gy and 0.003 ± 0.104 Gy.

CONCLUSIONS

The use of ERBs in the post-prostatectomy setting did improve geometric reproducibility of the target and surrounding normal tissues, however no improvement in dosimetric stability was observed for the margins employed.

Reducing rectal injury during external beam radiotherapy for prostate cancer

Rectal bleeding and faecal incontinence are serious injuries that men with prostate cancer who receive radiotherapy can experience. Although technical advances--including the use of intensity-modulated radiotherapy coupled with image-guided radiotherapy--have enabled the delivery of dose distributions that conform to the shape of the tumour target with steep dose gradients that reduce the dose given to surrounding tissues, radiotherapy-associated toxicity can not be avoided completely. Many large-scale prospective studies have analysed the correlations of patient-related and treatment-related parameters with acute and late toxicity to optimize patient selection and treatment planning. The careful application of dose-volume constraints and the tuning of these constraints to the individual patient's characteristics are now considered the most effective ways of reducing rectal morbidity. Additionally, the use of endorectal balloons (to reduce the margins between the clinical target volume and planning target volume) and the insertion of tissue spacers into the region between the prostate and anterior rectal wall have been investigated as means to further reduce late rectal injury. Finally, some drugs and other compounds are also being considered to help protect healthy tissue. Overall, a number of approaches exist that must be fully explored in large prospective trials to address the important issue of rectal toxicity in prostate cancer radiotherapy.

Dose-escalated simultaneous integrated-boost treatment of prostate cancer patients via helical tomotherapy

PURPOSE

The goal of this work was to assess the feasibility of moderately hypofractionated simultaneous integrated-boost intensity-modulated radiotherapy (SIB-IMRT) with helical tomotherapy in patients with localized prostate cancer regarding acute side effects and dose-volume histogram data (DVH data).

METHODS

Acute side effects and DVH data were evaluated of the first 40 intermediate risk prostate cancer patients treated with a definitive daily image-guided SIB-IMRT protocol via helical tomotherapy in our department. The planning target volume including the prostate and the base of the seminal vesicles with safety margins was treated with 70 Gy in 35 fractions. The boost volume containing the prostate and 3 mm safety margins (5 mm craniocaudal) was treated as SIB to a total dose of 76 Gy (2.17 Gy per fraction). Planning constraints for the anterior rectal wall were set in order not to exceed the dose of 76 Gy prescribed to the boost volume. Acute toxicity was evaluated prospectively using a modified CTCAE (Common Terminology Criteria for Adverse Events) score.

RESULTS

SIB-IMRT allowed good rectal sparing, although the full boost dose was permitted to the anterior rectal wall. Median rectum dose was 38 Gy in all patients and the median volumes receiving at least 65 Gy (V65), 70 Gy (V70), and 75 Gy (V75) were 13.5%, 9%, and 3%, respectively. No grade 4 toxicity was observed. Acute grade 3 toxicity was observed in 20% of patients involving nocturia only. Grade 2 acute intestinal and urological side effects occurred in 25% and 57.5%, respectively. No correlation was found between acute toxicity and the DVH data.

CONCLUSION

This institutional SIB-IMRT protocol using daily image guidance as a precondition for smaller safety margins allows dose escalation to the prostate without increasing acute toxicity.