Dose Coverage Of Target Volume Research Articles

Dosimetric Evaluation of Volumetric-Modulated Arc Therapy (RapidArc) for Primary Leiomyosarcoma in the Spine

This study aims to investigate the suitability of volumetric-modulated arc therapy (VMAT) with RapidArc for primary leiomyosarcoma (LMS) in the spine, and present a new method to improve the target coverage and organs at risk (OAR) sparing. Five patients with LMS were retrospectively reviewed. The intensity-modulated radiotherapy (IMRT) with five coplanar beams (5b-IMRT) or seven coplanar beams (7b-IMRT), and VMAT using four quasi-quarter coplanar arcs (4q-VMAT) or two full coplanar arcs (2f-VMAT) were generated. Planning target volume (PTV) dose coverage, OAR dose sparing, conformity index (CI), and homogeneity index (HI) were evaluated. A hollow-cylinder model (HCM) was also used for feasible optimal beam arrangements. The mean doses to PTV were 95.2% ± 1.0%, 93.0% ± 1.0%, 97.9% ± 1.0% and 96.2% ± 1.5% for 4q-VMAT, 2f-VMAT, 5b-IMRT and 7b-IMRT respectively, while the mean maximum doses to spinal cord (SC) were 43.7 ± 0.9 Gy, 42.0 ± 0.8 Gy, 41.4 ± 1.2 Gy and 40.6 ± 1.4 Gy. Compared to 5b-IMRT, the mean doses delivered to kidneys decreased by about 35.1% (8.5 Gy), 2.5% (0.6 Gy) and 35.5% (8.6 Gy) for 4q-VMAT, 2f-VMAT, and 7b-IMRT, respectively. The CI proposed by Baltas et al. was twice as good with IMRT than with 4q-VMAT, and the numbers of monitor units were increased five- and threefold with 7b-IMRT and with 5b-IMRT compared to VMAT. The unexpected results we presented here show that VMAT technique can't achieve highly conformal treatment plans while maintaining SC sparing for LMS in the spine. An approach is proposed based on a hollow-cylinder model, but it is difficult to apply to clinical practice. In this case, VMAT is not superior to IMRT except for significant reduction in delivery time.

SU‐F‐19A‐03: Dosimetric Advantages in Critical Structure Dose Sparing by Using a Multichannel Cylinder in High Dose Rate Brachytherapy to Treat Vaginal Cuff Cancer

Purpose:The multichannel cylindrical vaginal applicator is a variation of traditional single channel cylindrical vaginal applicator. The multichannel applicator has additional peripheral channels that provide more flexibility in the planning process. The dosimetric advantage is to reduce dose to adjacent organ at risk (OAR) such as bladder and rectum while maintaining target coverage with the dose optimization from additional channels.Methods:Vaginal HDR brachytherapy plans are all CT based. CT images were acquired in 2 mm thickness to keep integrity of cylinder contouring. The CTV of 5mm Rind with prescribed treatment length was reconstructed from 5mm expansion of inserted cylinder. The goal was 95% of CTV covered by 95% of prescribed dose in both single channel planning (SCP)and multichannel planning (MCP) before proceeding any further optimization for dose reduction to critical structures with emphasis on D2cc and V2Gy .Results:This study demonstrated noticeable dose reduction to OAR was apparent in multichannel plans. The D2cc of the rectum and bladder were showing the reduced dose for multichannel versus single channel. The V2Gy of the rectum was 93.72% and 83.79% (p=0.007) for single channel and multichannel respectively (Figure 1 and Table 1). To assure adequate coverage to target while reducing the dose to the OAR without any compromise is the main goal in using multichannel vaginal applicator in HDR brachytherapy.Conclusion:Multichannel plans were optimized using anatomical based inverse optimization algorithm of inverse planning simulation annealing. The optimization solution of the algorithm was to improve the clinical target volume dose coverage while reducing the dose to critical organs such as bladder, rectum and bowels. The comparison between SCP and MCP demonstrated MCP is superior to SCP where the dwell positions were based on geometric array only. It concluded that MCP is preferable and is able to provide certain features superior to SCP.

Plan Dose Evaluation of Three Dimensional Conformal Radiotherapy Planning (3D-CRT) of Nasopharyngeal Carcinoma (NPC): Experience of a Tertiary Care University Hospital in Pakistan

Radiation therapy is the mainstay of treatment for nasopharyngeal carcinoma. Importance of tumor coverage and challenges posed by its unique and critical location are well evident. Therefore we aimed to evaluate our radiation treatment plan through dose volume histograms (DVHs) to find planning target volume (PTV) dose coverage and factors affecting it. This retrospective study covered 45 histologically proven nasopharyngeal cancer patients who were treated with definitive 3D-CRT and chemotherapy between Feb 2006 to March 2013 at the Department of Oncology, Section Radiation Oncology ,Aga Khan University Hospital, Karachi, Pakistan. DVH was evaluated to find numbers of shrinking field (phases), PTV volume in different phases and its coverage by the 95% isodose lines, along with influencing factors. There were 36 males (80%) and 9 females (20%) in the age range of 12-84 years. Stage IVA (46.7%) was the most common stage followed by stage III (31.1). Eighty six point six-percent received induction, 95.5% received concurrent and 22.2% received adjuvant chemotherapy. The prescribed median radiation dose was 70Gy to primary, 60Gy to clinically positive neck nodes and 50Gy to clinically negative neck regions. Mean dose to spinal cord was 44.2Gy and to optic chiasma was 52Gy. Thirty seven point eight-percent patients completed their treatment in three phases while 62.2% required four to five phases. Mean volume for PTV3 was 247.8 cm3 (50-644.3), PTV4 173.8 cm3 (26.5-345.1) and PTV5 119.6 cm3 (18.9-246.1) and PTV volume coverage by 95% isodose lines were 74.4%, 85.7% and 100% respectively. Advanced T stage, intracranial extension and tumor volume >200 cm3 were found to be important factors associated with decreased PTV coverage by 95% isodose line. 3D CRT results in adequate PTV dose coverage by 95% isodose line. However advanced T stage, intracranial extension and large target volume require more advanced techniques like IMRT for appropriate PTV coverage.

Analysis of esophageal‐sparing treatment plans for patients with high‐grade esophagitis

We retrospectively generated IMRT plans for 14 NSCLC patients who had experienced grade 2 or 3 esophagitis (CTCAE version 3.0). We generated 11‐beam and reduced esophagus dose plan types to compare changes in the volume and length of esophagus receiving doses of 50, 55, 60, 65, and 70 Gy. Changes in planning target volume (PTV) dose coverage were also compared. If necessary, plans were renormalized to restore 95% PTV coverage. The critical organ doses examined were mean lung dose, mean heart dose, and volume of spinal cord receiving 50 Gy. The effect of interfractional motion was determined by applying a three‐dimensional rigid shift to the dose grid. For the esophagus plan, the mean reduction in esophagus V50, V55, V60, V65, and V70 Gy was 2.8, 4.1, 5.9, 7.3, and 9.5 cm3, respectively, compared with the clinical plan. The mean reductions in LE50, LE55, LE60, LE65, and LE70 Gy were 2.0, 3.0, 3.8, 4.0, and 4.6 cm, respectively. The mean heart and lung dose decreased 3.0 Gy and 2.4 Gy, respectively. The mean decreases in 90% and 95% PTV coverage were 1.7 Gy and 2.8 Gy, respectively. The normalized plans’ mean reduction of esophagus V50, V55, V60, V65, and V70 Gy were 1.6, 2.0, 2.9, 3.9, and 5.5 cm3, respectively, compared with the clinical plans. The normalized plans’ mean reductions in LE50, LE55, LE60, LE65, and LE70 Gy were 4.9, 5.2, 5.4, 4.9, and 4.8 cm, respectively. The mean reduction in maximum esophagus dose with simulated interfractional motion was 3.0 Gy and 1.4 Gy for the clinical plan type and the esophagus plan type, respectively. In many cases, the esophagus dose can be greatly reduced while maintaining critical structure dose constraints. PTV coverage can be restored by increasing beam output, while still obtaining a dose reduction to the esophagus and maintaining dose constraints.PACS number: 87.53 Tf

Retrospective analysis of target volume dose coverage using CBCT acquired for accelerated hypo-fractionated radiation therapy (AHFRT) in lung cancer

Objective To assess target volume dose coverage using cone-beam computed tomography scans (CBCT) acquired for AHFRT. Materials and Methods Nine patients were investigated in this study. Dose prescription was 60 Gy to the PTV (ITV plus 5 mm margins) delivered in 3, 5 or 8 fractions with volumetric modulated arc therapy (VMAT), using 3–5 arcs, 6 MV energy beam and the Novalis Tx linac. Patients were supine on a cushion or a thoracic support. All but 2 patients had arms over the head. ExacTrac was used on patient's bony structures enabling fine 6D table position movements followed by CBCT for ITV 3D positioning. During treatment, Exactrac snap verifications monitored the patient. Twenty CBCT were randomly selected to re-calculate dose coverage to the target in patient's treatment position, having tested the feasibility on phantom (results presented separately). DVHs were analyzed and for 3 patients, session dose distribution reproducibility was assessed. Results Mean median dose percentage difference coverage (CTCBCT) PTV and ITV was 0.01 ± 2.41 and −0.08 ± 2.67 respectively. Mean minimum and maximum dose percentage difference coverage (CT-CBCT) of ITV was 0.415 ± 2.83 and −1.21 ± 2.62 respectively. One patient showed systematic differences larger than −4%, due to image CBCT problems and without this patient, previous reported values changed to 0.81 ± 1.46, 0.88 ± 1.36, 1.23 ± 2.11and −0.2 ± 0.93. Dose targets coverage reproducibility, means standard deviation, was less than 1.8%. Conclusions This preliminary study showed that good patients positioning leads to dose distribution reproducibility respect to simulation as verified using CBCT.

An investigation of rotational issues for rectal carcinoma treated with radiotherapy

AbstractPurposeTo quantify the amount of inter-fractional pitch for rectal carcinoma patients, to investigate the dosimetric impact of pitch on the target volume and critical structures and to determine a tolerance where no pitch correction is required.Materials and methodsDaily pre-treatment images of rectal carcinoma patients were analysed to determine the residual pitch compared with the computed tomography (CT) planning scan. The dosimetric impact of pelvic rotation was modelled. The dose coverage of the clinical target volume (CTV) and small bowel were evaluated using dose–volume histograms.ResultsPre-treatment images had a mean of 0·27° and standard deviation was 2·23°. The volume of CTV receiving 95% of the prescription dose altered by 0·1% when up to ±10° of pitch was simulated.ConclusionsNo clinically significant change in CTV coverage was found (when ±10° of pitch was simulated). A tolerance of ±10° of pitch has been implemented for rectal carcinoma patients treated with three-dimensional conformal radiotherapy in our institution, when daily pre-treatment imaging with a zero action threshold for translational shifts is used.

Prostate rotation detected from implanted markers can affect dose coverage and cannot be simply dismissed

With implanted markers, daily prostate displacements can be automatically detected with six degrees of freedom. The reported magnitudes of the rotations, however, are often greater than the typical range of a six‐degree treatment couch. The purpose of this study is to quantify geometric and dosimetric effects if the prostate rotations are not corrected (ROT_NC) and if they can be compensated with translational shifts (ROT_C). Forty‐three kilovoltage cone‐beam CTs (KV‐CBCT) with implanted markers from five patients were available for this retrospective study. On each KV‐CBCT, the prostate, bladder, and rectum were manually contoured by a physician. The prostate contours from the planning CT and CBCT were aligned manually to achieve the best overlaps. This contour registration served as the benchmark method for comparison with two marker registration methods: (a) using six degrees of freedom, but rotations were not corrected (ROT_NC); and (b) using three degrees of freedom while compensating rotations into the translational shifts (ROT_C). The center of mass distance (CMD) and overlap index (OI) were used to evaluate these two methods. The dosimetric effects were also analyzed by comparing the dose coverage of the prostate clinical target volume (CTV) in relation to the planning margins. According to our analysis, the detected rotations dominated in the left–right axis with systematic and random components of 4.6° and 4.1°, respectively. When the rotation angles were greater than 10°, the differences in CMD between the two registrations were greater than 5 mm in 85.7% of these fractions; when the rotation angles were greater than 6°, the differences of CMD were greater than 4 mm in 61.1% of these fractions. With 6 mm/4 mm posterior planning margins, the average difference between the dose to 99% (D99) of the prostate in CBCTs and the planning D99 of the prostate was −8.0±12.3% for the ROT_NC registration, and −3.6±9.0% for the ROT_C registration (p=0.01). When the planning margin decreased to 4 mm/2 mm posterior, the average difference in D99 of the prostate was −22.0±16.2% and −15.1±15.2% for the ROT_NC and ROT_C methods, respectively (p<0.05). In conclusion, prostate rotation cannot be simply dismissed, and the impact of the rotational errors depends on the distance between the isocenter and the centroid of implanted markers and the rotation angle.PACS number: 87.55

Feasibility of an online adaptive replanning method for cranial frameless intensity-modulated radiosurgery

To introduce an approach for online adaptive replanning (i.e., dose-guided radiosurgery) in frameless stereotactic radiosurgery, when a 6-dimensional (6D) robotic couch is not available in the linear accelerator (linac). Cranial radiosurgical treatments are planned in our department using intensity-modulated technique. Patients are immobilized using thermoplastic mask. A cone-beam computed tomography (CBCT) scan is acquired after the initial laser-based patient setup (CBCTsetup). The online adaptive replanning procedure we propose consists of a 6D registration-based mapping of the reference plan onto actual CBCTsetup, followed by a reoptimization of the beam fluences (“6D plan”) to achieve similar dosage as originally was intended, while the patient is lying in the linac couch and the original beam arrangement is kept. The goodness of the online adaptive method proposed was retrospectively analyzed for 16 patients with 35 targets treated with CBCT-based frameless intensity modulated technique. Simulation of reference plan onto actual CBCTsetup, according to the 4 degrees of freedom, supported by linac couch was also generated for each case (4D plan). Target coverage (D99%) and conformity index values of 6D and 4D plans were compared with the corresponding values of the reference plans. Although the 4D-based approach does not always assure the target coverage (D99% between 72% and 103%), the proposed online adaptive method gave a perfect coverage in all cases analyzed as well as a similar conformity index value as was planned. Dose-guided radiosurgery approach is effective to assure the dose coverage and conformity of an intracranial target volume, avoiding resetting the patient inside the mask in a “trial and error” way so as to remove the pitch and roll errors when a robotic table is not available.

Utilization of cone-beam CT for offline evaluation of target volume coverage during prostate image-guided radiotherapy based on bony anatomy alignment

AimTo assess target volume coverage during prostate image-guided radiotherapy based on bony anatomy alignment and to assess possibility of safety margin reduction. BackgroundImplementation of IGRT should influence safety margins. Utilization of cone-beam CT provides current 3D anatomic information directly in irradiation position. Such information enables reconstruction of the actual dose distribution. Materials and methodsSeventeen prostate patients were treated with daily bony anatomy image-guidance. Cone-beam CT (CBCT) scans were acquired once a week immediately after bony anatomy alignment. After the prostate, seminal vesicles, rectum and bladder were contoured, the delivered dose distribution was reconstructed. Target dose coverage was evaluated by the proportion of the CTV encompassed by the 95% isodose. Original plans employed a 1cm safety margin. Alternative plans assuming a smaller 7mm margin between CTV and PTV were evaluated in the same way. Rectal and bladder volumes were compared with the initial ones. Rectal and bladder volumes irradiated with doses higher than 75Gy, 70Gy, 60Gy, 50Gy and 40Gy were analyzed. ResultsIn 12% of reconstructed plans the prostate coverage was not sufficient. The prostate underdosage was observed in 5 patients. Coverage of seminal vesicles was not satisfactory in 3% of plans. Most of the target underdosage corresponded to excessive rectal or bladder filling. Evaluation of alternative plans assuming a smaller 7mm margin revealed 22% and 11% of plans where prostate and seminal vesicles coverage, respectively, was compromised. These were distributed over 8 and 7 patients, respectively. ConclusionSufficient dose coverage of target volumes was not achieved for all patients. Reducing of safety margin is not acceptable. Initial rectal and bladder volumes cannot be considered representative for subsequent treatment.

Optimization and quality assurance of an image-guided radiation therapy system for intensity-modulated radiation therapy radiotherapy

To develop a quality assurance (QA) of XVI cone beam system (XVIcbs) for its optimal imaging-guided radiotherapy (IGRT) implementation, and to construe prostate tumor margin required for intensity-modulated radiation therapy (IMRT) if IGRT is unavailable. XVIcbs spatial accuracy was explored with a humanoid phantom; isodose conformity to lesion target with a rice phantom housing a soap as target; image resolution with a diagnostic phantom; and exposure validation with a Radcal ion chamber. To optimize XVIcbs, rotation flexmap on coincidency between gantry rotational axis and that of XVI cone beam scan was investigated. Theoretic correlation to image quality of XVIcbs rotational axis stability was elaborately studied. Comprehensive QA of IGRT using XVIcbs has initially been explored and then implemented on our general IMRT treatments, and on special IMRT radiotherapies such as head and neck (H and N), stereotactic radiation therapy (SRT), stereotactic radiosurgery (SRS), and stereotactic body radiotherapy (SBRT). Fifteen examples of prostate setup accounted for 350 IGRT cone beam system were analyzed. IGRT accuracy results were in agreement ± 1 mm. Flexmap 0.25 mm met the manufacturer's specification. Films confirmed isodose coincidence with target (soap) via XVIcbs, otherwise not. Superficial doses were measured from 7.2–2.5 cGy for anatomic diameters 15–33 cm, respectively. Image quality was susceptible to rotational stability or patient movement. IGRT using XVIcbs on general IMRT treatments such as prostate, SRT, SRS, and SBRT for setup accuracy were verified; and subsequently coordinate shifts corrections were recorded. The 350 prostate IGRT coordinate shifts modeled to Gaussian distributions show central peaks deviated off the isocenter by 0.6 ± 3.0 mm, 0.5 ± 4.5 mm in the X(RL)- and Z(SI)-coordinates, respectively; and 2.0 ± 3.0 mm in the Y(AP)-coordinate as a result of belly and bladder capacity variations. Sixty-eight percent of confidence was within ± 4.5 mm coordinates shifting. IGRT using XVIcbs is critical to IMRT for prostate and H and N, especially SRT, SRS, and SBRT. To optimize this modality of IGRT, a vigilant QA program is indispensable. Prostate IGRT reveals treatment accuracy as subject to coordinates' adjustments; otherwise a 4.5-mm margin is required to allow for full dose coverage of the clinical target volume, notwithstanding toxicity to normal tissues.

Usefulness of 4D-CT for Radiation Treatment Planning of Gastric MZBCL/MALT

It is well known that significant variations in stomach size, shape, and respiratory motion lead to uncertainties in target localization during treatment for gastric lymphoma. In this study, the usefulness of 4D-CT for radiation planning of gastric MZBCL/MALT was evaluated. Treatment planning using 4DCT (plan A) and conventional planning with a uniform margin (plan B) were compared using dose volume histograms (DVH) of the planning target volume (PTV) and the organ at risk, as well as the dose coverage of the clinical target volume (CTV) assessed by weekly online cone beam CT (CBCT) during the treatment course. In addition, regarding the image quality of CBCT , the interobserver agreement for the delineated volume of the CTV on CBCT was analyzed. The mean PTV of plan A was significantly smaller than that of plan B (p = 0.008). The mean doses to the liver and heart in plan A were significantly lower than those in plan B (p = 0.02 and 0.03, respectively). The reductions of V(20) of each kidney in plan A compared with those in plan B were 4.8 ± 2.4% in the right kidney and 16.3 ± 10.4% in the left. There was no significant difference in the dose coverage of the CTV between the plans during the treatment course. The interobserver agreement for the volume of the CTV was moderate correlation. Treatment planning using 4DCT for gastric MZBCL/MALT was useful for effective and safe irradiation with minimizing exposure of the organ at risk.

Multiphase-computed tomography-based target volume definition in conventional fractionated radiotherapy of lung tumors: Dosimetric and reliable comparison with the technique using addition of generic margins

The aim of the present study was to compare radiotherapeutic plans based on internal target volume determined by between multiphase computed tomography and addition of a generic margin in lung tumors and to evaluate the reliability of ITV determined by multiphase computed tomography during conventional fractionated radiotherapy. The radiotherapeutic plans based on internal target volume determined by between multiphase computed tomography and addition of a generic margin in 10 patients with lung tumors were applied. The difference of two planning target volumes (PTV) and irradiated dose and volume of normal lung tissue were compared. Weekly new targets were delineated on repeated computed tomography scans, and weekly dose coverage of clinical target volume under two different treatment plans was evaluated. For all patients, PTV3CT volume based on multiphase computed tomography was significantly smaller than that of PTVcon based on addition of a generic margin (t = 6.831, P <0.001). The volume receiving more than 20 Gy in Plan3CT and Plancon was 16.7 ± 5.2% and 20.0 ± 5.2% (t = 7.565, P <0.001), the volume receiving more than 5 Gy was 36.6 ± 7.2% and 42.7 ± 6.4% (t = 7.459, P <0.001), and mean lung dose was 1037.5 ± 275.0 cGy and 1246.8 ± 271.0 cGy (t = 8.078, P <0.001), respectively. Both Plan3CT and Plancon provided a satisfactory clinical target volume coverage weekly during conventional fractionated radiotherapy for 6-7 weeks, and the ratio of the volume receiving the prescription dose was 1.03 ± 0.02 and 1.04 ± 0.02, respectively. The radiotherapeutic plan based on internal target volume determined by multiphase computed tomography can ensure weekly target coverage during conventional fractionated radiotherapy in lung tumors, and it is better than the plan based on the addition of generic internal target volume, which can effectively reduce normal lung tissue irradiation.

TH‐E‐BRB‐06: Clinical Implementation of Electron Monte Carlo for Breast Boost Radiation Therapy: A Retrospective Study to Improve Target Volume Dose Coverage

Purpose: To retrospectively compare and improve target volume dose coverage for breast boosts using eMC. Methods: In this study, 101 retrospective breast patient cases were planned and treated for an electron boost (ranging from 6Gy/3F to 16Gy/8F). All patients were planned using Varian Eclipse (v8.9). Dose contributions from tangents were not taken into account. All plans were based on CT scan datasets. Breast boost CTVs were delineated with adequate expansion for PTV and insert size for treatment. All plans were delivered with treatment intent of 90% coverage to the CTV. Actual treatment prescriptions were based on CAX percentage depth dose with no dose distributions. Without changing any parameters (eMC algorithm, contours and treatment), dose distributions were generated. Actual delivered treatment monitor units were entered into the plans and analyzed for CTV dose coverage. For inadequate CTV coverage, plans were either normalized manually or via the TPS. Boost volume coverage was compared for delivered MU and EMC dose distribution MU. Dose coverage for V90 (volume receiving 90% of the prescribed dose) was set to 98%. Dose to 90% of the CTV (D90) and difference in monitor units for both plans were compared. Results: Majority (83%) of patients had been treated with 9 or 12MeV. Data consisted of a random sample of 51 right and 49 left boosts with a mean CTV=10.04cm2. The D90 increased from a mean of 89.7% to 92.5% (p=0.000) and similarly V90 increased from a mean of 85.8% to 96.6% (p=0.000). Monitor Units between both plans increased by 3% for adequate coverage of the CTV. Verification of insert output factors using eMC were within ±0.5% of measured. Conclusions: This retrospective study indicates improvement in target coverage requires a small and significant increase in MU for breast boost treatment without changing planning parameters or technique.

Comparison of high–dose rate prostate brachytherapy dose distributions with iridium-192, ytterbium-169, and thulium-170 sources

Purpose Recent studies have identified that among different available radionuclides, the dose characteristics and shielding properties of ytterbium-169 ( 169Yb) and thulium-170 ( 170Tm) may suit high–dose rate (HDR) brachytherapy needs. The purpose of this work was to compare clinically optimized dose distributions using proposed 169Yb and 170Tm HDR sources with the clinical dose distribution from a standard microSelectron V2 HDR iridium-192 ( 192Ir) brachytherapy source (Nucletron B.V., Veenendaal, The Netherlands). Methods and materials CT-based treatment plans of 10 patients having prostate volumes ranging from 17 to 92 cm 3 were studied retrospectively. Clinical treatment of these patients involved 16 catheters and a microSelectron V2 HDR 192Ir source. All dose plans were generated with inverse planning simulated annealing optimization algorithm. Dose objectives used for the 192Ir radionuclide source were used for the other two radionuclides. The dose objective parameters were adjusted to obtain the same clinical target (prostate) volume coverage as the original 192Ir radionuclide plan. A complete set of dosimetric indices was used to compare the plans from different radionuclides. A pairwise statistical analysis was also performed. Results and conclusions All the dose distributions optimized with specific 192Ir, 169Yb, and 170Tm sources satisfied the standard clinical criteria for HDR prostate implants, such as those for the Radiation Therapy Oncology Group clinical trial 0321, for combined HDR and external beam treatment for prostate adenocarcinoma. For equivalent clinical target volume dose coverage, the specific 169Yb and 170Tm sources resulted in a statistically significant dose reduction to organs at risk compared with microSelectron V2 HDR 192Ir source. This study indicates that a 170Tm or 169Yb radionuclide source may be an alternative to the 192Ir radionuclide sources in HDR brachytherapy.

Is it possible to get adequate dose coverage of a clinical target volume (CTV) to account for the possible extra capsular extension in early-stage prostate cancer by low dose rate (LDR) brachytherapy?

98 Background: There is no consensus for the definition and evaluation of CTV in post-implant setting for LDR prostate brachytherapy to account for extra capsular extension in clinically localized prostate cancer. In this study, we defined a CTV and evaluated its dosimetry in the post implant CT/MR scans done at 1 month after the LDR brachytherapy procedure. Methods: The initial consecutive 71 patients who underwent LDR brachytherapy under a single physician at Princess Margaret Hospital from June 2009 to July 2010 were included in this retrospective study. On the post implant MRI, the CTV was created by adding 3mm uniform margins around the prostate but respecting the anatomical boundaries like bone, bladder and rectum. Post implant dosimetry was based on CT/MR fusion using the dosimetric parameters V80, V90, V100, V150, V200, D80, D90 and D100. Implants were qualified as optimal if their V100 was &gt;85% and D90 was &gt;90% for both prostate and CTV. Univariate analysis was performed to evaluate associations of factors with V100 and D90 for the CTV using Wilcoxon rank sum test and Fisher's exact test. Results: The mean (SD) prostate V100 and D90 were 95.5% (4.2) and 117% (10) respectively with only 1 patient having sub optimal implant (V100 &lt;85% and D90 &lt;90%). The mean (SD) V100 and D90 for the CTV were also acceptable at 90.6% (4.9) and 103% (9), respectively. Six patients had V100 &lt;85% and 7 patients had D90 &lt;90% for the CTV. On univariate analysis, edema and seed implantation technique correlated with sub-optimal implant for the CTV. The mean (SD) edema for patients with V100 &lt;85% was 18% (10) and with D90 &lt;90% was 15% (12). The corresponding values for the optimal implants both in terms of V100 and D90 were 3% (13). Patients implanted with exclusively loose seeds (15 patients only) had higher incidence of sub-optimal implants (26%) compared to patients who had strands on the antero-lateral margins (56 patients; 3.5%). Conclusions: In this study, adequate dose coverage of CTV was achieved in most patients with current technique but implants with optimal dosimetry to prostate still may have sub-optimal D90 and V100 for the CTV. No significant financial relationships to disclose.

Total Marrow Irradiation With RapidArc Volumetric Arc Therapy

To develop a volumetric arc therapy (VMAT)-total marrow irradiation (TMI) technique for patients with hematologic malignancies. VMAT planning was performed for 6 patients using RapidArc technology. The planning target volume consisted of all the bones in the body from the head to the mid-femur, excluding the extremities, except for the humerus, plus a 3.0-mm margin. The organs at risk included the lungs, heart, liver, kidneys, bowels, brain, eyes, and oral cavity. The VMAT-TMI technique consisted of three plans: the head and neck, the chest, and the pelvis, each with three 330° arcs. The plans were prescribed to ensure, at a minimum, 95% planning target volume dose coverage with the prescription dose (percentage of volume receiving dose of ≥12 Gy was 95%). The treatments were delivered and verified using MapCheck and ion chamber measurements. The VMAT-TMI technique reported in the present study provided comparable dose distributions with respect to the fixed gantry linear accelerator intensity-modulated TMI. RapidArc planning was less subjective and easier, and, most importantly, the delivery was more efficient. RapidArc reduced the treatment delivery time to approximately 18 min from 45 min with the fixed gantry linear accelerator intensity-modulated TMI. When the prescription dose coverage was reduced to 85% from 95% and the mandible and maxillary structures were not included in the planning target volume as reported in a tomotherapy study, a considerable organ at risk dose reduction of 4.2-51% was observed. The average median dose for the lungs and lenses was reduced to 5.6 Gy from 7.2 Gy and 2.4 Gy from 4.5 Gy, respectively. The RapidArc VMAT technique improved the treatment planning, dose conformality, and, most importantly, treatment delivery efficiency. The results from our study suggest that the RapidArc VMAT technology can be expected to facilitate the clinical transition of TMI.

Sci-Sat AM(1): Planning - 06: SBRT Lung: From Homogeneous to Heterogeneous Dose Calculation

A viable treatment option for inoperable early stage non-small cell lung cancer is stereotactic body radiation therapy (SBRT) using 4DCT to incorporate tumour motion in the planning stage, hypofractionation, and cone beam CT for image guidance on the treatment unit. At our cancer centre, lung SBRT treatments were originally planned with no heterogeneity correction and prescribed to a point and with a penumbra. Even though no correction was applied to these plans, the monitor units delivered were injected into a separate plan where heterogeneity corrections were applied thus giving a true sense of the delivered dose. The goal was to plan future patients using heterogeneity correction only and the question became whether the original dose prescription of 48 Gy to the dose point should be changed. Ten plans had heterogeneity correction applied using monitor units calculated from the homogeneous distributions (Group A), and ten patients had heterogeneity correction only (Group B); 3D conformai plans in both groups. A comparison was made in terms of target volume dose coverage to determine whether the target coverage moving forward is comparable to what was delivered previously. ITV received a minimum dose of 50.5±1.4 Gy and 48.7±0.9 Gy, Group A and B respectively. The PTV received a minimum of 95% of prescribed dose in Group A 45.8±1.6 Gy, by comparison Group B was lower 44.4±0.7 Gy (p < 0.02). Overall target volume coverage is decreased as we move from homogeneous to heterogeneous dose calculation and the prescription should be modified as we move forward.

SU-GG-T-486: Sparing of Lung Function Using Perfusion SPECT Guided IMRT Treatment Planning for Lung Cancer Patients

Purpose: To develop a method for improving functional lung sparing utilizing perfusion SPECT information and Monte Carlo (MC) dose calculation in IMRT optimization of lung cancer radiotherapy. Methods and Materials: 99mTc-macroaggregated albumin (MAA) SPECT scans were acquired before radiotherapy (RT) for five non-small cell lung cancer patients treated with RT (60Gy / 30 fx). Perfusion SPECT images were reconstructed with attenuation, scatter correction and rigidly registered with planning CT afterwards. Beamlets generated from MC simulation were incorporated into direct aperture optimization (DAO) of static gantry step and shoot IMRT treatment planning. For each patient, three RT plans were generated. 1. Clinical plan: 3D-CRT plan generated in vendor's software which is clinically delivered. 2. DVH driven plan: IMRT plan generated using DAO and MC dose calculation without SPECT guidance. Conventional DVH constraints for targets and OARs were used in the treatment planning, 3. SPECT driven plan: IMRT plan generated using DAO and MC dose calculation with SPECT guidance. SPECT weighted mean dose (SWMD) and Equivalent uniform dose (EUD) constraints were incorporated into the objective function. SWMD constraints were applied to ipsilateral and contralateral lungs respectively as the metric of lung function sparing. EUD was adopted to optimize planning target volume (PTV) dose coverage. Results: 1. Compared to clinical plan, same target dose coverage was achieved in both DVH driven and SPECT driven plans. 95% iso-dose line covered more than 97% of PTV in both plans. 2. Compared to DVH driven plans, in SPECT driven plans, V5 and V20 were reduced by ∼8% and ∼3% respectively, SWMD were reduced by ∼1Gy. Thus superior sparing of both lung function and volume was achieved. Conclusion: Comparing to conventional DVH driven IMRT plans, superior lung sparing of both anatomical and functional volumes can be achieved in SPECT driven IMRT planning based on EUD and SWMD.

SU-GG-T-13: Efficacy of the Fiducial Marker Based IGRT Approach in Prostate Tomotherapy and an Inter-Fractional Motion Correction Optimization Method for Dose Coverage Improvement and Potential Margin Reduction

Purpose: To evaluate the efficacy of the target volume dose coverage in the fiducial marker based prostate Tomotherapy and to explore a correction optimization technique to improve the dose distribution. Method and Materials: The prostate and seminal vesicles were delineated by an experienced radiation oncologist on Tomo verification MVCT images acquired for 10 patients at different treatment fractions. The MVCT images were registered to planning kVCT images based on the implanted markers to correct for the inter-fracational deviations. This correction process is identical to that in clinical treatments. The treatment dose coverage was simulated by computing the dose coverage for the target volumes based on the MVCT verification images after the registration. The simulated treatment dose coverage was compared to corresponding plan to evaluate the efficacy of the fiducial marker based IGRT approach. Separately, a Hill-Climbing optimization algorithm was used to optimize the translational correction by maximizing a dose-based objective function. During this optimization process, the dose coverage was constantly recomputed as the translation correction was adjusted until an optimized dose coverage was reached. This optimized dose coverage was compared with the marker based treatment dose coverage to evaluate whether further dose coverage improvement and planning margin reduction could be achieved. Results: With the fiducial marker based IGRT approach, prostate was underdosed in about 5% of fractions and seminal vesicles were underdosed in about 10% of fractions. The amount of underdosing was much more pronounced for seminal vesicles than for prostate. In all the fractions, the translational correction optimization method improved the dose coverage beyond the prescription requirement and could prevent the underdosing, especially for seminal vesicles. Conclusions: The fiducial marker based IGRT approach is more reliable for prostate than for seminal vesicles. The developed correction optimization method can improve the dose coverage and holds potential to allow margin reduction.

Dosimetric Analysis of 3D Image-Guided HDR Brachytherapy Planning for the Treatment of Cervical Cancer: Is Point A-Based Dose Prescription Still Valid in Image-Guided Brachytherapy?

The purpose of this study was to analyze the dosimetric outcome of 3D image-guided high-dose-rate (HDR) brachytherapy planning for cervical cancer treatment and compare dose coverage of high-risk clinical target volume (HRCTV) to traditional Point A dose. Thirty-two patients with stage IA2–IIIB cervical cancer were treated using computed tomography/magnetic resonance imaging–based image-guided HDR brachytherapy (IGBT). Brachytherapy dose prescription was 5.0–6.0 Gy per fraction for a total 5 fractions. The HRCTV and organs at risk (OARs) were delineated following the GYN GEC/ESTRO guidelines. Total doses for HRCTV, OARs, Point A, and Point T from external beam radiotherapy and brachytherapy were summated and normalized to a biologically equivalent dose of 2 Gy per fraction (EQD2). The total planned D90 for HRCTV was 80–85 Gy, whereas the dose to 2 mL of bladder, rectum, and sigmoid was limited to 85 Gy, 75 Gy, and 75 Gy, respectively. The mean D90 and its standard deviation for HRCTV was 83.2 ± 4.3 Gy. This is significantly higher ( p < 0.0001) than the mean value of the dose to Point A (78.6 ± 4.4 Gy). The dose levels of the OARs were within acceptable limits for most patients. The mean dose to 2 mL of bladder was 78.0 ± 6.2 Gy, whereas the mean dose to rectum and sigmoid were 57.2 ± 4.4 Gy and 66.9 ± 6.1 Gy, respectively. Image-based 3D brachytherapy provides adequate dose coverage to HRCTV, with acceptable dose to OARs in most patients. Dose to Point A was found to be significantly lower than the D90 for HRCTV calculated using the image-based technique. Paradigm shift from 2D point dose dosimetry to IGBT in HDR cervical cancer treatment needs advanced concept of evaluation in dosimetry with clinical outcome data about whether this approach improves local control and/or decreases toxicities.