The Impact of Varying Volumes in Rectal Balloons on Rectal Dose Sparing in Conformal Radiation Therapy of Prostate Cancer

BACKGROUND AND PURPOSE: The use of a rectal balloon leads to a protection of the posterior rectal wall in irradiation of prostate cancer. The purpose of this analysis was to quantitatively assess the optimal volume in rectal balloons concerning rectal dose sparing in different clinical target volumes (CTVs) in prostate cancer irradiation. PATIENTS AND METHODS: 14 patients with localized prostate cancer undergoing external beam radiotherapy were investigated. The prostate, the entire and the proximal seminal vesicles were delineated as CTV. Treatment plans without a rectal balloon and with a rectal balloon inflated with 40 ml (six patients) or 60 ml air (eight patients) were generated for each CTV and compared concerning rectal dose volume histograms. RESULTS: The use of a rectal balloon filled with 40 ml air led to no significant advantage in radiation exposure of the rectal wall in all CTVs. The use of a rectal balloon filled with 60 ml air resulted in a significant decrease of the exposed rectal wall volume in all CTVs with a reduced estimated risk for chronic toxicity in case of inclusion of the proximal or entire seminal vesicles into the CTV. CONCLUSION: The use of a rectal balloon filled with 60 ml air led to a significantly decreased proportion of the irradiated rectal wall for all CTVs. This volume filled in rectal balloons is therefore recommended for use. In case of irradiation of the prostate without the seminal vesicles, the use of a rectal balloon should be considered carefully concerning the patients' imaginable discomfort using a rectal balloon and a questionable advantage concerning the estimated risk for chronic toxicity.     

Dosimetric advantage and clinical implication of a micro-multileaf collimator in the treatment of prostate with intensity-modulated radiotherapy.

This paper investigates the dosimetric benefits of a micro-multileaf (4-mm leaf width) collimator (mMLC) for intensity-modulated radiation therapy (IMRT) treatment planning of the prostate cancer and its potential application for dose escalation and hypofractionation. We compared treatment plans for IMRT delivery using 2 different multileaf collimator (MLC) leaf widths (4 vs. 10 mm) for 10 patients with prostate cancer. Treatment planning was performed on the XknifeRT2 treatment planning system. All beams and optimization parameters were identical for the mMLC and MLC plans. All of the plans were normalized to ensure that 95% of the planning target volume (PTV) received 100% of the prescribed dose (74 Gy). The differences in dose distribution between the 2 groups of plans using the mMLC and the MLC were assessed by dose-volume histogram (DVH) analysis of the target and critical organs. Significant reductions in the volume of rectum receiving medium to higher doses were achieved using the mMLC. The average decrease in the volume of the rectum receiving 40, 50, and 60 Gy using the mMLC plans was 40.2%, 33.4%, and 17.7%, respectively, with p-values less than 0.0001 for V40 and V50 and 0.012 for V60. The mean dose reductions for D17 and D35 for the rectum were 20.0% (p < 0.0001) and 18.3% (p < 0.0002), respectively, when compared to those with the MLC plans. There were consistent reductions in all dose indices studied for the bladder. The target dose inhomogeneity was improved in the mMLC plans by an average of 32%. In the high-dose range, there was no significant difference in the dose deposited in the "hottest" 1 cc of the rectum between the 2 MLC plans for all cases (p > 0.78). Because of the reduction of rectal volume receiving medium to higher doses, dose to the prostate target can be escalated by about 20 Gy to over 74 Gy, while keeping the rectal dose (either denoted by D17 or D35) the same as those with the use of the MLC. The maximum achievable dose, derived when the rectum is allowed to reach the tolerance level, was found to be in the range of 113-172 Gy (using the tolerance value of D17). We conclude that the use of the mMLC for IMRT of the prostate may facilitate dose hypofractionation due to its dosimetric advantage in significantly improving the DVH parameters of the prostate and critical organs. When used for conventional fractionation scheme, mMLC for IMRT of the prostate may reduce the toxicity to the critical organs.     

Dosimetric evaluation of MapCHECK 2 and 3DVH in the IMRT delivery quality assurance process

Based on per-field data, 3 dose-volume histogram (DVH) is designed to calculate 3-dimensional (3D) dose using patient-computed tomography (CT) for an intensity-modulated radiation therapy (IMRT) delivery quality assurance (DQA) process measured with diode detector arrays. In this study, the characteristics and suitability of 3DVH for an IMRT DQA process were evaluated by assessment of clinically applied results. The DQA plans were prepared with a 2D diode detector array (MapCHECK 2) for 20 IMRT patients (10 with head and neck cancer and 10 with prostate cancer), and the matching rate between the calculated dose and the measured dose was analyzed. The relative dose differences between the original IMRT plan and the recalculated plan by 3DVH were evaluated for tumor target and organs at risk (OARs). Confidence limits (CLs) were determined for quantification of the degree of agreement that should be expected in each DQA method. The individual CLs for DQA accuracy of 3D dosimetric evaluation in the tumor target and OARs were also calculated to evaluate the differences compared with conventional 2D-based DQA results. The matching rates were sufficient to validate the accuracy of IMRT DQA analyses and the calculated CL values were reasonable values considering the normal criteria of our institution. The 3DVH analysis generally had better matching rate than per-field measurement analysis and the CL values for OARs were better than those for the planning target volume (PTV). The usefulness of the 3DVH tool was verified for IMRT DQA from the point of view of dose evaluation in the corresponding patient's CT data. It also enables dosimetric evaluation in each important structure (PTV and OARs), which can promote more effective evaluation of DQA error in terms of tumor treatment and side effects.     

Intensity-Modulated Radiotherapy (IMRT) of Localized Prostate Cancer

PURPOSE: The present status of intensity-modulated radiation therapy (IMRT) for treatment of localized prostate cancer is discussed. METHODS: The technological basis of IMRT and the rationale for the use in treatment of prostate cancer are described. Clinical results from the literature are presented and treatment strategies for further reduction of safety margins are outlined. RESULTS AND DISCUSSION: Multiple planning studies demonstrated the dosimetric advantage of IMRT compared to three-dimensional conformal radiotherapy. Though randomized studies are missing, retrospective studies indicate that improved dose distributions of IMRT transfer into improved rates of local control and/or lower rates of rectal toxicity. However, with standard safety margins the benefit of IMRT seems to be limited. Image guidance is considered to be essential to reduce errors of patient setup and internal motion of the prostate.     

Predictive factors of long-term rectal toxicity following permanent iodine-125 prostate brachytherapy with or without supplemental external beam radiation therapy in 2216 patients

PurposeWe analyzed factors associated with rectal toxicity after iodine-125 prostate brachytherapy (BT) with or without external beam radiation therapy (EBRT).Methods and MaterialsIn total, 2216 prostate cancer patients underwent iodine-125 BT with or without EBRT between 2003 and 2013. The median followup was 6.9 years. Cox proportional hazards modeling was used for univariate and multivariate analyses to assess clinical and dosimetric factors associated with rectal toxicity. Dosimetric parameters from 1 day after implantation (Day 1) and 1 month after implantation (Day 30) were included in the analyses.ResultsThe 7-year cumulative incidence of Grade 2 or higher rectal toxicity was 5.7% in all patients. The multivariate analysis revealed that antiplatelet or anticoagulant therapy, neoadjuvant androgen deprivation therapy, treatment modality, Day 1 rectal volume receiving 100% of the prescribed dose (RV₁₀₀), and the Day 30 minimal percent of the prescribed dose delivered to 30% of the rectum (RD₃₀) were associated with rectal toxicity. Day 1 RV₁₀₀ was a common predictor in both BT-alone and the BT + EBRT groups. The 5-year cumulative incidence of Grade 2 or higher rectal toxicity was 12.6%, 5.9%, and 1.7% for BT + 3-dimensional conformal radiation therapy, BT + intensity-modulated radiation therapy, and the BT-alone groups, respectively (p < 0.001).ConclusionsRectal dosimetric parameters in BT were associated with late rectal toxicity. Although the risk of rectal toxicity was higher when EBRT was combined with BT, with proper and achievable rectal dose constraints intensity-modulated radiation therapy yielded less toxicity than 3-dimensional conformal radiation therapy.     

VMAT vs. 7-Field-IMRT: Assessing the Dosimetric Parameters of Prostate Cancer Treatment with a 292-Patient Sample

We compared normal tissue radiation dose for the treatment of prostate cancer using 2 different radiation therapy delivery methods: volumetric modulated arc therapy (VMAT) vs. fixed-field intensity-modulated radiation therapy (IMRT). Radiotherapy plans for 292 prostate cancer patients treated with VMAT to a total dose of 7740 cGy were analyzed retrospectively. Fixed-angle, 7-field IMRT plans were created using the same computed tomography datasets and contours. Radiation doses to the planning target volume (PTV) and organs at risk (bladder, rectum, penile bulb, and femoral heads) were measured, means were calculated for both treatment methods, and dose-volume comparisons were made with 2-tailed, paired t-tests. The mean dose to the bladder was lower with VMAT at all measured volumes: 5, 10, 15, 25, 35, and 50% (p < 0.05). The mean doses to 5 and 10% of the rectum, the high-dose regions, were lower with VMAT (p < 0.05). The mean dose to 15% of the rectal volume was not significantly different (p = 0.95). VMAT exposed larger rectal volumes (25, 35, and 50%) to more radiation than fixed-field IMRT (p < 0.05). Average mean dose to the penile bulb (p < 0.05) and mean dose to 10% of the femoral heads (p < 0.05) were lower with VMAT. VMAT therapy for prostate cancer has dosimetric advantages for critical structures, notably for high-dose regions compared with fixed-field IMRT, without compromising PTV coverage. This may translate into reduced acute and chronic toxicity.     

Clinical experience with intensity-modulated radiation therapy (IMRT) for prostate cancer with the use of rectal balloon for prostate immobilization.

The implementation of intensity-modulated radiation therapy (IMRT) is the result of advances in imaging, radiotherapy planning technologies, and computer-controlled linear accelerators. IMRT allows both conformal treatment of tumors and conformal avoidance of the surrounding normal structures. The first patient treated with Peacock IMRT at Baylor College of Medicine took place in March 1994. To date, more than 1500 patients have been treated with IMRT; more than 700 patients were treated for prostate cancer. Our experience in treating prostate cancer with IMRT was reviewed. Patient and prostate motions are important issues to address in delivering IMRT. The Vac-Lok bag-and-box system, as well as rectal balloon for immobilization of patient and prostate gland, respectively, are employed. Treatment planning also plays a very important role. IMRT as a boost after conventional external beam radiotherapy is not our treatment strategy. To derive maximal benefits with this new technology, all patients received full course IMRT. Three separate groups of patients receiving (1) primary IMRT, (2) combined radioactive seed implant and IMRT, and (3) post-prostatectomy IMRT were addressed. Overall, toxicity profiles in these patients were very favorable. IMRT has the potential to improve treatment outcome with dose escalation while minimizing treatment-related toxicity.     

Die bedeutung eines rektumballons als interne immobilisation bei der konformalen strahlentherapie des prostatakarzinoms

BACKGROUND: As known from the literature, prostate motion depends on different bladder and/or rectum fillings. The aim of this study was to analyze the influence of a rectum balloon catheter, used as an internal immobilization device, on prostate and rectum motion during the treatment course. Moreover we have analyzed if the balloon enables an increase of the distance between the prostate and the posterior rectum wall. PATIENTS AND METHODS: Ten patients with localized prostate cancer (T1 to T3) underwent computed tomographic examinations with and without rectal balloon (filled with 40 ml air) at 3 times during treatment course (at the start, middle and end of treatment). Edges of prostate, rectum and bladder were measured in relation to bony reference structures and compared for both examination series (with and without balloon). RESULTS: An increase of the distance between the prostate and the posterior rectal wall of 8 mm was observed at the base of the prostate when using the rectum balloon (Figures 1a,b and 2). Moreover prostate motion in the ventrodorsal direction > or = 4 mm (1 SD) was reduced from 6/10 patients (60%) to 1/10 patients (10%) using the rectal balloon (Table 3, Figure 3). In general, deviations in the latero-lateral and cranio-caudal directions were less (mean < or = 2 mm, 1 SD), no difference between both examination series (with and without balloon) was observed. CONCLUSION: Rectal balloon catheter offers a possibility to reduce prostate motion and rectum filling variations during treatment course. In addition it enables an increase in the distance between prostate and posterior rectal wall, which could enable an improved protection of the posterior rectal wall.     

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.     

An in silico comparative dosimetric study of postmastectomy locoregional irradiation using intensity-modulated vs 3-dimensional conventional radiotherapy

An in silico dosimetric evaluation of intensity-modulated radiation therapy (IMRT) vs 3-dimensional conventional radiation therapy (3D-CRT) treatment plans in postmastectomy radiation therapy (PMRT) to the chest wall and regional lymphatics was conducted. Twenty-five consecutive patients with breast cancer referred for locoregional PMRT, stages T_2-4 with N_1-3, were planned to receive 50?Gy in 25 fractions with IMRT. Additionally, a 3D-CRT plan was generated using identical contours for the clinical target volumes (CTV), planning target volumes (PTV), and organs at risk (OAR). Treatment plans were assessed using dose-volume histogram (DVH) parameters of D₉₈, D₉₅, D₅₀, D₂, and homogeneity index for individual CTVs and PTVs. OARs evaluated were ipsilateral and contralateral lungs, heart, spinal cord, and opposite breast. Most DVH parameters pertaining to CTVs and PTVs significantly favored IMRT. V₂₀ for ipsilateral and contralateral lungs, D₃₃ of heart and maximum dose to spinal cord favored IMRT (all p?<?0.001). The mean dose to the opposite breast was significantly lesser with 3D-CRT (5.8?±?1.8?Gy vs 2.0?±?1.0?Gy, p?<?0.001). Thus, except for the mean dose to the opposite breast, the compliance to DVH constraints applied to PTV and OARs were significantly better with IMRT. At a median follow-up of 76 months (7-91), none had locoregional failure or pulmonary or cardiac morbidity. For PMRT, requiring comprehensive irradiation to both chest wall and regional lymphatics, IMRT offers superior dosimetric advantages over 3D-CRT. This was also corroborated by long-term outcomes in these patients treated with IMRT.     

Laparoscopic Seminal Vesicle and Pelvic Lymph Node Resection before High-Dose Three-Dimensional Conformal Radiation Therapy for Localized Prostate Cancer

PURPOSE: To analyze the dosimetric implications of seminal vesicle (SV) resection at the time of laparoscopic pelvic lymph node (PLN) dissection in patients presenting with high-risk prostate cancer regarding PLN and SV involvement. PATIENTS AND METHODS: Between June and September 2005, twelve patients underwent laparoscopic SV resection and PLN dissection before delivering a total dose of 80 Gy through a three-dimensional conformal radiation therapy (3D-CRT). Dose-volume histograms (DVHs; rectum, bladder, femoral heads) were obtained from G1 (group 1 with SV) and G2 (group 2 without SV) according to either three- dimensional or intensity-modulated treatment planning. All DVH medians were compared using the nonparametric sign test. RESULTS: SV resection during laparoscopic PLN dissection was performed in all twelve patients without major complications. DVH obtained with three-dimensional and intensity-modulated treatment planning showed that the median doses of RV(25%) (25% of rectal volume), RV(50%), RFHV(5%) (5% of right femoral head volume) and LFHV(5%) (5% of left femoral head volume) in G1 were significantly higher compared to those obtained in G2. CONCLUSION: For patients presenting with high-risk prostate cancer, SV resection performed at the time of laparoscopic PLN dissection allows to significantly decrease the dose delivered to the rectum, using either three-dimensional or intensity-modulated treatment planning, and to reduce the risk of acute and late rectal toxicity.     

Organ Movements and Dose Exposures in Teletherapy of Prostate Cancer using a Rectal Balloon

BACKGROUND AND PURPOSE: During radiotherapy of localized prostate cancer, organ movements for the dose exposure of organs at risk like rectum, urinary bladder and urethra play, inter alia, a significant role. One possibility of internal organ stabilizing is offered by the usage of a rectal balloon during radiotherapy. The influence on organ movements and dose allocation of the organs at risk is unknown. PATIENTS AND METHODS: Twelve patients (Table 1) were characterized based on planning-CT's regarding organ movements and organ doses using a rectal balloon, inflated with 0 ml and 60 ml air. For the determination of the organ doses, three-dimensional conformal radiation plans (3-field-pelvis box) with a cumulative dose of 59.4 Gy were created, and the dose-volume-histograms for the anterior rectal wall, the posterior rectal wall, the rectal mucosa, the whole rectum, as well as the urinary bladder were compared (Figures 1 and 2). RESULTS: The application of a 60 ml air-filled rectal balloon during each fraction of teletherapy led to significant organ movements of the anterior and posterior rectal wall and to a reduction of the transversal prostate diameter, as well as to a changed organ dose exposure of the organs at risk. A ventral shift of the anterior rectal wall (maximum 0.8 cm, mean 0.4 cm) was shown, as well as a dorsal shift of the posterior rectal wall (maximum 1.2 cm, mean 0.7 cm), associated with a transversal prostate diameter decrease (maximum 0.8 cm, mean 0.3 cm) (Table 2, Figure 3). The organ dose of the anterior rectal wall increased significantly (maximum 1.3 Gy, mean 0.5 Gy) during application of a rectal balloon, the one of the posterior rectal wall decreased significantly (maximum 18.6 Gy, mean 6.5 Gy). Related to the entire rectal mucosa and the rectum as a complete organ, a decrease of the maximum doses was shown (rectal mucosa: maximum 9.1 Gy, mean 3.0 Gy; rectum: maximum 9.4 Gy, mean 3.7 Gy). The organ dose of the urinary bladder did not show significant changes (Tables 3 and 4, Figures 4 to 7). CONCLUSION: The application of a rectal balloon in teletherapy of localized prostate cancer leads to significantly changed dose exposition of organs at risk. The decreased dose exposure of the posterior rectal wall and the rectal mucosa is opposed by the higher organ dose of the anterior rectal wall. It has to be shown weather documented organ dose exposure is associated with short and long-term consequences.     

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.     

Impact of Endorectal Balloon in the Dosimetry of Prostate and Surrounding Tissues in Prostate Cancer Patients Treated with IMRT

The dosimetric effect of endorectal balloon repositioning or failure was assessed in 10 prostate cancer patients treated with intensity modulated radiation therapy (IMRT). Three extreme clinical scenarios were simulated by placing the balloon in the most superior and inferior positions within the rectum and by removing the balloon. Treatment planning was performed by obtaining a computed tomography (CT) image with the balloon in the most superior position (plan 1). Subsequently, the isodose lines of plan 1 were superpositioned over the anatomy of 2 other CTs, one obtained with the balloon in the most inferior position and another without the balloon (plans 2 and 3, respectively). Dose-volume histograms (DVHs) of the prostate and surrounding tissues were generated and compared for all 3 plans. The prescribed radiation dose to the prostate and seminal vesicles was 70 Gy in 35 fractions. Balloon repositioning resulted in significant changes only for the seminal vesicles, where the minimum doses decreased from 70.39 to 61.58 Gy, and the percent volume below 70 Gy increased from 1.62% to 8.39%. Balloon failure resulted in significant decreases in mean and minimum doses for prostate from 74.36 to 72.84 Gy and 67.62 to 50.96 Gy, respectively. Similar decreases in the mean and minimum doses were also observed for seminal vesicles from 74.21 to 64.43 Gy and 70.39 to 41.74 Gy, respectively. Balloon repositioning did not affect normal tissue doses, while balloon failure significantly decreased the upper rectum mean doses from 30.79 to 19.38 Gy. This study demonstrates that repositioning of the endorectal balloon results in increased dose inhomogeneity for seminal vesicles, while balloon failure causes significant prostate and seminal vesicle underdosing without overdosing normal tissues.     

Impact of Endorectal Balloon in the Dosimetry of Prostate and Surrounding Tissues in Prostate Cancer Patients Treated with IMRT

The dosimetric effect of endorectal balloon repositioning or failure was assessed in 10 prostate cancer patients treated with intensity modulated radiation therapy (IMRT). Three extreme clinical scenarios were simulated by placing the balloon in the most superior and inferior positions within the rectum and by removing the balloon. Treatment planning was performed by obtaining a computed tomography (CT) image with the balloon in the most superior position (plan 1). Subsequently, the isodose lines of plan 1 were superpositioned over the anatomy of 2 other CTs, one obtained with the balloon in the most inferior position and another without the balloon (plans 2 and 3, respectively). Dose-volume histograms (DVHs) of the prostate and surrounding tissues were generated and compared for all 3 plans. The prescribed radiation dose to the prostate and seminal vesicles was 70 Gy in 35 fractions. Balloon repositioning resulted in significant changes only for the seminal vesicles, where the minimum doses decreased from 70.39 to 61.58 Gy, and the percent volume below 70 Gy increased from 1.62% to 8.39%. Balloon failure resulted in significant decreases in mean and minimum doses for prostate from 74.36 to 72.84 Gy and 67.62 to 50.96 Gy, respectively. Similar decreases in the mean and minimum doses were also observed for seminal vesicles from 74.21 to 64.43 Gy and 70.39 to 41.74 Gy, respectively. Balloon repositioning did not affect normal tissue doses, while balloon failure significantly decreased the upper rectum mean doses from 30.79 to 19.38 Gy. This study demonstrates that repositioning of the endorectal balloon results in increased dose inhomogeneity for seminal vesicles, while balloon failure causes significant prostate and seminal vesicle underdosing without overdosing normal tissues.     

Difference in VMAT dose distribution for prostate cancer with/without rectal gas removal and/or adaptive replanning

We investigated differences in the volumetric-modulated arc therapy (VMAT) dose distribution in prostate cancer patients treated by rectal gas removal and/or adaptive replanning. Cone-beam computed tomography (CBCT) scans were performed daily for 22 treatments in eight prostate cancer patients with excessive rectal gas, and the CBCT images were analyzed. Rectal gas removal was performed, and irradiation was delivered after prostate matching. We compared dose-volume histograms for the daily CBCT images before and after rectal gas removal. Plan A was the original plan on CBCT images before rectal gas removal. Plan B was a single reoptimized plan on CBCT images before rectal gas removal. Plan C was the original plan on CBCT images after rectal gas removal. Plan D was a single reoptimized plan on CBCT images after rectal gas removal. D₉₅ of the planning target volume (PTV) minus the rectum of Plan C (94.7% ± 6.6%) was significantly higher than that of Plan A (88.5% ± 10.4%). All dosimetric parameters of Plan C were improved by rectal gas removal compared with Plan A, regardless of the initial rectal gas volume. Dosimetric parameters of PTV minus the rectum of Plan B were significantly improved compared with Plan C. Additionally, the V₇₈ of the rectal wall of Plan B (0.2% ± 0.5%) was significantly improved compared with Plan C (3.9% ± 6.3%, p = 0.003). The dosimetric parameters of Plan D were not significantly different from Plan B. The dose distribution of prostate VMAT was improved by rectal gas removal and/or adaptive replanning. An adaptive replanning on daily CBCT images might be a better method than rectal gas removal for prostate cancer patients with excessive rectal gas.