Changes In Rectal Volume Research Articles

Daily Online Adaptive Radiation Therapy of Postoperative Endometrial and Cervical Cancer With PTV Margin Reduction to 5 mm: Dosimetric Outcomes, Acute Toxicity, and First Clinical Experience

PurposeThis study evaluated the first clinical implementation of daily iterative cone-beam computed tomography (iCBCT)-guided online adaptive radiotherapy (oART) in the postoperative treatment of endometrial and cervical cancer. Methods and MaterialsSeventeen consecutive patients treated with daily iCBCT-guided oART were enrolled in this prospective study, with a reduced uniform three-dimensional PTV margin of 5 mm. Treatment plans were designed to deliver 45 or 50.4 Gy in 1.8 Gy daily fractions to PTV. Pre- and posttreatment ultrasound and iCBCT scans were performed to record intrafractional bladder and rectal volume changes. The accuracy of contouring, oART procedure time, dosimetric outcomes and acute toxicity were evaluated. ResultsThe average time from first iCBCT acquisition to completion of treatment was 22 min and 26 s. During this period, bladder volume increased by 44 cm3 using iCBCT contouring, while rectal volume remained stable (62.9 cm3 pretreatment vs. 61.9 cm3 posttreatment). A total of 91.6% of influencers and 88.1% of CTVs required no or minor edits. The adapted plan was selected in all (434) fractions and significantly improved the dosimetry coverage for CTV and PTV, especially the vaginal PTV coverage by nearly 7% (p<0.05). The adapted bladder Dmean was 104.61 cGy, and the rectum Dmean was 123.67 cGy, significantly lower than the scheduled plan of 108.24 and 128.19 cGy, respectively. The bone marrow, femur head left and right dosimetry were also improved with adaptation. Grade 2 acute gastrointestinal and genitourinary toxicities were 24% and 0, respectively. There was a Grade 3 acute toxicity of decreased white blood cell count in one patient. ConclusionsDaily oART was associated with favourable dosimetry improvement and low acute toxicity, supporting its safety and efficacy for postoperative treatment of endometrial and cervical cancer. These results need to be validated in a larger prospective randomized controlled cohort.

The use of dose surface maps as a tool to investigate spatial dose delivery accuracy for the rectum during prostate radiotherapy.

This study aims to address the lack of spatial dose comparisons of planned and delivered rectal doses during prostate radiotherapy by using dose-surface maps (DSMs) to analyze dose delivery accuracy and comparing these results to those derived using DVHs. Two independent cohorts were used in this study: twenty patients treated with 36.25Gy in five fractions (SBRT) and 20 treated with 60Gy in 20 fractions (IMRT). Daily delivered rectum doses for each patient were retrospectively calculated using daily CBCT images. For each cohort, planned and average-delivered DVHs were generated and compared, as were planned and accumulated DSMs. Permutation testing was used to identify DVH metrics and DSM regions where significant dose differences occurred. Changes in rectal volume and position between planning and delivery were also evaluated to determine possible correlation to dosimetric changes. For both cohorts, DVHs and DSMs reported conflicting findings on how planned and delivered rectum doses differed from each other. DVH analysis determined average-delivered DVHs were on average 7.1%±7.6% (p ≤ 0.002) and 5.0±7.4% (p ≤ 0.021) higher than planned for the IMRT and SBRT cohorts, respectively. Meanwhile, DSM analysis found average delivered posterior rectal wall dose was 3.8±0.6Gy (p=0.014) lower than planned in the IMRT cohort and no significant dose differences in the SBRT cohort. Observed dose differences were moderately correlated with anterior-posterior rectal wall motion, as well as PTV superior-inferior motion in the IMRT cohort. Evidence of both these relationships were discernable in DSMs. DSMs enabled spatial investigations of planned and delivered doses can uncover associations with interfraction motion that are otherwise masked in DVHs. Investigations of dose delivery accuracy in radiotherapy may benefit from using DSMs over DVHs for certain organs such as the rectum.

A comparative study of rectal volume variation in patients with prostate cancer: A tertiary care center study

IntroductionEvery day variations in rectal filling in prostate cancer radiotherapy can significantly alter the delivered dose distribution from what was intended. The goal of this study was to see if the time of treatment delivery affected the rectal filling. MethodsThis is a retrospective study which included 50 patients with localized prostate cancer treated with volumetric modulated arc therapy (VMAT) to the primary and regional lymph nodes. Cone Beam Computed Tomography (CBCT) image-sets were done for all patient's daily setup verification. The radiation therapist contoured the rectum on all CBCT image sets. The rectal volumes delineated on CBCT and the planning CT image sets were compared. The change in rectal volumes between morning and afternoon treatments were calculated and compared. ResultsA total of 1000 CBCT image sets were obtained on 50 patients in the morning and afternoon. The percentage variation of the CBCT rectal volumes over the planning CT scan was 16.57% in the AM group and 24.35% in the PM group. ConclusionThe percentage change in rectal volume was significantly lesser in AM group compared to PM group and therefore morning treatments may result in dose distribution that is close to the intended dose distribution. Implications for practiceIn prostate cancer radiotherapy our study suggests that a simple technique of changing the time of treatment from afternoon to morning can help to reduce the rectal volume.

Dosimetric impacts of cone-beam computed tomography (CBCT)-based anatomic changes in intensity-modulated radiotherapy for cervical cancer.

To evaluate the effects of dose to tumors and organs at risk (OARs) on inter-fractional anatomic changes. We evaluated nine patients with cervical cancer treated with intensity-modulated radiotherapy (IMRT) (45 Gy in 25 fractions) using kV cone-beam computed tomography (CBCT) image guidance once or twice a week before treatment. For each patient, the original plan on the computed tomography (CT) image was copied to merged images, and then the fractional doses were calculated. Subsequently, deformable accumulated doses were obtained by summing the fractional absolute doses into a single dose in MIM Maestro software. The volume changes in the target and OARs were compared between the original CT and merged CBCT images, and the differences in the fractional and accumulated doses were also evaluated. Sixty-nine merged CBCT images were obtained and analyzed in this study. For the target areas, the volume changes in the clinical target volume (CTV) and planning target volume (PTV) reached -18.05% and -24.11% at most, respectively. The fractional D2% of the CTV and PTV was generally higher than the original plans, and the accumulated deviations were 2.27%±0.82% (P<0.01) and 2.42%±1.28% (P<0.01), respectively. The fractional D98% of the PTV was underdosed up to 18.28% for 78% of patients, and the accumulated deviations were -2.06% to -17.29% (P<0.05). For the OARs, the bladder volume changes were the most dramatic, reducing up to 93.60%. The fractional Dmean and D2cc of the bladder were generally higher than the original plans, and there were significant differences in their accumulated values (P<0.05). There was no obvious trend of rectal volume change with -69.65% to 74.20%. The rectum Dmean and D2cc of the accumulated were not significantly different from the planned dose (P>0.05). For patients with cervical cancer, the changes in bladder and rectal volume were greater than in the target volume. Although the volume changes in the bladder and rectum had no significant effect on D98% of the CTV and PTV, they had a significant effect on their own D2cc and the D2% of the CTV and PTV. More attention should be paid to the volume changes in the bladder and rectum in clinical work.

Focal Dose Intensification to the Dominant Intraprostatic Lesion (DIL) Using an MR-Linac Adaptive-Planning Approach for Prostate Cancer: Dosimetric Outcomes and Early Toxicity

<h3>Purpose/Objective(s)</h3> Focal boosting of the dominant intraprostatic lesion (DIL) improves biochemical control. The MR-linear accelerator (MRL) may improve the precision of DIL boosting with ultrahypofractionation by accounting for daily changes in the target and organs at risk (OAR). We report our early experience using this approach. <h3>Materials/Methods</h3> Fifteen patients with intermediate risk prostate cancer and a dominant lesion were treated on the MRL with stereotactic body radiation (SBRT) to 40Gy in 5 fractions, with the DIL boosted to 45Gy. A daily adaptive plan was created on initial daily imaging starting with bladder empty using adapt-to-shape (ATS) method. A verification image was acquired and further adjustments made using ATS or adapt-to-position (ATP) to account for target motion during initial planning. Contours were retrospectively adjusted on verification and post treatment scans to account for anatomic changes. The daily adapted plan was recalculated on these scans to estimate true dose delivered to the prostate and DIL. Prostate and DIL coverage on the pre-treatment plan (plan 1), was compared with recalculated plans on the verification (plan 2) and post treatment (plan 3) scans, based on values averaged over patients' 5 fractions (n=15). Differences across all fractions (n=75) were evaluated using a linear mixed model. Dosimetric impact of covariables including use of secondary ATS/ATP, and bladder and rectal volume changes were evaluated. Bladder filling was evaluated from fraction 1 to 5 to evaluate for any trends as treatment progresses. <h3>Results</h3> The median prostate D95 for plan 1, 2 and 3 was 40.3Gy, 40.5Gy and 40.3Gy respectively and DIL D95 was 45.7Gy, 45.2Gy and 44.6Gy. On a per fraction level, prostate coverage did not significantly change during planning time (plan 1 vs plan 2) or treatment delivery time (plan 2 vs plan 3), however there was a trend towards reduced DIL coverage between plan 1 vs plan 2, and plan 2 vs plan 3 (p=0.08, 0.06 respectively). Increased bladder volume reduced DIL coverage (p=0.03, plan 1 vs 2), and prostate coverage (p= 0.03, plan 2 vs 3). Bladder filling was associated with fraction number, fraction 1 being higher than the other 4 (all p-values < 0.02). Treatment time on table was associated with change in bladder filling (p=0.01) and after accounting for this, the associations between change in bladder filling and fraction number were weaker. At a median follow-up of 9 mo, the maximum reported toxicity was G2 GU (n=2) and G2 GI (n=1). The median change in IPSS was 3.5. <h3>Conclusion</h3> SBRT with focal boosting of the DIL on the MRL with daily adaptive planning is feasible and results in excellent planned and delivered dosimetry and acceptable toxicity. Intra-fractional bladder filling may impact target coverage. When considering focal boosting with SBRT, adaptive planning with MRL may provide the greatest confidence for delivering the prescribed dose with least normal tissue exposure.

A prospective observational study to analyse the influence of bladder and rectal volume changes on prostate radiotherapy using IMRT.

To analyse the interfractional bladder and rectal volume changes and the influence on prostate position. Interfractional displacement of prostate due to variation in bladder and rectal volume is usual. It is only rational to study the bladder and rectal volume changes and their effects on prostate position during intensity modulated radiotherapy of prostate cancer. A prospective study was conducted on twenty patients with localized prostate cancer during the first phase of radiotherapy, where 50 gray in 25 fractions was delivered by the IMRT technique with daily cone beam computed tomography Bladder and rectum volumes were delineated on CBCT images and their volumes were noted. Prostate position was noted on each set of CBCT images with respect to specific reference points defined on the ileum and coccyx, and daily prostate displacement was noted. Mean setup errors in vertical, longitudinal and lateral directions were noted as 1.49, 0.498 and 0.17 cm, respectively. Mean change in bladder and rectal volumes in daily CBCT images with respect to that on the first day CT images was noted as 101.94 and 10.22, respectively. Mean lateral and vertical displacement in prostate position was noted as 0.53 and 0.49 cm respectively. No considerable changes in dosimetric parameters were observed because of bladder and rectal volume changes. Daily CBCT should be done for accurate treatment delivery by the IMRT technique for prostate radiotherapy as prostate shifts physiologically with changes in rectal and bladder volumes.

Dosimetric and volumetric effects in clinical target volume and organs at risk during postprostatectomy radiotherapy.

To assess the reproducibility of the dose-volume distribution of the initial simulation CT, generated using volumetric modulated arc therapy (VMAT) planning, during the radiotherapy of the prostatic bed based on weekly cone beam CTs (CBCT). Twenty-three patients, after radical prostatectomy were treated with adjuvant or salvage radiotherapy between July and December 2016 and considered for this evaluation. Weekly CBCT scans (n = 138) were imported into the treatment planning system, and the clinical tumor volume (CTV), the rectum and the bladder were contoured. The initially calculated dose distribution and the dose-volume histograms generated from weekly CBCTs were compared. The prostatic fossa dose coverage was assessed by the proportion of the CTV fully encompassed by the 95% and 98% isodose lines. Rectal and bladder volumes receiving 50, 60 and 65 Gy during the treatment were compared to the initial plan, with statistical significance determined using the one-sample t‑test. Marked variations in the total organ volume of the rectum and the bladder were observed. The correlation between rectum volume and V50 was not significant (p = 0.487), while the bladder volume and V50 demonstrated asignificant correlation. There was no correlation between urinary bladder volume and CTV. The change in rectal volume correlated significantly with CTV. The dose coverage (D98% and D95%) to the prostatic bed could be achieved for all patients due to the ventral shift in the volume differences of the rectum. Weekly CBCTs can be considered as adequate verification tools to assess the interfractional variability of the CTV and organs at risk. The proven volume changes in the urinary bladder and the rectum do not compromise the final delivered dose in the CTV.

Dosimetric Impact of Changes in Organs at Risk during the Course of HDR Brachytherapy for Gynecologic Malignancies

The use of 3D image based treatment planning for high-dose-rate interstitial brachytherapy (HDR ISBT) allows for the delineation of organs at risk (OAR) to better estimate and minimize dose to these structures. Daily CT is also useful as a tool for daily verification of the relative position of the interstitial template and target volume. It is common practice to assume that OARs receive roughly the same dose on average over the course of treatment; therefore, it is not typical to contour these structures after initial treatment planning on day 1 (d1). Subsequently, organ position change, bowel gas, and variation in bladder volume are not specifically accounted for in a standardized fashion. Given the potential for normal tissue injury from large doses per treatment with HDR ISBT, our aim was to investigate the changes in OARs over the course of ISBT for gynecological cancers. Single institution retrospective analysis evaluating CT scans obtained for ISBT of patients with primary and recurrent gynecological cancer treated from 2014 through 2016. Patients received a d1 CT with or without post-implant MRI fusion for HRCTV delineation prior to fraction 1 and a verification CT prior to fractions 2 (d2) and 4 (d3). OARs rectum and bladder were re-contoured for all d1 CT scans and new contours were created for all d2 and d3 verification CT scans by one physician to minimize contour variability. The d1 plan was replicated with the d2 and d3 CT with updated catheter positions, but the same dwell times from the original plan. Actual and expected ISBT alone and external beam radiotherapy plus ISBT (EBRT-ISBT) doses to the OARs were calculated on d1 – d3 CTs. Doses to OARs for fractions 3 and 5 were assumed to be the same as the prior same-day treatment. OAR volumes on d1 – d3 CT were also calculated. Single institution retrospective analysis evaluating CT scans obtained for ISBT of patients with primary and recurrent gynecological cancer treated from 2014 through 2016. Patients received a d1 CT with or without post-implant MRI fusion for HRCTV delineation prior to fraction 1 and a verification CT prior to fractions 2 (d2) and 4 (d3). OARs rectum and bladder were re-contoured for all d1 CT scans and new contours were created for all d2 and d3 verification CT scans by one physician to minimize contour variability. The d1 plan was replicated with the d2 and d3 CT with updated catheter positions, but the same dwell times from the original plan. Actual and expected ISBT alone and external beam radiotherapy plus ISBT (EBRT-ISBT) doses to the OARs were calculated on d1 – d3 CTs. Doses to OARs for fractions 3 and 5 were assumed to be the same as the prior same-day treatment. OAR volumes on d1 – d3 CT were also calculated. Expected versus actual bladder and rectal doses were highly correlated over the course of ISBT. Changes in rectal volume during treatment may result in an undesirable dose increase in some patients.

PEG spacer gel and adaptive planning vs single plan in external prostate radiotherapy--clinical dosimetry evaluation.

Spacer gel is used to reduce the rectal dose in prostate radiotherapy. It is injected to increase the distance between the prostate and rectum. During the course of external radiotherapy treatment, physiological changes in rectal volume exist. When using polyethylene glycol material, such as DuraSeal(®) (Covidien, Mansfield, MA), gel resorption also occurs. Together, these factors alter the original dose plan distribution. External dose planning and calculations were simulated using images acquired from 10 patients who were treated with brachytherapy and gel. The CT series was taken relative to gel injection: pre 1 day, post 1 day, post 1 month and post 2 months. Adaptive planning was compared with a single plan. Adaptive planning shows better results compared with the single plan used in the total treatment course; however, the effect is minor. Gel usage is clearly favourable to rectal DVH. Using adaptive planning with gel improves rectal DVH but is not necessary according to this study. Spacer gel is used in prostate radiotherapy to increase distance between the prostate and the rectum, thus reducing the rectal doses. During the treatment course, gel resorption exists which affects the rectal doses. The usefulness of adaptive planning to compensate this resorption effect has not been studied before.

Dosimetric Changes during Daily MR-Guided Pulse-Dose-Rate Brachytherapy for Cervix Cancer

Dosimetric Changes during Daily MR-Guided Pulse-Dose-Rate Brachytherapy for Cervix Cancer

A Novel Biodegradable Balloon (Bioprotect Spaceguard) Provides Reproducible Geometry and Significantly Reduces Rectal Dose in Patients Receiving IMRT for Prostate Cancer: Single Arm Phase I Multi-Institutional International Study

Results: The procedure was tolerated well without any significant discomfort from the procedure or placement. There was no procedure or balloon related Adverse Effect. The balloon displaced both the prostate post wall (mean: 11.4mm; range: 5mm18.7mm) and anterior rectal wall (mean: 10.8mm; range: 6mm-16mm) along the interface while the anterior surface of the prostate and posterior surface of the rectum got displaced negligibly (mean: 4.8mm; range: 0.2mm-12.3mm). The prostate was elongated in the superior-inferior dimension but its volume was constant throughout. There was non-significant change in balloon volume and position, prostate posterior wall and rectal anterior wall throughout treatment. The balloon seemed to serve as buffer for prostate inter-fraction displacement from changes in rectal volume. The mean gap introduced by the balloon was 25.7 mm (range: 13.6mm-32.7mm) and it remained constant during treatment. This gap resulted in a mean V65 of 4.255% (range: 0%11.69%) compared to 14.45% (range: 11.14%-20.52%) before implant. Similarly, V70 changed from 12.15% (range: 9.3%17.11%) to 2.95% (range: 0%-9.69%). Follow up scans showed degradation of the balloon 90 days post treatment. Conclusions:Bioprotect is safe, easy to insert and creates a highly reproducible and uniform space between prostate and anterior rectal wall. This technique has tremendous implications for dose escalation and hypofractionation in radiation therapy. Author Disclosure: M.K. Garg: None. G. Kovacs: F. Consultant/Advisory Board; Bioprotect. D.M. Blakaj: None. N. Tsai: None. E. Gez: None. G. Scarzello: None. I. Koziol: None. M. Anscher: None. A. Paz: E. Ownership Interest; Co-founder Bioprotect. S. Kalnicki: None.

Can diet combined with treatment scheduling achieve consistency of rectal filling in patients receiving radiotherapy to the prostate?

Background and purposeThis pilot study investigates whether an individualized fluid and fibre prescription combined with a constant treatment can improve rectal filling consistency during radiotherapy. Methods and materialsFibre, fluid intake and bowel function were assessed in 22 patients at a standard planning scan (SCT) and individualized dietary advice was prescribed to regularize bowel habit. Patients were requested to record frequency and type of bowel movements, fibre and fluid intake daily. Two subsequent CT scans were acquired at 7 (CCT1) and 10days (CCT2) after SCT at a similar time. Rectal volume and gas were measured planning CT’s and ‘on treatment’ cone beam CT scans. We hypothesised that the difference in volume between CCT1 and CCT2 would be less than the difference between SCT and CCT1. ResultsThe mean (SD) change in volume between SCT to CCT1 and CCT1 to CCT2 was 5.68cm3 (26.2) and −8.6cm3 (40.1), respectively (p=0.292). Of the 22 patients scanned 20 provided a complete record of dietary intake and bowel motion. The majority of patients either achieved or exceeded prescription. Change in rectal gas was the only correlation with change in rectal volume. ConclusionPatient self reporting of bowel motion, fibre, fluid intake was achievable but consistency of rectal filling was not improved. Improved understanding of the aetiology and management of rectal gas is indicated.

A significant decrease in rectal volume and diameter during prostate IMRT

PurposeTo record changes in rectal volume (RV) and diameter (RD) of patients with prostate adenocarcinoma prior to and at an interim period during radiotherapy, which could potentially affect treatment toxicity and tumor control. MethodsThree hundred and fifteen patients treated with intensity modulated radiotherapy (IMRT) underwent planning CT scans before radiation and after 45Gy. For each scan, RV and RD were recorded and compared using a two-tailed paired t-test. Robust linear regression analysis assessed correlation between initial RV and percent RV change. ResultsThe mean change in RV was −8.62cm3 and in RD was −0.19cm3, (p<0.05). A decrease ⩾10% in RV and RD was seen in 159 patients (50.5%) and 117 patients (37.1%), respectively. Patients with ⩾10% volume change had larger initial RVs than those with <10% decrease, (78.1 vs. 50.8cm3, p<0.0001). ConclusionsA significant decrease in RV and RD occurs during prostate IMRT delivery. More than half of patients had decreased RV and over a third had decreased RD. This observation is pertinent to prostate localization, planning margins, and implies that dose–volume histogram (DVH) analysis of rectal irradiation based on pre-treatment CT scanning may inaccurately estimate the risk of rectal toxicity when the initial RV is larger than 70cm3.

Changes in rectal volume and prostate localization due to placement of a rectum-emptying tube

Rectal volume variation has a crucial effect on prostate localization during external beam radiotherapy for prostate cancer. This study investigated the effect of rectal volume reduction by a rectum-emptying tube (RET) on prostate immobilization. The study group comprised 21 patients who underwent proton beam treatment for prostate cancer. Sigmoid-shaped flexible plastic RETs were used to drain gases from the rectum. Computed tomography (CT) was performed before and after RET placement at the treatment planning stage and at the beginning of treatment. Prostate displacement and changes in rectal volume were measured on the CT images. The feasibility of RET placement was evaluated during and after the procedure. All 21 patients tolerated the procedure. The rectal volume was significantly lower with a RET than without a RET. The differences in rectal volume between the treatment planning stage and the beginning of treatment were significantly lower with a RET than without. RET placement significantly decreased prostate displacement in the anteroposterior and superoinferior directions but not in the left-right direction. RET placement reduced both rectal volume and variation in rectal volume. The procedure reduced displacement of the prostate. RET placement thus appears to be an effective technique for immobilizing the prostate.

Motion and deformation of the target volumes during IMRT for cervical cancer: What margins do we need?

Background and Purpose For cervical cancer patients the CTV consists of multiple structures, exhibiting complex inter-fraction changes. The purpose of this study is to use weekly MR imaging to derive PTV margins that accommodate these changes. Materials and methods Twenty patients with cervical cancer underwent a T2-weighted MRI exam before and weekly during IMRT. The CTV, GTV and surrounding organs were delineated. PTV margins were derived from the boundaries of the GTV and CTV in the six main directions and correlated with changes in the volumes of organs at risk. Results Around the GTV a margin of 12, 14, 12, 11, 4 and 8 mm to the anterior, posterior, right lateral, left lateral, superior and inferior directions was needed. The CTV required margins of 24, 17, 12, 16, 11 and 8 mm. The shift of the GTV and CTV in the AP directions correlated weakly with the change in rectal volume. For the bladder the correlations were even weaker. Conclusions We used weekly MRI scans to derive inhomogeneous PTV margins that accommodate changes in GTV and CTV. The weak correlations with rectum and bladder volume suggest that measures to control filling status of these organs may not be very effective.

Is a 3-mm intrafractional margin sufficient for daily image-guided intensity-modulated radiation therapy of prostate cancer?

Purpose To determine whether a 3-mm isotropic target margin adequately covers the prostate and seminal vesicles (SVs) during administration of an intensity-modulated radiation therapy (IMRT) treatment fraction, assuming that daily image-guided setup is performed just before each fraction. Materials and methods In-room computed tomographic (CT) scans were acquired immediately before and after a daily treatment fraction in 46 patients with prostate cancer. An eight-field IMRT plan was designed using the pre-fraction CT with a 3-mm margin and subsequently recalculated on the post-fraction CT. For convenience of comparison, dose plans were scaled to full course of treatment (75.6 Gy). Dose coverage was assessed on the post-treatment CT image set. Results During one treatment fraction (21.4 ± 5.5 min), there were reductions in the volumes of the prostate and SVs receiving the prescribed dose (median reduction 0.1% and 1.0%, respectively, p < 0.001) and in the minimum dose to 0.1 cm 3 of their volumes (median reduction 0.5 and 1.5 Gy, p < 0.001). Of the 46 patients, three patients’ prostates and eight patients’ SVs did not maintain dose coverage above 70 Gy. Rectal filling correlated with decreased percentage-volume of SV receiving 75.6, 70, and 60 Gy ( p < 0.02). Conclusions The 3-mm intrafractional margin was adequate for prostate dose coverage. However, a significant subset of patients lost SV dose coverage. The rectal volume change significantly affected SV dose coverage. For advanced-stage prostate cancers, we recommend to use larger margins or improve organ immobilization (such as with a rectal balloon) to ensure SV coverage.

4042 POSTER Rectal volume changes during treatment: the case for ansisotropic safety margins around the clinical tumor volume in radiotherapy for prostate cancer

4042 POSTER Rectal volume changes during treatment: the case for ansisotropic safety margins around the clinical tumor volume in radiotherapy for prostate cancer

SU-FF-J-111: Assessing Prostate, Bladder and Rectal Doses During Image Guided Prostate IMRT Using Kilo Voltage Cone Beam CT

Purpose: To evaluate dosimetric uncertainties due to interfraction organ variation during Image Guided IMRT using Elekta Synergy System. On board Kilovoltage Cone Beam CT scans were evaluated for prostate, bladder and rectal volume changes and planned dose was compared against the actual delivered dose. Clinical feasibility for an offline correction protocol for an adaptive process was investigated. Methods & Materials: We investigated for five patients the actual dose delivered to prostate, bladder and rectum by using the cone beam CT as the study set and recomputing dose from IMRT fields. In order to do achieve this one full volumetric Cone Beam CT study set was randomly chosen for each patient from each week and thus a total of 9 CT (Week 1 to Week 9) study sets were used for each patient to analyze the prostate, bladder and rectal volume changes and hence their impact on dosimetry. The contoured Cone Beam CT volumes were reviewed by the Radiation Oncologist for accuracy. The CBCT scans were fused with the KV CT scans and the dose was recomputed. Results: KV CBCT was shown to be useful in characterizing Prostate, Bladder and rectal volume changes. There was a large variation in bladder and rectal volumes and hence the constraints defined in RTOG-0126 protocol varied from week to week. Conclusion: The current work describes an adaptive process for Image Guided Prostate IMRT. Replanning will ultimately allow one to reoptimize the IMRT beams based on the dose variation from original plan. However at this point this process is prohibitively time consuming and for future implementation of adaptive radiotherapy or dose escalation this process needs to be automated with a deformable registration model.

SU-FF-J-109: Conformal Prostate Treatment - a Closer Look at Dose Delivery Uncertainty and Adaptive Options

Purpose: To evaluate uncertainties in Tomotherapy conformal prostate treatment due to inter‐fraction organ variation. On board MVCT images were evaluated for reconstruction, comparing planned dose and delivered dose. Clinical feasibility of a closed feed back process for image guided adaptive process was studied. Method and Materials: Five prostate patients were randomly chosen for this study. The positional variations of inter‐fraction organ motion for each treatment fraction were systematically monitored and characterized using onboard MVCT images. Rectum and bladder were re‐contoured on MVCT images after each treatment and delivered dose was reconstructed using a sinogram that was generated during kVCT based treatment planning optimization. Reconstructeddoses were compared with treatment planningdoses for individual organs through dose volume histograms. Results: MVCT images were shown to be useful for dosereconstruction purposes. Three patients' treatment delivery showed large dose delivery variations for individual treatment fractions. Rectal doses were significantly higher (10–50%) due to large changes in rectal volume. Bladder dose varied less compared to rectal dose. However, two patients' delivered dose was close to planned dose.Conclusions: The current report describes an adaptive process for Tomotherapy prostate radiation treatment. Re‐planning with dose differences will ultimately allow one to adjust for any dose variation (in order to correct for under dosing or overdosing). However, at this point, re‐contouring, re‐planning, and verification of quality assurance are very labor intensive and require strategies for offline patient specific adaptive treatment. One may, however, sum dose differences for all fractions to evaluate actual total dose delivery. In the future, the development of automatic adaptive processes may pave the way for true closed loop intervention.

Po-Thur Eve General-09: Dynamic evaluation for the treatment outcome incorporating prostate organ motion

This paper is to calculate the cumulative dose considering internal organ motion and patient repositioning of prostate patient using the one fractional treatment plan method for a specific phantom and patients. In the multi‐fractional radiation treatment, it is desirable to consider the internal organ motion from fraction to fraction. The PTV positional variations can be measured and expressed as the fractional equivalent uniform dose (FEUD) deviation by randomly picking up the FEUD using Monte Carlo method in every fraction. The average EUD deviations for PTV with hundreds of simulations are 3.3% for phantom and 1.8% for 5 prostate patients. The changes of rectal volume and position are also simulated by shifting the geometric center of the rectum and changing the rectal radius to model the rectal deformation using Pinnacle3 planning system and FEUD for rectum are estimated. The prostate margin, together with the inter‐fraction variations in rectal position and shape, affects the delivered dose to the rectum. The effect of the dose fluctuations is larger for the normal tissues than the target, and the change of rectal position would cause more complication compared to the rectal volume change. The method provides a simple way to estimate the tumor control probability and normal tissue complication probability dynamically with considering internal organ motion and repositioning errors throughout the whole treatment course. It is possible to test a treatment plan against a range of possible patterns of organ motion with this dynamic evaluation for the treatment outcome.