Coplanar Fields Research Articles

Negative Margin Technique: A New Planning Strategy for Improving Dose Conformation in SBRT with Dynamic Conformal Arc Delivery

To introduce a new planning strategy for dose conformation improvement and evaluate its effectiveness in stereotactic body radiation therapy (SBRT) using dynamic conformal arc technique (DCAT). With gantry rotation, in the DCAT, the field aperture is continuously conformed to the planning target volume (PTV) with an “aperture margin” to compensate for penumbra effect. It is a common belief that the field aperture should be larger than or at least the same as the PTV to get adequate target coverage and most treatment planning systems do not allow aperture margins below zero. However, the radial penumbra in the arc plane may significantly decrease due to the continuous dose overlapping nature of the arc delivery. Thus, we hypothesize that if “negative margin (NM)” in the radial direction is allowed contrary to the common belief, it would improve the PTV dose conformation while reducing the surrounding normal tissue dose. To verify the hypothesis, plans were made using the NM technique (NMT) and compared with “zero margin (ZM)” plans for five actual lung cases representing 5 different situations; when the PTV was located [Case1] with enough distance from each organ at risk, [Case2] in a moderate distance from the spinal cord, [Case3] close to the spinal cord, [Case4] close to the skin, and [Case5] too laterally thus a full arc was not feasible due to collision. All plans met 95% PTV coverage with the prescription dose (12×4 Gy) and spared the spinal cord below the tolerance (volumes receiving 20.8 and 13.6 Gy were less than 0.35 and 1.2 cc, respectively). Two conformality indices, CI-100 [the ratio of the prescription isodose volume to the PTV] and CI-50 [the ratio of 50% prescription isodose volume to the PTV] were investigated. Between the ZM and NMT plans, the CI-100 values were reduced from 1.27, 1.18, 1.34, 1.38, and 1.30 to 1.16, 1.13, 1.19, 1.17, and 1.20 for the Case1, Case2, Case3, Case4, and Case5, respectively. The CI-50 values were also reduced from 4.67, 4.54, 5.11, 5.35, and 4.81 to 4.21, 4.40, 4.73, 5.21, and 4.49 for the Case1, Case2, Case3, Case4, and Case5, respectively. In overall, the amounts of reduction were 9.4 ± 4.1% and 5.9 ± 3.1% for the CI-100 and CI-50, respectively, representing significant increase in target dose conformation and decrease in normal tissue dose. The NMT turned out to be an effective planning strategy that could bring significant improvement of dose conformation in the SBRT. The NMT is expected to help planners achieve plan objectives with coplanar field(s) only, resulting in more accurate and efficient beam delivery. The NMT can be easily implemented in most clinics with no prerequisite.

The superiority of hybrid-volumetric arc therapy (VMAT) technique over double arcs VMAT and 3D-conformal technique in the treatment of locally advanced non-small cell lung cancer – A planning study

PurposeTo compare the dosimetric performance of three different treatment techniques – conformal radiotherapy (CRT), double arcs volumetric modulated arc therapy (RapidArc, RA) and Hybrid-RapidArc (H-RA) for locally-advanced non-small cell lung cancer (NSCLC). Material and methodsCRT, RA and H-RA plans were optimized for 24 stage III NSCLC patients. The target prescription dose was 60Gy. CRT consisted of 5–7 coplanar fields, while RA comprised of two 204o arcs. H-RA referred to two 204o arcs plus 2 static fields, which accounted for approximately half of the total dose. The plans were optimized to fulfill the departmental plan acceptance criteria. ResultsRA and H-RA yielded a 20% better conformity compared with CRT. Lung volume receiving >20Gy (V20) and mean lung dose (MLD) were the lowest in H-RA (V20 1.7% and 2.1% lower, MLD 0.59Gy and 0.41Gy lower than CRT & RA respectively) without jeopardizing the low-dose lung volume (V5). H-RA plans gave the lowest mean maximum spinal cord dose (34.4Gy, 3.9Gy<CRT and 2.2Gy<RA plans) and NTCP of lung. Higher average MU per fraction (addition 52.4MU) was observed with a reduced treatment time compared with CRT plans. ConclusionThe H-RA technique was superior in dosimetric outcomes for treating locally-advanced NSCLC compared to CRT and RA.

Normal liver tissue sparing by intensity-modulated proton stereotactic body radiotherapy for solitary liver tumours

Background. Stereotactic body radiotherapy (SBRT) is often the preferred treatment for the advanced liver tumours which owing to tumour distribution, size and multi-focality are out of range of surgical resection or radiofrequency ablation. However, only a minority of patients with liver tumours may be candidates for conventional SBRT because of the limited radiation tolerance of normal liver, intestine and other normal tissues. Due to the favourable depth-dose characteristics of protons, intensity-modulated proton therapy (IMPT) may be a superior alternative to photon-based SBRT. The purpose of this treatment planning study was therefore to investigate the potential sparing of normal liver by IMPT compared to photon-based intensity-modulated radiotherapy (IMRT) for solitary liver tumours. Material and methods. Ten patients with solitary liver metastasis treated at our institution with multi-field SBRT were retrospectively re-planned with IMRT and proton pencil beam scanning techniques. For the proton plans, two to three coplanar fields were used in contrast to five to six coplanar and non-coplanar photon fields. The same planning objectives were used for both techniques. A risk adapted dose prescription to the PTV surface of 12.5–16.75 Gy × 3 was used. Results. The spared liver volume for IMPT was higher compared to IMRT in all 10 patients. At the highest prescription dose level, the median liver volume receiving less than 15 Gy was 1411 cm3 for IMPT and 955 cm3 for IMRT (p < 0.005); also the mean liver dose was lower with IMPT compared to IMRT (median 9.1 Gy vs. 20.0 Gy; p < 0.005). All IMPT and IMRT plans met the VD < 15 Gy > 700 cm3 constraint. For the Dmean ≤ 15 Gy constraint, nine of 10 cases could be treated at the highest dose level using IMPT whereas with IMRT, only two cases met this constraint at the highest dose level and six at the lowest dose level. Conclusion. A considerable sparing of normal liver tissue can be obtained using proton-based SBRT for solitary liver tumours.

Experimental verification of IMPT treatment plans in an anthropomorphic phantom in the presence of delivery uncertainties

Clinically relevant intensity modulated proton therapy (IMPT) treatment plans were measured in a newly developed anthropomorphic phantom (i) to assess plan accuracy in the presence of high heterogeneity and (ii) to measure plan robustness in the case of treatment uncertainties (range and spatial). The new phantom consists of five different tissue substitute materials simulating different tissue types and was cut into sagittal planes so as to facilitate the verification of co-planar proton fields. GafChromic films were positioned in the different planes of the phantom, and 3D-IMPT and distal edge tracking (DET) plans were delivered to a volume simulating a skull base chordoma. In addition, treatments planned on CTs of the phantom with HU units modified were delivered to simulate systematic range uncertainties (range-error treatments). Finally, plans were delivered with the phantom rotated to simulate spatial errors. Results show excellent agreement between the calculated and the measured dose distribution: >99% and 98% of points with a gamma value <1 (3%/3 mm) for the 3D-IMPT and the DET plan, respectively. For both range and spatial errors, the 3D-IMPT plan was more robust than the DET plan. Both plans were more robust to range than to the spatial uncertainties. Finally, for range error treatments, measured distributions were compared to a model for predicting delivery errors in the treatment planning system. Good agreement has been found between the model and the measurements for both types of IMPT plan.

SU-E-T-557: Dose Calculation Accuracy Using Cone-Beam CT (CBCT) for Lung Stereotactic Body Radiotherapy (SBRT)

Purpose: To evaluate the dose calculation accuracy using Varian's cone‐beam CT(CBCT) for lung stereotactic body radiotherapy(SBRT). Methods: Treatment plans of 2 lungSBRT patients (1 and 2) were retrospectively studied. Both plans used 3 or 4 6MV coplanar IMRT fields, generated on an in‐house Treatment Planning System (TPS) using Pencil‐Beam (PB) dose calculation with radiological path‐length correction. CBCTimages were taken prior to each treatment and a total 7 sets of CBCTimages were exported into TPS. Each CBCTimages set was registered with the planning CT based on matching of PTV PTV. Patient contours from planning CT were also imported into CBCT and only external contours were adjusted accordingly. CT numbers (HU+1000) were compared at chosen spots between CBCT and planning CT. Treatment fields were copied to CBCT and CBCT plans were generated. Results: CBCT CT‐numbers agree with planning CT‐number at low CT‐number (< 1100) region. For CT‐number 99, 887 and 1049, CBCT shows 83±24, 836±48 and 1054±26. For high CT‐ number 1571 and 1981, the CBCT CT‐numbers are 1730±55 and 2248±174. Mean dose for CBCT plans are slightly higher than those in treatment plans, which are 101.8% and 100.8% vs 102.0±1.1% and 102.1±0.1% of prescribed dose. Average D95 in CBCT plans are closed to those in treatment plan: 100.2±0.2%, 100.2±0.8% in CBCT 99.3% and 100.8% in treatment plan. However target inhomogeneity is higher in the CBCT plans, with average PTV D05 of 104.1±0.5% and 103.9±1.4% compared to 102.4% and 103.0% respectively. Critical normal tissuedoses in CBCT plans were approximately the same as the acceptable values predicted by treatment plan. Conclusions: The dosimetric differences between CBCT and planing‐CT were within 2% of each other. CBCT may be used to patient radiation dose verification besides patient setup and target verification for SBRT patients.

SU-E-T-629: Dosimetric Effect of Different Commissioning Data on the SBRT of Small Lung Tumors

Purpose: To investigate the dosimetric effect of different linac commissioning data on the stereotactic body radiation therapy(SBRT) of small lungtumors. Methods: Small‐sized scanning ion chambers such as CC13 and CC04 (IBA Dosimetry) are commonly used to collect beam data for TPS commissioning. However, their volume‐averaging effect is non‐ negligible. Their dosimetric effect on the SBRT of lungtumors was investigated by commissioning the TPS (Pinnacle ver. 9.0) using CC13 and CC04 scanned beam data and compared with the beam model using a diode scanned beam data. Treatment plans were generated for 6 SBRT of lungtumors using each beam model, with a dose grid of 4×4×4 mm3. An additional plan for each case was generated using the diode model with a dose grid of 2×2×2 mm3 and served as the standard. Six to eight conformal coplanar fields using 6 MV photon beams were used for all the plans. PTV volumes ranged from 15.7 cc to 86.1 cc, with the average dimensions ranged from 2.2 cm to 5.2 cm. Important dosimetric parameters, including target coverage, conformal index, and normal structure sparing, were evaluated. Differences of more than 5% were considered significant. Results: Significant differences were seen among plans with different beam models. Differences in PTV coverage of up to 10% were seen, and differences in conformal indices were up to 23%. Significant differences were also seen in normal organ sparing, including the lung and chest wall. In general, the smaller the PTV was, the larger the difference. Dose grid of 4×4×4 mm3 was found to be adequate, resulting in less than 3% difference in most cases when compared with the standard. Conclusions: For SBRT of lungtumors, it is important to minimize the volume‐averaging effect of the beam data used for TPS commissioning to ensure consistent results.

Potential for dose escalation in the postprostatectomy setting with intensity-modulated radiation therapy: a dosimetric study using EORTC consensus guidelines for target volume contours

PurposeRadiation therapy (RT) is delivered as adjuvant and salvage therapy after radical prostatectomy for prostate cancer. Interest in dose escalation in this setting may necessitate more advanced RT techniques, such as intensity modulation. This study was designed to compare intensity-modulated radiation therapy (IMRT) versus 3-dimensional conformal radiation therapy (3DCRT) planning. MethodsTwenty patients were identified who received post--radical prostatectomy RT with 4-field, 3DCRT plans to 68.4 Gy. Contours were revised to comply with the European Organization for Research and Treatment of Cancer consensus guidelines. The IMRT plans with 5 versus 9 coplanar fields were compared for 10 patients. Then the 9-field IMRT plans were compared to 3DCRT in all 20 patients. Differences in dose-volume histogram values were evaluated using 2-tailed paired t tests. Cone beam computed tomographic images were analyzed to evaluate rectum doses in the treatment position during the RT course. The IMRT plans to 72.0 Gy were compared to 3DCRT to 68.4 Gy to be able to consider potential use in dose escalation. ResultsThe 9-field IMRT plans (vs 3DCRT) improved bladder mean dose and volume receiving 65 Gy or more (V65), as well as rectum mean dose (31.6 Gy vs 36.1 Gy; P < .001), volume receiving 75% or more of the prescription dose (24.4% vs 31.0%; P < .001), and V65 (10.5% vs 20.0%; P < .001). Advantages of IMRT were at the cost of small increases in maximum point doses delivered to the bladder and rectum. Cone beam computed tomographic images (n = 132) were analyzed for 8 patients; rectum mean dose and V65 were also improved by IMRT on these scans. IMRT allowed increasing dose to 72.0 Gy with similar bladder and rectum mean doses, V65, and V40 compared to 3DCRT to a total dose of 68.4 Gy. ConclusionsThe IMRT improves dosimetric parameters for the rectum and bladder, which may allow dose escalation after radical prostatectomy. Future studies should determine whether these advantages translate into improved clinical outcomes for prostate cancer patients.

Bone marrow-sparing intensity-modulated radiation therapy for Stage I seminoma

Background. A direct association between radiotherapy dose, side-effects and secondary cancers has been described in patients with seminoma. A treatment planning study was performed in order to compare computed tomography-based traditional radiotherapy (CT-tRT) versus bone marrow-sparing intensity-modulated radiation therapy (BMS-IMRT) in patients with Stage I seminoma. Material and methods. We optimized in 10 patients a CT-tRT and a BMS-IMRT treatment plan to deliver 20 Gy to the para-aortic nodes. CT-tRT and IMRT consisted of anteroposterior-posterioranterior parallel-opposed and seven non-opposed coplanar fields using 16 and 6-MV photon energies, respectively. Dose-Volume Histograms for clinical target volume (CTV), planning target volume (PTV) and organs at risk (OARs) were compared for both techniques using Wilcoxon matched-pair signed rank-test. Results. Dmean to CTV and PTV were similar for both techniques, even if CT-tRT showed a slightly improved target coverage in terms of PTV-D95% (19.7 vs. 19.5 Gy, p = 0.005) and PTV-V95% (100 vs. 99.7%, p = 0.011) compared to BMS-IMRT. BMS-IMRT resulted in a significant reduction (5.2 Gy, p = 0.005) in the Dmean to the active bone marrow (ABM). The V100% and V75% of the OARs were reduced with BMS-IMRT by: ABM-V100% = 51.7% and ABM-V75% = 42.3%; bowel-V100% = 15.7% and bowel-V75% = 16.8%; stomach-V100% = 22% and stomach-V75% = 27.7%; pancreas-V100% = 37.1% and pancreas-V75% = 35.9% (p = 0.005 for all variables). Conclusions. BMS-IMRT reduces markedly the dose to the OARs compared to CT-tRT. This should translate into a reduction in acute and long-term toxicity, as well as into the risk of secondary solid and hematological cancers.

Combined intensity-modulated radiation therapy vs. three-dimensional highly conformal radiotherapy after 125I prostate permanent seed brachytherapy: A comparative treatment planning study

Purpose A comparative treatment planning study was conducted to analyze combined intensity-modulated radiation therapy (IMRT) vs. three-dimensional highly conformal radiotherapy (3D-hCRT) after 125I permanent seed brachytherapy in patients with localized prostate cancer. Methods and Materials We optimized an IMRT and a 3D-hCRT treatment plan to 45 Gy in 9 patients 2 months after 125I seed brachytherapy (110 Gy) implant. IMRT and 3D-hCRT were planned using eight and seven nonopposed coplanar fields of 6- and 23-MV. Dose–volume histograms for target volume and organs at risk were compared for both techniques. Results IMRT provided similar conformality but better homogeneity ( p = 0.021) than 3D-hCRT plans, despite slightly higher maximal point dose rates (101.9% vs. 100.9%, p = 0.050). 3D-hCRT plans required fewer monitor units than did IMRT (289 vs. 607, p = 0.008). Dose to bladder, penile bulb, and femoral heads were lower with IMRT than with 3D-hCRT ( p = 0.008–0.028). No differences were observed for dose to urethra and the entire rectum. Mean dose to rectal wall was lower with IMRT than with 3D-hCRT (25.1 vs. 26.5 Gy, p = 0.028). Conclusion IMRT plans result in more homogeneous dose distribution and in reduced doses to most organs at risk at the expense of needing more than twice the monitor units compared with 3D-hCRT.

Conformal Radiotherapy in the Treatment of Advanced Juvenile Nasopharyngeal Angiofibroma With Intracranial Extension: An Institutional Experience

To describe the results of conformal radiotherapy in advanced juvenile nasopharyngeal angiofibroma in a tertiary care institution. Retrospective chart review was conducted for 8 patients treated with conformal radiotherapy between 2006 and 2009. The median follow-up was 17 months. All patients had Stage IIIB disease with intracranial extension. Radiotherapy was considered as treatment because patients were deemed inoperable owing to extensive intracranial/intraorbital extension or proximity to optic nerve. All but 1 patient were treated with intensity-modulated radiotherapy using seven coplanar fields. Median (range) dose prescribed was 39.6 (30-46) Gy. Actuarial analysis of local control and descriptive analysis of toxicity profile was conducted. Despite the large and complex target volume (median planning target volume, 292 cm(3)), intensity-modulated radiotherapy achieved conformal dose distributions (median van't Reit index, 0.66). Significant sparing of the surrounding organs at risk was obtained. No significant Grade 3/4 toxicities were experienced during or after treatment. Actual local control at 2 years was 87.5%. One patient died 1 month after radiotherapy secondary to massive epistaxis. The remaining 7 patients had progressive resolution of disease and were symptom-free at last follow-up. Persistent rhinitis was the only significant toxicity, seen in 1 patient. Conformal radiotherapy results in good local control with minimal acute and late side effects in juvenile nasopharyngeal angiofibromas, even in the presence of advanced disease.

Clinical Application of Aperture-based VMAT with Ring-shaped Region of Interest for Planning in Localized Prostate Cancer

Clinical Application of Aperture-based VMAT with Ring-shaped Region of Interest for Planning in Localized Prostate Cancer

A Co-Planar Type Lateral Field Emission Device by Carbon Nanotubes

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Primary application of active breathing control system in conformal radiotherapy for patients with non-small cell lung cancer

Objective To evaluate the feasibility of active breathing control (ABC) in conformal radiotherapy (CRT) for patients with non-small cell lung cancer (NSCLC). Methods From Feb 2005 to Mar 2008, 29 patients with inoperable NSCLC (stage Ⅱ-Ⅳ) were evaluated. For each patient, two series of CT scans were obtained with free breathing (FB) and ABC system during simulation, respectively. Then two confonnal radiotherapy (CRT) plans were finished based on the two sets of reconstructed images. The pattern of post-inspiratory breath-hold was triggered at 80% of the peak of inspiration curve. The margin of clinical target volume (CTV) to planning target volume (PTV) was 0. 6 cm for lesions of the superior lobe, and 1.0 cm for the lesions of middle and inferior lobes. Three to five coplanar fields were performed in conformal radiotherapy. The gross tumor volume (GTV), CTV, PTV, volume of the bilateral lungs (Volume_(lung)), V_(20) and mean lung dose (MLD) of two plans were evaluated by dose-volume histogram (DVH). The World Health Organization criteria and National Cancer Institute Common Toxicity Criteria 3.0 (NCI-CTC3.0) scale were used to assess the immediate response and acute side-effect, respectively. Results Significant differences of GTV, CTV, FIN, Volum_(lung), V_(20) and MDL were observed between the two plans (36. 35 cm~3 vs. 31.40 cm~3, t = 9. 70, P <0. 001 ;82. 33 cm~3 vs. 70. 83 cm~3, t = 8. 19, P < 0. 001 ; 230. 73 cm~3 vs. 197.59 cm~3 ,t=5.72,P <0. 001 ;21.66% vs. 18. 76% ,t = 11.16,P <0. 001 ;1329. 07 Gy vs. 1143. 14 Gy, t = 13. 24, P < 0. 001). With ABC, all patients completed their treatment successfully except one patient for financial problems. The median radiation dose to the GTV was 64 Gy (60 -64 Gy). The overall immediate response rate was 64% (18/28). According to the NCI-CTC 3.0, grade 1 and 2 acute radiation-related toxicities occurred in 68% (19/28) and 18% (5/28) of patients for esophagitis, 82% (23/28) and 7% (2/28) for pneumonitis, respectively. Grade 1, 2 and 3 bone marrow suppression occurred in 57% (16/28), 25% (7/28) and 14% (4/28) of patients, respectively. Grade 1 and 2 acute cardiac injuries occurred in 86% (24/28) and 14% (4/28) of patients. Conclusions During CRT for patients with NSCLC, the use of ABC can decrease the radiation dose and acute complications of normal tissues. Key words: Carcinoma, non-small cell lung/radiotherapy; Radiotherapy, conformal; Active breathing control

A treatment planning comparison between proton beam therapy and intensity-modulated x-ray therapy for recurrent nasopharyngeal carcinoma

A comparative treatment planning study was undertaken between proton beam therapy (PBT) and intensity-modulated x-ray therapy (IMXT) for patients with recurrent nasopharyngeal carcinoma (NPC), to assess the potential benefits and limitations of the two treatment modalities. We retrospectively analyzed seven patients with recurrent NPC treated at our proton center with PBT from 2004 to 2007. Eclipse proton treatment planning system and Eclipse inverse treatment planning system for IMXT were employed. For each patient, the IMXT plan was optimized with nine evenly spaced coplanar fields. Three coplanar beams and passive scattering mode were used for PBT. The dose prescription in cobalt Gray equivalent (CGE) for gross tumor volume (GTV) was 66CGE and for planning target volume (PTV), 62.7CGE. Dose-volume histograms (DVH) were used to evaluate the difference in dosimetric distributions for the target volume and the organs at risk (OARs). Optimal target volume coverage and similar target conformation were achieved in both PBT and IMXT. Median conformity index was 0.72 and 0.75 (p=0.15) and median inhomogeneity coefficient was 0.14 and 0.10 (p=0.08) for PBT and IMXT, respectively. Dose to OARs was significantly lower in PBT plans than IMXT. Median maximal dose to the brainstem was 27.89CGE(cobalt Gray equivalent) and 42.45Gy (p< 0.01), and the dose to 5% of the brainstem(D5) was 12.83CGE and 19.47Gy (p< 0.001 ), for PBT and IMXT, respectively. Median maximal dose to the spinal cord was 8.38CGE and 22.91Gy (p< 0.004), and the dose to 5% of the spinal cord was 2.18CGE and 13.62Gy (p< 0.001), for PBT and IMXT, respectively. The use of PBT, when compared with IMXT, resulted in similar levels of tumor conformation. PBT, however, exposed the OARs to a significantly lower dose, effectively sparing the brainstem, spinal cord, optic nerve and chiasm, temporal lobes and parotid glands. The superior dose distributions possible with PBT should translate into reduced morbidity and improved quality of life.

Unilateral radiotherapy for tonsil cancer: Potential dose distribution optimization with a simple two-field intensity-modulated radiation therapy beam arrangement

Background and purpose To evaluate the feasibility and dosimetric optimization potential of a unilateral two-field intensity-modulated radiotherapy (IMRT) technique in the curative treatment of lateralized tonsil cancer. Materials and methods Six patients with lateralized tonsillar carcinoma were treated unilaterally with a two-field IMRT technique (oblique-anterior and oblique-posterior fields, with or without collimator and couch rotation). Alternative IMRT plans using seven non-opposed coplanar fields were compared with the two-field plans for each patient. Results Planning target volume (PTV) coverage was excellent with the two-field technique, using a relatively low number of monitor units (MU) (median, 441; range, 309–550). Dose constraints were respected for all organs at risk (OAR). Mean doses to contralateral parotid and submandibular glands were 3.9 and 17.7 Gy, respectively. Seven-field IMRT provided similar PTV coverage, with statistically significant better dose homogeneity and conformality. However, the mean delivered dose to the contralateral parotid (3.9 vs. 9.0 Gy, p = 0.001) as well as the mean number of MU (437 vs. 814, p = 0.002) and consequently machine time were lower with two-field IMRT. Conclusions Unilateral two-field IMRT is a simple and feasible technique providing excellent tumor coverage and optimal OAR sparing while reducing the number of MU and treatment time.

Dosimetric Comparison of Split Field and Fixed Jaw Techniques for Large IMRT Target Volumes in the Head and Neck

Some treatment planning systems (TPSs), when used for large-field (>14 cm) intensity-modulated radiation therapy (IMRT), create split fields that produce excessive multiple-leaf collimator segments, match-line dose inhomogeneity, and higher treatment times than nonsplit fields. A new method using a fixed-jaw technique (FJT) forces the jaw to stay at a fixed position during optimization and is proposed to reduce problems associated with split fields. Dosimetric comparisons between split-field technique (SFT) and FJT used for IMRT treatment is presented. Five patients with head and neck malignancies and regional target volumes were studied and compared with both techniques. Treatment planning was performed on an Eclipse TPS using beam data generated for Varian 2100C linear accelerator. A standard beam arrangement consisting of nine coplanar fields, equally spaced, was used in both techniques. Institutional dose-volume constraints used in head and neck cancer were kept the same for both techniques. The dosimetric coverage for the target volumes between SFT and FJT for head and neck IMRT plan is identical within ±1% up to 90% dose. Similarly, the organs at risk (OARs) have dose-volume coverage nearly identical for all patients. When the total monitor unit (MU) and segments were analyzed, SFT produces statistically significant higher segments (17.3 ± 6.3%) and higher MU (13.7 ± 4.4%) than the FJT. There is no match line in FJT and hence dose uniformity in the target volume is superior to the SFT. Dosimetrically, SFT and FJT are similar for dose-volume coverage; however, the FJT method provides better logistics, lower MU, shorter treatment time, and better dose uniformity. The number of segments and MU also has been correlated with the whole body radiation dose with long-term complications. Thus, FJT should be the preferred option over SFT for large target volumes.

Dosimetric Comparison between 3D-Conformal Radiotherapy and Intensity-modulated Radiation Therapy (IMRT) in Stereotactic Body Radiotherapy

Dosimetric Comparison between 3D-Conformal Radiotherapy and Intensity-modulated Radiation Therapy (IMRT) in Stereotactic Body Radiotherapy

7051 Clinical predictors of late gastrointestinal and genitourinary toxicity after three-dimensional conformal radiotherapy using seven coplanar fields to localize prostate cancer

It has become apparent of late that even in tamoxifen and/or aromatase resistant breast cancers, ERα remains a bona fide therapeutic target. Not surprisingly, therefore, there has been considerable interest in developing Selective ER Degraders (SERDs), compounds that target the receptor for degradation. Currently, ICI 182,780 (ICI, fulvestrant) is the only SERD approved for the treatment of breast cancer. However, the poor pharmaceutical properties of this injectable drug and its lack of superiority over second line aromatase inhibitors in late stage breast cancer have negatively impacted its clinical use. These findings have provided the impetus to develop second generation, orally bioavailable SERDs with which quantitative turnover of ERα in tumors can be achieved. Interestingly however, the contribution of SERD activity to fulvestrant efficacy is unclear, making it difficult to define the characteristics desired of the next generation of ER antagonists. It is of significance therefore, that we have determined that the antagonist activity of ICI and its ability to induce ERα degradation are not coupled processes. Specifically, our results indicate that it is the ability of ICI to interact with ERα and to (a) competitively displace estradiol and (b) induce a conformational change in ER incompatible with transcriptional activation that are likely to be the most important pharmacological characteristics of this drug. Collectively, these data argue for a renewed emphasis on the development of high affinity, orally bioavailable pure antagonists and suggest that SERD activity though proven effective may not be required for ERα antagonism in breast cancer.

SU-FF-T-515: NCTP Of Bladder And Rectum : Does Simultaneous Integrated Boost IMRT Score Over Other Techniques

Purpose: The main objective of this study was to analyze the radiobiological effect of different treatment strategies in high risk adenocarcinoma prostate. Method and Materials: Ten cases of high risk adenocarcinoma prostate were selected for this dosimetric study. Four different treatment strategies used for treating prostate cancer were compared. Conventional four‐field box technique covering prostate and nodal volumes followed by three‐field conformal boost (3D+3DCRT), four‐field box technique followed by intensity modulated radiotherapy(IMRT) boost (3D+IMRT), IMRT followed by IMRT boost (IMRT+IMRT) and simultaneous integrated boost IMRT (SIBIMRT) were compared in terms of tumorcontrol probability (TCP) and normal tissue complication probability (NTCP). The dose prescription except for SIBIMRT was 45Gy in 25 fractions for the prostate and nodal volumes in the initial phase and 27Gy in 15 fractions to the prostate in the boost phase. For SIBIMRT, equivalent doses were calculated using biologically equivalent dose (BED) assuming α/β of 1.5Gy with dose prescription of 60.75Gy to GTV and 45Gy to CTV in 25 fractions. IMRT plans were made with 15MV equi‐spaced seven coplanar fields. NTCP was calculated using Lyman‐Kutcher‐Burman (LKB) model. Results: A NTCP of 10.7±0.99%, 8.36±0.66%, 6.72±0.85%,1.45±0.11% for bladder and 14.9±0.99%, 14.04±0.66%, 11.38±0.85%, 5.12±0.11% for rectum was seen with 3D+3DCRT, 3D+IMRT, IMRT+IMRT and SIB‐IMRT respectively. Conclusions: SIBIMRT had the least NTCP over all other strategies with reduced treatment time (3 weeks less). It should be the technique of choice for dose escalation in prostate carcinoma.

SU-FF-J-136: The Impact of Attenuation and Scatter Correction On the SPECT Guided Radiation Therapy for Lung Cancer Patients: Comparison of SPECT Weighted Mean Dose and Functional Lung Segmentation

Purpose: To investigate the impact of different types of image reconstruction and attenuation/scatter (A/S) correction on the calculation of dosimetric indices proposed to be used for Single Photon Emission Computed Tomography (SPECT) guided dose escalation in lung cancer patients. Methods and Materials: SPECT lung perfusion scans were obtained for nine lung cancer patients using 99mTc-macroaggregated albumin. Four image sets were reconstructed from each scan: one using a vendor provided ordered subsets expectation maximization (OSEM) algorithm, two quantitative SPECT reconstructions using OSEM methods with different types of A/S corrections and the fourth an OSEM reconstruction without any A/S correction. SPECT weighted mean dose (SWMD), dose function histogram, and functional lung volume have been calculated from dose distributions and regional perfusion maps. To investigate the dependence of SWMDs on gantry angle, twelve equally spaced co-planar open field radiation beams delivering the same MU were centered on the PTV. Three field sizes, 5×5, 7.5×7.5 and 10×10cm2 were considered. SWMDs were calculated for each field and reconstruction. Functional lung volumes were segmented in each reconstruction using 10, 20, …, 90% of maximum SPECT uptake as a threshold. Results: SWMDs calculated from reconstructions without A/S correction showed more than 5% average difference compared to those with corrections. With A/S corrections, more consistent SWMDs were found in all the three OSEM reconstructions (average difference ∼2%). However, a large variation was observed between segmented functional lung volumes and the V20 of these volumes in all four reconstructions. The difference between the volumes reached over 50% regardless of whether A/S correction was applied in the reconstruction. Conclusion: Functional volume segmentation is sensitive to the type of A/S correction. In contrast, SPECT weighted mean dose calculation produces more consistent results and appears to be a more robust choice for clinical outcome analysis and SPECT guided treatment planning.