Dose-volume Histograms Of Planning Target Volume Research Articles

A Risk-Adjusted Control Chart to Evaluate Intensity Modulated Radiation Therapy Plan Quality

PurposeThis study aimed to develop a quality control framework for intensity modulated radiation therapy plan evaluations that can account for variations in patient- and treatment-specific risk factors. Methods and MaterialsPatient-specific risk factors, such as a patient’s anatomy and tumor dose requirements, affect organs-at-risk (OARs) dose-volume histograms (DVHs), which in turn affects plan quality and can potentially cause adverse effects. Treatment-specific risk factors, such as the use of chemotherapy and surgery, are clinically relevant when evaluating radiation therapy planning criteria. A risk-adjusted control chart was developed to identify unusual plan quality after accounting for patient- and treatment-specific risk factors. In this proof of concept, 6 OAR DVH points and average monitor units serve as proxies for plan quality. Eighteen risk factors are considered for modeling quality: planning target volume (PTV) and OAR cross-sectional areas; volumes, spreads, and surface areas; minimum and centroid distances between OARs and the PTV; 6 PTV DVH points; use of chemotherapy; and surgery. A total of 69 head and neck cases were used to demonstrate the application of risk-adjusted control charts, and the results were compared with the application of conventional control charts. ResultsThe risk-adjusted control chart remains robust to interpatient variations in the studied risk factors, unlike the conventional control chart. For the brainstem, the conventional chart signaled 4 patients with unusual (out-of-control) doses to 2% brainstem volume. However, the adjusted chart did not signal any plans after accounting for their risk factors. For the spinal cord doses to 2% brainstem volume, the conventional chart signaled 2 patients, and the adjusted chart signaled a separate patient after accounting for their risk factors. Similar adjustments were observed for the other DVH points when evaluating brainstem, spinal cord, ipsilateral parotid, and average monitor units. The adjustments can be directly attributed to the patient- and treatment-specific risk factors. ConclusionsA risk-adjusted control chart was developed to evaluate plan quality, which is robust to variations in patient- and treatment-specific parameters.

Dosimetric Comparison of Pelvic Bone Marrow Preservation between Volumetric-Modulated Arc Therapy and Intensity-Modulated Radiotherapy in Radical Radiotherapy of High-Risk Prostate Cancer

Background: Pelvic bone marrow (PBM) preservation is one of the factors that should be taken into consideration while choosing a technique for radiotherapy of pelvic malignancies. Aim: To dosimetrically compare between volumetric-modulated arc therapy (VMAT) and intensity-modulated radiotherapy (IMRT) in PBM preservation in radical treatment of high-risk prostate cancer. Methods: In 26 patients with high-risk prostatic carcinoma, dual arc VMAT and 7 fields IMRT plans were generated. In every patient, two targets were defined, clinical target volume (CTV) including the prostate and seminal vesicles (CTV-PSV) and CTV including pelvic lymph nodes (CTV-LN). The organs at risk delineated were the rectum, urinary bladder, small intestine, bulb of the penis, femoral heads bilaterally and PBM. The dose prescribed to the CTV-PSV was 76 Gy in 38 fractions given over 7.5 weeks and the dose to CTV-LN was 54 Gy in 38 fractions given over 7.5 weeks. Planning target volume (PTV) was created from the CTV with a margin of 5 mm in all direction. For assessment of PBM dose, V10, V20, V30, V40, V50 and mean dose were calculated. The dose volume histogram of PTV and PBM for both techniques was compared. Results: The mean dose of PTV 54 Gy was achieved in both techniques adequately with better sparing of organs at risk with the VAMT technique. The mean dose for PBM in the VMAT technique was significantly less than that in the IMRT (21.7 Gy vs. 25.8 Gy, respectively; p < 0.001). The significant differences in PBM doses were in the range of 20 Gy to 40 Gy. Conclusion: In radical treatment of prostate cancer, VMAT technique can offer comparable conformality to IMRT with better PBM preservation. Awareness of PBM delineation and reduction of its doses using VMAT can help to decrease the hematological toxicity.

Impact of dose calculation algorithms on the dosimetric and radiobiological indices for lung stereotactic body radiotherapy (SBRT) plans calculated using LQ–L model

AbstractPurposeTo investigate discrepancies in dose calculation algorithms used for lung stereotactic body radiotherapy (SBRT) plans.Methods and materialsIn total, 30 patients lung SBRT treatment plans, initially generated using BrainLab Pencil Beam (BL_PB) algorithm for 10 Gy×5 Fractions to the planning target volume (PTV) were included in the study. These plans were recalculated using BrainLab Monte Carlo (BL_MC), Eclipse AAA (EC_AAA), Eclipse Acuros XB (EC_AXB) and ADAC Pinnacle CCC (AP_CCC) algorithms. Dose volume histograms of PTV were used to calculate dosimetric and radiobiological quality indices, and equivalent dose to 2 Gy per fraction using linear-quadratic-linear model. The BL_MC algorithm is considered gold standard tool to compare PTV parameters and quality indices to investigate dose calculation discrepancies of abovementioned plans.ResultsBL_PB overestimates doses that may be due to inability of the algorithm to properly account for electron scattering and transport in inhomogeneous medium. Compared with BL_MCNO plans, the EC_AAA and EC_AXB yield lower homogeneity indices and overestimate the dose in the penumbra region, whereas AP_CCC plans were comparable for small PTV (≈8 cc) and had significant difference for large PTV.ConclusionBL_PB algorithm overestimates PTV doses than BL_MC calculated doses. The EC_AAA, EC_AXB and AP_CCC algorithms calculate doses within acceptable limits of radiotherapy dose delivery recommendations.

Dosimetric comparison between the prostate intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans using the planning target volume (PTV) dose–volume factor

AbstractBackgroundWe demonstrated that our proposed planning target volume (PTV) dose–volume factor (PDVF) can be used to evaluate the PTV dose coverage between the intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) plans based on 90 prostate patients.PurposePDVF were determined from the prostate IMRT and VMAT plans to compare their variation of PTV dose coverage. Comparisons of the PDVF with other plan evaluation parameters such as D5%, D95%, D99%, Dmean, conformity index (CI), homogeneity index (HI), gradient index (GI) and prostate tumour control probability (TCP) were carried out.Methods and materialsProstate IMRT and VMAT plans using the 6 MV photon beams were created from 40 and 50 patients, respectively. Dosimetric indices (CI, HI and GI), dose–volume points (D5%, D95%, D99% and Dmean) and prostate TCP were calculated according to the PTV dose–volume histograms (DVHs) of the plans. All PTV DVH curves were fitted using the Gaussian error function (GEF) model. The PDVF were calculated based on the GEF parameters.ResultsFrom the PTV DVHs of the prostate IMRT and VMAT plans, the average D99% of the PTV for IMRT and VMAT were 74·1 and 74·5 Gy, respectively. The average prostate TCP were 0·956 and 0·958 for the IMRT and VMAT plans, respectively. The average PDVF of the IMRT and VMAT plans were 0·970 and 0·983, respectively. Although both the IMRT and VMAT plans showed very similar prostate TCP, the dosimetric and radiobiological results of the VMAT technique were slightly better than IMRT.ConclusionThe calculated PDVF for the prostate IMRT and VMAT plans agreed well with other dosimetric and radiobiological parameters in this study. PDVF was verified as an alternative of evaluation parameter in the quality assurance of prostate treatment planning.

SU‐E‐T‐642: PTV Is the Voxel‐Wise Worst‐Case of CTV in Prostate Photon Therapy

Purpose:To examine the adequacy of the planning target volume (PTV) dose distribution as the worst‐case representation of clinical target volume (CTV) dose distribution in prostate volumetric‐modulated arc therapy (VMAT) plans.Methods:Ten intact prostate cancer cases treated by VMAT at our institution were randomly selected. Isocenter was shifted in the three cardinal directions by a displacement equal to the PTV expansion on the CTV (±3 mm) for a total of six shifted plans per original plan. Rotationally‐perturbed plans were generated with a couch rotation of ±1° to simulate patient yaw. The eight perturbed dose distributions were recalculated in the treatment planning system using the same, fixed fluence map as the original plan. The voxel‐wise worst‐case CTV dose distribution was constructed from the minimum value per voxel from the eight perturbed doses. The resulting dose volume histograms (DVH) were evaluated for statistical correlation between the worst‐case CTV and nominal PTV dose distributions based on D95% by Wilcoxon signed‐rank test with significance level p ≤ 0.05.Results:Inspection demonstrates the PTV DVH in the nominal dose distribution is bounded by the CTV DVH in the worst‐case dose distribution. Comparison of D95% for the two dose distributions by Wilcoxon signed‐rank test gives p = 0.131. Therefore the null hypothesis cannot be rejected since the difference in median values is not statistically significant.Conclusion:The assumption that the nominal dose distribution for PTV represents the worst‐case dose distribution for CTV appears valid for the ten plans under examination. Although the worst‐case dose distribution is unphysical since the dose per voxel is chosen independently, it serves as a lower bound for the possible CTV coverage. Furthermore, this is consistent with the unphysical nature of the PTV. Minor discrepancies between the two dose distributions are expected since the dose cloud is not strictly static.Funding Support: NIH/NCI K25CA168984, Eagles Cancer Research Career Development, The Lawrence W. and Marilyn W. Matteson Fund for Cancer Research, Mayo ASU Seed Grant, and The Kemper Marley Foundation

Feasibility study of deep inspiration breath-hold based volumetric modulated arc therapy for locally advanced left sided breast cancer patients.

The purpose of this study was to assess the feasibility of deep inspiration breath-hold (DIBH) based volumetric modulated arc therapy (VMAT) for locally advanced left sided breast cancer patients undergoing radical mastectomy. DIBH immobilizes the tumor bed providing dosimetric benefits over free breathing (FB). Ten left sided post mastectomy patients were immobilized in a supine position with both the arms lifted above the head on a hemi-body vaclock. Two thermoplastic masks were prepared for each patient, one for normal free breathing and a second made with breath-hold to maintain reproducibility. DIBH CT scans were performed in the prospective mode of the Varian real time position management (RPM) system. The planning target volume (PTV) included the left chest wall and supraclavicular nodes and PTV prescription dose was 5000cGy in 25 fractions. DIBH-3DCRT planning was performed with the single iso-centre technique using a 6MV photon beam and the field-in-field technique. VMAT plans for FB and DIBH contained two partial arcs (179o-300oCCW/CW). Dose volume histograms of PTV and OAR's were analyzed for DIBH-VMAT, FB-VMAT and DIBH-3DCRT. In DIBH mode daily orthogonal (0o and 90o) KV images were taken to determine the setup variability and weekly twice CBCT to verify gating threshold level reproducibility. DIBH-VMAT reduced the lung and heart dose compared to FB-VMAT, while maintaining similar PTV coverage. The mean heart V30Gy was 2.3% ±2.7, 5.1% ±3.2 and 3.3% ±7.2 and for left lung V20Gy was 18.57% ±2.9, 21.7% ±3.9 and 23.5% ±5.1 for DIBH-VMAT, FB-VMAT and DIBH-3DCRT respectively. DIBH-VMAT significantly reduced the heart and lung dose for left side chest wall patients compared to FB-VMAT. PTV conformity index, homogeneity index, ipsilateral lung dose and heart dose were better for DIBH-VMAT compared to DIBH-3DCRT. However, contralateral lung and breast volumes exposed to low doses were increased with DIBH-VMAT.

Multi-institutional comparison of treatment planning using stereotactic ablative body radiotherapy for hepatocellular carcinoma – benchmark for a prospective multi-institutional study

IntroductionSeveral single institution phase I and phase II trials of stereotactic ablative body radiotherapy (SABR) for liver tumors have reported promising results and high local control rates of over 90%. However, there are wide variations in dose and fractionation due to different prescription policies and treatment methods across SABR series that have been published to date.This study aims to assess and minimize inter-institutional variations in treatment planning using SABR for hepatocellular carcinoma (HCC) in preparation for a prospective multi-institutional study.MethodsFour institutions (A-D) participated in this study. Each institution was provided with data from four cases, including planning and diagnostic CT images and clinical information, and asked to implement three plans (a practice plan and protocol plans 1 and 2). Practice plans were established based on the current treatment protocols at each institution. In protocol plan 1, each institution was instructed to prescribe 40 Gy in five fractions within 95% of the planning target volume (PTV). After protocol plan 1 was evaluated, we made protocol plan 2, The additional regulation to protocol plan 1 was that 40 Gy in five fractions was prescribed to a 70% isodose line of the global maximum dose within the PTV. Planning methods and dose volume histograms (DVHs) including the median PTV D50 (Dm50) and the median normal liver volume that received 20 Gy or higher (Vm20) were compared.ResultsIn the practice plan, Dm50 was 48.4 Gy (range, 43.6-51.2 Gy). Vm20 was 15.9% (range, 12.2-18.9%). In protocol plan 1, the Dm50 at institution A was higher (51.2 Gy) than the other institutions (42.0-42.2 Gy) due to differences in dose specifications. In protocol plan 2, variations in DVHs were reduced. The Dm50 was 51.9 Gy (range, 51.0-53.1 Gy), and the Vm20 was 12.3% (range, 10.4-13.2%). The homogeneity index was nearly equivalent at all institutions.ConclusionsThere were notable inter-institutional differences in practice planning using SABR to treat HCC. The range of PTV and normal liver DVH values was reduced when the dose was prescribed to an isodose line within the PTV. In multi-institutional studies, detailed dose specifications based on collaboration are necessary.

A Dosimetric Study of Inhomogeneous Dose Delivery: A Novel Approach to Dose Escalation

Materials/Methods: The study included volumetric modulated arc plans obtained with an Eclipse planning system from 11 localized prostatecancer cases. The clinical targetvolume (CTV) was the prostategland. The planningtargetvolume (PTV) was an expansion of CTV by 3 mm posteriorly and 4 mm in other directions. The prescribed dose (PD) to the PTV was 40 Gy (100%) to be delivered in 5 fractions with a RapidArc technique, similar to an ongoing SBRT protocol in our department. The intent in this exercise was to perform dose escalation within the PTV by increasing mean doses, rather than the minimum or peripheral doses. To create PTV dose inhomogeneity, a PTV sub-volume was defined to receive a higher dose. This high-dose volume (HDV) was created by shrinking the CTV by 5 mm and subtracting the urethral volume with a margin of 3 mm. The HDV was intended to receive at least 150% of the PD with no maximal dose limit. The mean PTV was 90 cc (range 54 to 127 cc). The HDV represented an average of 24% of the prostate PTV volume (range 15 to 38%). Rectal and bladder constraints were the same at: 50%\40 Gy, 20%\32 Gy, 12%\36 Gy, and 8%\40 Gy. Results: The profiles of the PTV dosevolume histograms resemble those achieved with high dose rate brachytherapy of the prostate. With respect to the target areas, the average PTV dose was 132% of the PD (range 110 to 140%). The HDV received an average of 155% of the PD (range 120 to 183%). The average of the HDV maximal doses was 195% of the PD (range 128.3 to 236.3%). Critical structure constraints were met in all cases. Overall, the mean rectal volume receiving 36 Gy was\5% (range 3 to 11%); the mean bladder volume receiving 36 Gy was\3% (range 1 to 12%) The urethral mean dose was 106% of the PD (range 91 to 128%). Conclusions:In this study, allowing dose inhomogeneity within the prostategland resulted in large dose escalation without compromiseofcriticalnormaltissues.Brachytherapytargetdose/volumeprofilescanbesafelyachievedwiththisnovelexternalbeam technique. Author Disclosure: N.G. Nickols: None. N. Agazaryan: None. C. King: None. P. Lee: None. M. Steinberg: None. P. Kupelian: None.

SU-GG-T-580: Parameterization of Dose Volume Histograms (DVHs) of Planning Target Volume (PTV) Among Patients Undergoing Radiation Therapy for Prostate Cancer

Purpose: To propose and validate a method to parameterize Dose Volume Histograms (DVHs) of the Planning Target Volume (PTV) for prostate cancer patients undergoing radiation therapy. Methods and Materials: There are generally two different formats of DVH: differential DVH (dDVH) and cumulative DVH (cDVH). Although cDVHs are more commonly used for radiotherapy treatment plan evaluation, dDVHs were used in our study to test the parameterization, as mathematically, the deferential format preserves more details than the cumulative/integral format. Prostate adenocarcinoma patients previously irradiated at our institution were selected for the study. For these patients, the total dose was 7400 cGy prescribed to cover 95%–98% of the PTV. The treatments were planned using a 7–9 field IMRT technique. It was found that the PTV dDVHs could be characterized by a normal function. Fifteen patients were selected and their PTV DVHs were fitted with a normal function. Parameters regularly used for the treatment plan evaluations, e.g. D95, mean dose and generalized mean dose (GMD), from the original DVHs were compared with those from fitted DVHs. Results: The fittings of the rectal DVHs yielded high goodness of fit with adjusted R-square greater than 0.97. The mode of the peak was 7575.4 ± 57.1 cGy. The standard deviation was 43.3 ± 10.5 cGy. For D95, mean dose and GMD, the relative differences between original and fitted DVHs were within 3%. Conclusions: DVHs could be represented by several parameters. The PTV DVH of patients undergoing irradiation to the prostate gland could successfully be parameterized using a normal function.

SU‐GG‐T‐572: Are Constant Dose Rate VMAT Plans Clinically Acceptable? — A Site‐By‐Site Plan Comparison Study

Purpose: To evaluate the adequacy of constant dose rate (CDR) VMAT plans for clinical use of commonly treated sites. The clinical acceptability of CDR VMAT plans was evaluated based on plan quality comparisons against clinically used IMRT plans. Methods: Several patient treatments planned using IMRT were considered for the study and replanned using Pinnacle 9.0 (SmartArc) treatment planning system. Several clinical sites were considered for this study — prostate and prostate bed (4 patients), pelvis (1), breast (1) and head and neck (H&amp;N) (2). The quality of CDR plans normalized to PTV mean dose was compared against static field IMRT plans based on dose conformality, planning target volume (PTV) coverage and dose volume histograms (DVHs) of organs at risk (OAR). Results: Plan qualities of prostate, prostate bed and pelvis plans were evaluated based on rectum and bladder DVHs, PTV coverage and dose conformality. Prostate and prostate bed: CDR plan was found to be clinically acceptable with better OAR sparing and delivery time. Pelvis: CDR plan was lacking coverage and conformality and didn't have a significant advantage over the IMRT plan. Breast: Plan quality was evaluated based on PTV coverage, isodose conformality and DVHs generated for OARs — lungs, heart and cord. The CDR plan was clinically unacceptable, primarily due to low doses (&lt;10Gy) covering a large volume of the lungs. H&amp;N plans were evaluated based on PTV coverage, dose conformality and dose to parotids, cord and submandibular glands. The CDR plans were clinically unacceptable and didn't show significant advantage over IMRT plans beyond the reduced treatment time. Conclusion: The CDR plans for prostate and prostate bed were clinically acceptable and found to be equivalent, if not superior to IMRT plans. For the other sites investigated ‐ pelvis, breast, and H&amp;N, static field IMRT plans proved to be superior to CDR plans.

Dose distribution in 3-dimensional conformal radiotherapy for prostate cancer: Comparison of two treatment techniques (six coplanar fields and two dynamic arcs)

Purpose To compare dose distribution for two techniques of 3-dimensional conformal radiotherapy (RT): 6-field technique (6F) and 2-dynamic arc therapy (2DA). Methods and materials Thirty nonmetastatic prostate cancer patients were included. In each patient, two treatment plans were prepared: with six coplanar fields (45°, 90°, 135°, 225°, 270°, 315°) and with two dynamic lateral 100°-wide arcs (40–140°, 220–320°). Dose–volume histograms (DVHs) were computed and mean area under curve (AUC) values were calculated for the DVHs of Planning Target Volume (PTV), rectum, urinary bladder and femoral heads. Doses given to 30% of rectum (DR 30), to 60% of rectum (DR 60), to 50% of bladder (DB 50), to 50% of femoral head (DF 50) and to 95% of PTV (DPTV 95) were reported as a percentage of the total dose. Results Mean DR 30 and DR 60 for 6F and 2DA were 75.8%, 51.5% and 72.2%, 37.2%, respectively. Mean DB 50 for 6F and 2DA were 68% and 64.2%, respectively. Mean right DF 50 for 6F and 2DA were 35.4% and 45.5%, respectively. Mean DPTV 95 for 6F and 2DA were 99% and 99.2%, respectively. Mean AUCs of DVHs of rectum and urinary bladder were significantly higher for 6F (this was more evident for small PTV and in the intermediate dose range). Mean AUC of DVHs of PTV and femoral heads were significantly higher for 2DA. Conclusions Both 6F and 2DA offer good dose distribution for PTV. 2DA allows for significantly better sparing of rectum and urinary bladder with slightly worse femoral head dose distribution. Further study is warranted in order to establish the clinical relevance of these differences.

SU-FF-T-241: Feasibility and Accuracy of Using Cone-Beam Computed Tomography (CBCT) Scans for Stereotactic Radiosurgery (SRS) Planning and Dose Verification

Purpose: To evaluate the feasibility and accuracy of using on‐board imager (OBI) cone‐beam computed tomography(CBCT) scans for stereotactic radiosurgery(SRS) planning and dose verification. Method and Materials: The simulation CT scan and OBI CBCT scan of a Rando head phantom were imported into a commercial SRStreatment planning system, BRW‐coordinate localized and co‐registered. A simulated clinical target volume (CTV), planning target volume (PTV) and critical organs were contoured on the simulation CT scan and transferred to the CBCT scan. A SRS treatment plan with 10 static conformal fields was created using the simulation CT volume. Beam parameters of this CT plan were also imported into a second treatment plan using the CBCT volume. Dose volume histogram (DVH), mean dose, monitor unit (MU), and equivalent depth of the CBCT plan were compared with those of the CT plan, with and without the inhomogeneity correction. Results: The DVH of CTV, PTV and critical organs for these two plans were indistinguishable without inhomogeneity correction. The MU and equivalent depth of the CBCT plan were on average 0.2 % (standard deviation (SD)=0.4%) higher and 0.3% (SD=1.3%) lower, respectively, than the CT plan. Although the DVH of these two plans were comparable with inhomogeneity correction, significant discrepancy was observed for the MU (mean=1.4% and SD=1.3%) and equivalent depth (mean=5.3% and SD=3.9%) due to the lower Hounsfield numbers of the CBCT scan. However, when the CT plan was applied to the CBCT volume, mean doses to PTV and critical organs were only slightly (1.2% and on average 1.0% with SD=3.2%, respectively) higher than those of the CT plan. Conclusion: The OBI CBCT scans can be used for SRS planning and dose verification. More accurate Hounsfield number calibration is needed for inhomogeneity correction.

Organ sparing by conformal avoidance intensity-modulated radiation therapy for anal cancer: Dosimetric evaluation of coverage of pelvis and inguinal/femoral nodes

To describe a novel and straightforward conformal avoidance intensity-modulated radiation therapy (IMRT) technique for coverage of pelvis and inguinal/femoral nodes and to compare the dosimetry of the new method with that of other traditional methods of radiation treatment. Data of 2 patients with anal cancer were used as example cases to illustrate details and advantages of conformal avoidance IMRT technique. Conventional photons with enface electrons design was created first, thereby providing "outermost boundaries" defined as planning target volume (PTV) for subsequent conformal avoidance IMRT design. Organs at risk (OARs), including femoral head and neck and external genitalia, were contoured as conformal avoidance structures. A step-and-shoot inverse IMRT planning was subsequently generated. For dosimetric comparison, a recently published technique by modified segmental boost was also generated. These treatment techniques were evaluated by dose-volume histogram (DVH) of PTV and OARs. Dose profiles at four different depths from each treatment planning were generated for comparison. The DVH of PTV showed that coverage of the PTV was comparable among three treatment techniques. Percent volume of PTV receiving more than 90% prescription dose was in the range 94-98% for the three treatment techniques, and all had only 0-2% of PTV receiving more than 110% of prescription dose. The DVH of OARs confirmed that both femoral head and neck and external genitalia could be spared well by conformal avoidance IMRT as compared with the other two techniques. Although greater inhomogeneity of dose distribution within the PTV was noted by conformal avoidance IMRT technique, as shown by dose profiles at four different depths, the maximum doses at different depths were less than 115%, which was comparable to those planned by modified segmental boost technique. Planning by photons and enface electrons technique, however, showed a greater dose variation up to 134% of the prescription dose at 1.5 cm depth along photon-electron match-line. To cover pelvis and inguinal/femoral nodes, conformal avoidance IMRT is technically simple to simulate, plan, and execute. Dosimetric study has demonstrated that it achieves comparable PTV coverage compared with other approaches while at the same time significantly sparing the surrounding OARs.

Organ sparing by conformal avoidance intensity modulated radiation therapy for anal cancer: Dosimetric evaluation of coverage of pelvis and inguinal/femoral nodes

4257 Background: To describe a novel and straightforward conformal avoidance IMRT technique for coverage of pelvis and inguinal/femoral nodes and to compare the dosimetry of the new method with that of other methods of radiation treatment. Methods: Data of 2 patients with anal cancer were used as example cases to illustrate details and advantages of conformal avoidance IMRT technique. Conventional photons with enface electrons design was created first, thereby providing “outermost boundaries” defined as planning target volume (PTV) for subsequent conformal avoidance IMRT design. Organs at risk (OARs) including femoral head and neck and external genitalia were contoured as conformal avoidance structures. A step-and-shoot inverse IMRT planning was generated. For dosimetric comparison, technique by modified segmental boost was also generated. These treatment techniques were evaluated by dose volume histogram (DVH) of PTV and OARs. Dose profiles at 4 different depths from each treatment planning were generated. Results: DVH of PTV shows that coverage of the PTV is comparable among 3 treatment techniques. Volume of PTV receiving more than 90% prescription dose are in the range of 94 to 98% for the 3 treatment techniques and all have only 0 to 2% of PTV receiving more than 110% prescription dose. DVH of OARs confirms that both femoral head and neck and external genitalia could be spared well by conformal avoidance IMRT. Although greater inhomogeneity of dose distribution within the PTV is noted by conformal avoidance IMRT as shown by dose profiles at 4 depths, the maximum doses at different depths are less than 115% which are comparable to the planning by modified segmental boost technique. Planning by photons and enface electrons technique however shows a greater dose variation up to 134% of the prescription dose at 1.5 cm depth along photon-electron match-line. Conclusions: To cover pelvis and inguinal/femoral nodes conformal avoidance IMRT is technically simple to simulate, plan and execute. Dosimetric study demonstrated that it achieves comparable PTV coverage compared with other approaches while at the same time significantly spares the surrounding OARs. No significant financial relationships to disclose.

Intensity-modulated radiation therapy as a means of reducing dose to bone marrow in gynecologic patients receiving whole pelvic radiation therapy

Purpose: Whole pelvic radiation therapy (WPRT) is commonly used in gynecologic malignancies. However, WPRT results in irradiation of a considerable volume of the surrounding normal tissues including small bowel and pelvic bone marrow (BM). While BM suppression is not typically significant following WPRT alone, it is common in patients who receive chemotherapy. Recent data have demonstrated that intensity-modulated WPRT (IM-WPRT) significantly reduces the volume of small bowel, bladder and rectum receiving the prescription dose in gynecology patients. The purpose of this study is to evaluate IM-WPRT as a means to reduce the volume of pelvic BM irradiated. Materials and methods: Ten women with cervical or endometrial cancer previously treated using IM-WPRT were selected for this analysis. Using the treatment planning computed tomography scan, the clinical target volume (CTV) was defined to encompass the gross tumor, parametrial tissues, uterus (if present) and the regional lymph nodes. The CTV was expanded by a 1 cm margin to form the planning target volume (PTV). The bladder, rectum, small bowel were delineated in each patient. BM in the iliac crests was also contoured since studies have shown that the majority of the pelvic BM is located in these sites. Two plans were created for each patient: a standard 4-field WPRT plan with apertures shaped to the PTV in each beam’s-eye view, and an IM-WPRT plan designed to conform dose to the PTV while minimizing the dose to the normal tissues including the BM. Dose volume histograms (DVHs) for the PTV, small bowel and BM were compared for each patient. Results: For each of the ten patients, IM-WPRT treatment plans demonstrated a significant reduction of the volume of the BM receiving greater than 30% of the prescription dose (13.5 Gy). The attached table shows the average volume of the BM in the iliac crests receiving the specified dose (or greater) for the 4-field WPRT and the BM-sparing IM-WPRT plans. On average, IM-WPRT resulted in a 50% reduction in the volume of BM irradiated to doses greater than 22.5 Gy (p < 0.001), and more than 60% reduction of the BM irradiated to doses greater than 31.5 Gy (p < 0.001). Furthermore, as expected, the BM-sparing IM-WPRT plans resulted in significant sparing of all other normal tissues that was comparable to the original IM-WPRT that did not specifically consider BM as a constraint. In all ten cases, the BM-sparing treatment plan did not result in any significant differences in the PTV and small bowel DVHs. Conclusion: IM-WPRT significantly reduces the volume of pelvic BM irradiated compared to conventional WPRT. In addition, BM-sparing IM-WPRT did not compromise the improvements previously seen in IM-WPRT treatment plans that did not consider BM. Currently patients are being treated with this BM-sparing technique to determine if the dosimetric improvements seen here translate into clinical benefits. Tabled 1Dose Gy (%)WPRT (% BM Volume)BM-Sparing IM-WPRT (% BM volume)p-value9.0 (20%)95.8%98.5%< 0.00113.5 (30%)92.3%90.4%0.3522.5 (50%)87.0%43.7%< 0.00131.5 (70%)52.8%25.9%< 0.00140.5 (90%)42.5%12.1%< 0.00145.0 (100%)33.0%4.5%< 0.001 Open table in a new tab