Tomotherapy Plans Research Articles

SU-E-T-303: Dosimetric Comparison of a LINAC Fallback Plan Generated From Tomotherapy System

Purpose: Quantitatively evaluate the Multi Criteria Optimization (MCO) based MLC step and shoot (sMLC) fallback plan derived from Tomotherapy of multiple lesions lung SBRT Methods: Inter-comparison of various IMRT planning systems tends to be difficult due to the vendor-specific functionalities. The methodology of defining dose constraints and beam geometries is challenging. Raysearch™ planning system offers an alternative replanning to deliver same intensity map from Tomotherapy without modifying original fluence. This intuitive comparison comes from the final fluence map converted without any embedded system dependent dose optimization. This planner independent approach could be utilized to study the merits of individual machines. The term “fallback” was utilized to (A) transfer plans in among treatment delivery systems; (B) maintain acceptable plan qualities; and (C) minimize the biological dose impact due to machine breakdown. The Tomotherapy specific DICOM RT dose and plan are retrieved into Raystation’s pre-defined sMLC protocol. Based on specific machine characteristics, same fluence maps were converted to generate equivalent deliverable segments (Fig.1). Therefore, the treatment plans were evaluated among two planning tools, Tomotherapy and MCO based sMLC delivery plans. Results: By converting the fluence map with the pre-defined machine characteristics, the 9-fields fallback plan has similar ITV coverage compared to the original Tomotherapy plan. ITV average doses, the D95 consisted of 0.9% variation. The total lung doses of fallback plan drifted from 17.4% to 30.5% which represents the limitations of the static beam delivery. D2 of fallback spinal cord increased from 22.4% to 36.4% but still within tolerances (Fig.2). Ipsilateral lung changed from 11.0% to 22.6%. Low dose region between ITVs presented increased dose to the normal lung tissues (Fig.3). Conclusion: Acceptable fallback plan for Tomotherapy SBRT has similar ITVs coverage, but lack of the normal tissues avoidance due to the limitation of geometrical factors to reduce doses in between ITVs.

SU‐E‐T‐532: Left‐Sided Breast Cancer Irradiation Using Volumatric Modulated Arc Therapy: An Evaluation of Multiple Commercial Systems

Purposes:There has been growing interest in treating breast cancer using VMAT technique. Our goal is to compare the dosimetry and treatment delivery parameters for the left‐sided breast cancer treatment using various VMAT platforms from commercially available planning systems.Methods:Five consecutive left‐sided breast cancer patients initially treated with conventional 3D‐conformal radiotherapy (3DCRT) were selected. Four VMAT plans using most popular treatment planning systems, including Eclipse (Version 11, Varian), Pinnacle (Version 9.8, Philips), Monaco (Version 2.03, Elekta) and helical Tomotherapy (V4.0, Accuray). The same structure set and same planning goals were used for all VMAT plans. The dosimetric parameters including target coverage and minimum/maximum/mean, dose‐volume endpoints for the selected normal structures: the heart, ipsilateral‐/contralateral lung and breast, were evaluated. Other dosimetric indices including heterogeneity index (HI) were evaluated. The treatment delivery parameters, such as monitor unit (MUs) and delivery time were also compared.Results:VMAT increases dose homogeneity to the treated volume and reduces the irradiated heart and left‐lung volumes. Compared to the 3DCRT technique, all VMAT plans offer better heart and left‐lung dose sparing; the mean heart doses were 4.5±1.6(Monaco), 1.2±0.4(Pinnacle), 1.3± (Eclipse) and 5.6±4.4(Tomo), the mean left‐lung doses were 5.9±1.5(Monaco), 3.7±0.7(Pinnacle), 1.4± (Eclipse) and 5.2±1.6 (Tomo), while for the 3DCRT plan, the mean heart and left‐Lung doses were 2.9±2.0, and 6.8±4.4 (Gy) respectively. The averaged contralateral‐breast and lung mean doses were higher in VMAT plans than the 3DCRT plans but were not statistically significant. Among all the VMAT plans, the Pinnacle plans often yield the lowest right‐lung/breast mean doses, and slightly better heterogeneity indices that are similar to Tomotherapy plans. Treatment delivery time of the VMAT plans (except helical Tomotherapy IMRT) is estimated to be comparable with the conventional 3DCRT.Conclusion:VMAT achieves equal or better PTV coverage and comparable OARs sparing compared to the conventional 3DCRT techniques.

SU‐E‐T‐472: Improvement of IMRT QA Passing Rate by Correcting Angular Dependence of MatriXX

Purpose:Multi‐channel planar detector arrays utilized for IMRT‐QA, such as the MatriXX, exhibit an incident‐beam angular dependent response which can Result in false‐positive gamma‐based QA results, especially for helical tomotherapy plans which encompass the full range of beam angles. Although MatriXX can use with gantry angle sensor to provide automatically angular correction, this sensor does not work with tomotherapy. The purpose of the study is to reduce IMRT‐QA false‐positives by correcting for the MatriXX angular dependence.Methods:MatriXX angular dependence was characterized by comparing multiple fixed‐angle irradiation measurements with corresponding TPS computed doses. For 81 Tomo‐helical IMRT‐QA measurements, two different correction schemes were tested: (1) A Monte‐Carlo dose engine was used to compute MatriXX signal based on the angular‐response curve. The computed signal was then compared with measurement. (2) Uncorrected computed signal was compared with measurements uniformly scaled to account for the average angular dependence. Three scaling factor (+2%, +2.5%, +3%) were tested.Results:The MatriXX response is 8% less than predicted for a PA beam even when the couch is fully accounted for. Without angular correction, only 67% of the cases pass the >90% points γ<1 (3%, 3mm). After full angular correction, 96% of the cases pass the criteria. Of three scaling factors, +2% gave the highest passing rate (89%), which is still less than the full angular correction method. With a stricter γ(2%,3mm) criteria, the full angular correction method was still able to achieve the 90% passing rate while the scaling method only gives 53% passing rate.Conclusion:Correction for the MatriXX angular dependence reduced the false‐positives rate of our IMRT‐QA process. It is necessary to correct for the angular dependence to achieve the IMRT passing criteria specified in TG129.

SU‐E‐P‐37: Helical Tomotherapy to LINAC Plan Conversion Utilizing RayStation Fallback Planning

Purpose:RayStation Fallback (RSF) plan was developed to switch patient's treatment from Helical Tomotherapy (HT) to Varian TrueBeam. Helical Tomotherapy plans were compared with the corresponding RSF plans by looking at the following dosimetric parameters: PTV coverage (Dmean and D95), Paddick conformity index (CI), uniformity index (UI=(D98‐D2)/Drx) and organ‐at‐risk (OAR) doses.Methods:Five patients with five tumor sites including larynx, head and neck, esophagus, lung, and prostate previously treated with HT were re‐planned using RSF treatment planning station(TPS) to best match the HT treatment plans. IMRT technique with nine fields 6 MV X‐ray beams and a static multi‐leaf collimator (MLC) were used for RSF plans without any additional attempt to further optimize the RSF plans.Results:A physician evaluated and confirmed the clinical acceptability of the RSF plans. Both HT and RSF plans gave comparable PTV coverage with a maximum percentage difference for D95 of 0.7%. HT plans had a better CIs ranging from 2%–15% higher conformity compared to RSF plans. HT plans also had better UIs for all tumor sites except the esophagus. The UIs were 1.6% to 8.9% lower for HT plans compared to RSF plans. Both HT and RSF plans gave comparable and acceptable OAR doses. However, the integral dose was 1%–15% higher for HT plans compared to RSF plans.Conclusion:RSF plans were simple and fast to generate. In the study cases of 5 various tumor sites, RSF efficiently created clinically acceptable plans without further optimization. However, further optimization for any IMRT plan can be done if clinically warranted. The absolute differences in CI and UI were small between RSF and HT plans.

Pediatric Cranio-spinal Axis Irradiation

Pediatric cranio-spinal axis irradiation (CSI) is a valuable treatment for many central nervous system (CNS) diseases, but due to the life expectancies and quality of life expectations for children, the minimization of the risk for radiation-induced secondary malignancies must be a high priority. This study compared the estimated CSI-induced secondary malignancy risks of three radiation therapy modalities using three different models. Twenty-four (n = 24) pediatric patients previously treated with CSI for tumors of the CNS were planned using three different treatment modalities: three-dimensional conformal radiation therapy (3D-CRT), volume modulated arc therapy (VMAT), and Tomotherapy. Each plan was designed to deliver 23.4 Gy (1.8 Gy/fraction) to the target which was defined as the entire brain and spinal column with a 0.7 cm expansion. The mean doses as well as the dose volume histograms (DVH) of specific organs were analyzed for secondary malignancy risk according to three different methods: the effective dose equivalent (EDE), the excess relative risk (ERR), and the linear quadratic (LQ) models. Using the EDE model, the average secondary risk was highest for the 3D-CRT plans (37.60%), compared to VMAT (28.05%) and Tomotherapy (27.90%). The ERR model showed similarly that the 3D-CRT plans had considerably higher risk (10.84%) than VMAT and Tomotherapy, which showed almost equal risks (7.05 and 7.07%, respectively). The LQ model requires organ-specific cell survival parameters, which for the lungs, heart, and breast relevant values were found and applied. The lung risk for secondary malignancy was found to be 1.00, 1.96, and 2.07% for 3D-CRT, VMAT, and Tomotherapy, respectively. The secondary cancer risk for breast was estimated to be 0.09, 0.21, and 0.27% and for heart it was 9.75, 6.02 and 6.29% for 3D-CRT, VMAT, and Tomotherapy, respectively. Based on three methods of secondary malignancy estimation, the 3D-CRT plans produced highest radiation-induced secondary malignancy risk, and the VMAT and Tomotherapy plans had nearly equal risk. Pediatric patients must be treated with reducing long term sequelae as a priority.

Specific aspects of Tomotherapy and CyberKnife planning

Specific aspects of Tomotherapy and CyberKnife planning

Comparing the quality of passively-scattered proton and photon tomotherapy plans for brain and head and neck disease sites

We compare the quality of photon IMRT (helical tomotherapy) with classic proton plans for brain, head and neck tumors, in terms of target dose uniformity and conformity along with organ-at-risk (OAR) sparing.Plans were created for twelve target volumes among eight cases. All patients were originally planned and treated using helical tomotherapy. Proton plans were generated using a passively-scattered beam model with a maximum range of 32 g cm−2 (225 MeV), range modulation in 0.5 g cm−2 increments and range compensators with 4.8 mm milling tool diameters. All proton plans were limited to two to four beams. Plan quality was compared using uniformity index (UI), conformation number (CN) and a EUD-based plan quality index (fEUD).For 11 of the 12 targets, UI was improved for the proton plan; on average, UI was 1.05 for protons versus 1.08 for tomotherapy. For 7 of the 12 targets, the tomotherapy plan exhibited more favorable CN. For proximal OARs, the improved dose conformity to the target volume from tomotherapy led to a lower maximum dose. For distal OARs, the maximum dose was much lower for proton plans. For 6 of the 8 cases, near-total avoidance for distal OARs provided by protons leads to improved fEUD. However, if distal OARs are excluded in the fEUD calculation, the proton plans exhibit better fEUD in only 3 of the 8 cases.The distal OAR sparing and target dose uniformity are generally better with passive-scatter proton planning than with photon tomotherapy; proton therapy may be preferred if the clinician deems those attributes critical. However, tomotherapy may serve equally as well as protons for cases where superior target dose conformity from tomotherapy leads to plan quality nearly identical to or better than protons and for cases where distal OAR sparing is not concerning.

Dosimetric Evaluation of The Tomotherapy, Non-coplanar and Coplanar IMRT Plannings for Total Scalp Irradiation

Abstract In this study, the dosimetric effects of the Tomotherapy, noncoplanar and coplanar IMRT plannings were evaluated for the total scalp irradiation. Tomotherapy based and linac based IMRT plans were compared for 61 years old female patient with multipl myeloma. Two different linac based IMRT plans which are coplanar and noncoplanar were performed for total scalp irradiation. 20 Gy was given to the PTV volume. Firstly, homogeneity index and conformity index values were calculated for PTV. Secondly, the doses of organ at risk (OaR) which are lenses, brain, brainstem and parotid glands were dosimetrically evaluated. While, tomotherapy plan was more homogenous, noncoplanar plan was more conformal than the others. Our study figure out that the OaR doses can be decreased by using tomotherapy based IMRT. However, the treatment time was increased when the tomotherapy based IMRT used for treatment. Tomotherapy based IMRT can be used for total scalp irradiation with respect to OaR. Keywords: IMRT, tomotherapy, coplanar, noncoplanar, OaR

Evaluation of 3DVH Software for the Patient Dose Analysis in TomoTherapy

† Department of Radiation Oncology, Chonnam National University Hwasun Hospital, Hwasun, Korea The new function of 3DVH software for dose calculation inside the patient undergoing TomoTherapy treatment by applying the measured data obtained by ArcCHECK was recently released. In this study, the dosimetric accuracy of 3DVH for the TomoTherapy DQA process was evaluated by the comparison of measured dose distribution with the dose calculated using 3DVH. The 2D diode detector array MapCHECK phantom was used for the TomoTherapy planning of virtual patient and for the measurement of the compared dose. The average pass rate of gamma evaluation between the measured dose in the MapCHECK phantom and the recalculated dose in 3DVH was 92.6±3.5%, and the error was greater than the average pass rate, 99.0±1.2%, in the gamma evaluation results with the dose calculated in TomoTherapy planning system. The error was also greater than that in the gamma evaluation results in the RapidArc analysis, which showed the average pass rate of 99.3± 0.9%. The evaluated accuracy of 3DVH software for TomoTherapy DQA process in this study seemed to have some uncertainty for the clinical use. It is recommended to perform a proper analysis before using the 3DVH software for dose recalculation of the patient in the TomoTherapy DQA process considering the initial application stage in clinical use.

The sensitivity of ArcCHECK-based gamma analysis to manufactured errors in helical tomotherapy radiation delivery.

Three‐dimensional measurement arrays are an efficient means of acquiring a distribution of data for patient plan delivery QA. However, the tie between plan integrity and traditional gamma‐based analysis of these data are not clear. This study explores the sensitivity of such analysis by creating errors in Helical Tomotherapy delivery and measuring the passing rates with an ArcCHECK cylindrical diode array. Errors were introduced in each of the couch speed, leaf open time, and gantry starting position in increasing magnitude while the resulting gamma passing rates were tabulated. The error size required to degrade the gamma passing rate to 90% or below was on average a 3% change in couch speed, 5° in gantry synchronization, or a 5 ms in leaf closing speed for a 3%/3 mm Van Dyk gamma analysis. This varied with plan type, with prostate plans exhibiting less sensitivity than head and neck plans and with gamma analysis criteria, but in all cases the error magnitudes were large compared to actual machine tolerances. These findings suggest that the sensitivity of ArcCHECK‐based gamma analysis to single‐mode errors in tomotherapy plans is dependent upon plan and analysis type and at traditional passing thresholds unable to detect small defects in the plan.PACS number: 87.55.Qr

Influence of different boost techniques on radiation dose to the left anterior descending coronary artery.

PurposeThe purpose of this study is to compare the dosimetry of electron beam (EB) plans and three-dimensional helical tomotherapy (3DHT) plans for the patients with left-sided breast cancer, who underwent breast conserving surgery.Materials and MethodsWe selected total of 15 patients based on the location of tumor, as following subsite: subareolar, upper outer, upper inner, lower lateral, and lower medial quadrants. The clinical target volume (CTV) was defined as the area of architectural distortion surrounded by surgical clip plus 1 cm margin. The conformity index (CI), homogeneity index (HI), quality of coverage (QC) and dose-volume parameters for the CTV, and organ at risk (OAR) were calculated. The following treatment techniques were assessed: single conformal EB plans; 3DHT plans with directional block of left anterior descending artery (LAD); and 3DHT plans with complete block of LAD.Results3DHT plans, regardless of type of LAD block, showed significantly better CI, HI, and QC for the CTVs, compared with the EB plans. However, 3DHT plans showed increase in the V1Gy at skin, left lung, and left breast. In terms of LAD, 3DHT plans with complete block of LAD showed extremely low dose, while dose increase in other OARs were observed, when compared with other plans. EB plans showed the worst conformity at upper outer quadrants of tumor bed site.Conclusion3DHT plans offer more favorable dose distributions to LAD, as well as improved target coverage in comparison with EB plans.

Successful delivery of adjuvant external beam radiotherapy for ependymoma in a patient with Ondine׳s curse

Ondine׳s curse is a rare, potentially life-threatening disorder characterized by loss of automatic breathing during sleep and preserved voluntary breathing. It is seldom encountered in the radiotherapy clinic but can pose significant technical challenges and safety concerns in the delivery of a prescribed radiation course. We report a unique case of successful delivery of radiotherapy for ependymoma in a patient with Ondine׳s curse. A 53-year-old gentleman presented with vertigo when lying down. Brain magnetic resonance imaging revealed an enhancing mass in the floor of the fourth ventricle. He underwent maximal safe resection. Pathology revealed ependymoma. The patient was referred for radiotherapy. Computed tomography simulation was performed in supine position with 3-point thermoplastic mask immobilization. Sequential TomoTherapy plans were developed. At first scheduled treatment, shortly after mask placement, his arms went limp and he was unresponsive. Vitals showed oxygen saturation 83%, pulse 127, and blood pressure 172/97mmHg. He was diagnosed with Ondine׳s curse thought secondary to previous brainstem damage; the combination of lying flat and pressure from the mask was causing him to go into respiratory arrest. As supine positioning did not seem clinically advisable, he was simulated in prone position. A RapidArc plan and a back-up conformal plan were developed. Prescriptions were modified to meet conservative organs-at-risk constraints. Several strategies were used to minimize uncertainties in set-up reproducibility associated with prone positioning. He tolerated prone RapidArc treatments well. The report highlights the importance of applying practical patient safety and treatment planning/delivery strategies in the management of this challenging case.

Feasibility of PET-CT based hypofractionated accelerated dose escalation in oropharyngeal cancers: Final dosimetric results of the VORTIGERN study. (Secondary endpoint of UK NCRI portfolio: MREC No: 08/H0907/127, UKCRN ID 7341).

Technological advances have enabled clinicians to explore dose escalation strategies in various tumor sites. Intermediate and high risk oropharyngeal cancers have poor 5 year outcomes. This study aimed to assess the feasibility and dosimetric safety of 9% dose escalation in these tumors and compare the dose received by organs at risk (OAR) in escalated plans (67.2 Gy/28 fractions) versus (65 Gy/30 fractions) standard dose plans. FDG-PET fused datasets were used to delineate gross, clinical and planning target volumes. Standard dose plans were created using two non IMRT techniques (conventional and field in field plans) whilst the patient was treated using a helical tomotherapy plan. A fourth dose escalation plan was obtained allowing comparison between the 20 plans of oropharyngeal cancer patients. It was feasible to escalate dose to the FDG-PET avid tumor within the set constraints to that of planning target volume and OAR. Comparison of the escalated dose to that of standard plans showed a statistically significant (P < 0.05) sparing of the mastication apparatus (MA) with escalated plans. Dose to the other critical and functional organs were comparable between the four plans. Hypofractionated, slightly accelerated dose escalation in oropharyngeal cancers is likely to be safe and the chance of trismus is not any higher than when standard dose radiotherapy is used. Active measures to reduce dose to the MA achieves acceptable dose volume parameters even at escalated doses.

Efficacy of the Dynamic Jaw Mode in Helical Tomotherapy With Static Ports for Breast Cancer.

The recently developed dynamic jaw technology of tomotherapy can reduce craniocaudal dose spread without much prolonging the treatment time. This study aimed to investigate the efficacy of the dynamic jaw mode for tomotherapy of breast cancer. Static tomotherapy plans of the whole breast and supraclavicular regional lymph nodes, and plans for the whole breast only were generated in 25 patients with left-sided breast cancer. Plans with a field width of 2.5 or 5 cm with the dynamic or fixed jaw modes were made for each patient. The prescribed dose was 50 Gy in 25 fractions. In whole breast and supraclavicular nodal radiotherapy, dose distributions and homogeneity of the planning target volume (PTV) with the dynamic jaw mode were slightly inferior to those with the fixed jaw mode with a 5-cm field width (P < .05). However, lung low-dose volumes and mean doses of the larynx, thyroid, skin, and all the healthy tissues combined were smaller with the dynamic jaw mode than with the fixed jaw mode with a 5-cm field width (P < .001). In whole breast radiotherapy, mean doses of the skin and healthy tissues were lower with the dynamic jaw mode than with the fixed jaw mode with a 5-cm field width (P < .001) without significant differences in PTV dose distributions, homogeneity, and conformity. The dynamic jaw mode provided better sparing of organs at risks with minimal disturbance of dose–volume indices of PTV. Considering the treatment time, the 5-cm-field dynamic jaw mode is more efficient than the 2.5-cm fixed jaw mode.

Effect of CT contrast on volumetric arc therapy planning (RapidArc and helical tomotherapy) for head and neck cancer

The objectives of the study were to evaluate the effect of intravenous contrast in the dosimetry of helical tomotherapy and RapidArc treatment for head and neck cancer and determine if it is acceptable during the computed tomography (CT) simulation to acquire only CT with contrast for treatment planning of head and neck cancer. Overall, 5 patients with head and neck cancer (4 men and 1 woman) treated on helical tomotherapy were analyzed retrospectively. For each patient, 2 consecutive CT scans were performed. The first CT set was scanned before the contrast injection and secondary study set was scanned 45 seconds after contrast. The 2 CTs were autoregistered using the same Digital Imaging and Communications in Medicine coordinates. Tomotherapy and RapidArc plans were generated on 1 CT data set and subsequently copied to the second CT set. Dose calculation was performed, and dose difference was analyzed to evaluate the influence of intravenous contrast media. The dose matrix used for comparison included mean, minimum and maximum doses of planning target volume (PTV), PTV dose coverage, and V45Gy, V30Gy, and V20Gy organ doses. Treatment planning on contrasted images generally showed a lower dose to both organs and target than plans on noncontrasted images. The doses for the points of interest placed in the organs and target rarely changed more than 2% in any patient. In conclusion, treatment planning using a contrasted image had insignificant effect on the dose to the organs and targets. In our opinion, only CT with contrast needs to be acquired during the CT simulation for head and neck cancer. Dose calculations performed on contrasted images can potentially underestimate the delivery dose slightly. However, the errors of planning on a contrasted image should not affect the result in clinically significant way.

Dosimetric effect of small bowel oral contrast on conventional radiation therapy, linear accelerator–based intensity modulated radiation therapy, and helical tomotherapy plans for rectal cancer

PurposeThis study evaluated the dosimetric effect of small bowel oral contrast on conventional radiation therapy, linear accelerator--based intensity modulated radiation therapy (IMRT), and helical tomotherapy (HT) treatment plans. Methods and materialsThirteen patients with rectal cancer underwent computed tomography (CT) simulation with oral contrast (CCT) in preparation for chemoradiation therapy. The contrast in the small bowel was contoured, and a noncontrast CT scan (NCCT) was simulated by reassigning a CT number of 0 Hounsfield units to the contrast volume. Conventional, IMRT, and HT plans were generated with the CCT. The plan generated on the CCT was then recalculated on the NCCT, maintaining the same number of monitor units for each field, and the plans were not renormalized. Dosimetric parameters representing coverage of the planning target volume with 45 Gy (D98%, D95%, D50%, and D2%) and sparing of the bladder and peritoneal cavity (D50%, D30%, and D10%) were recorded. The ratio of dose calculated in the presence of contrast to dose with contrast edited out was recorded for each parameter. A paired Student t test was used for comparison of plans. ResultsFor conventional plans, there was <0.1% variance in the dose ratio for all volumes of interest. For IMRT plans, there was a 1% decrease in the mean dose ratio, and the range of dose ratios for all volumes was greater than that for HT or conventional plans. For HT plans, for all volumes of interest, the mean dose ratio was <0.2%, and the range for all patients was <1%. For all IMRT dosimetric parameters, the difference was in the order of 1% of the mean dose (P < .05). The dose difference was not statistically significant for the conventional or HT plans. ConclusionsThe use of CCT during CT simulation has no clinically significant effect on dose calculations for conventional, IMRT, and HT treatment plans and may not require replacement of the contrast with a CT number of 0 Hounsfield units.

The optimization of superficial planning target volumes (PTVs) with helical tomotherapy.

When the planning target volume (PTV), extends close to or beyond the skin, inverse‐planned intensity‐modulated radiotherapy can boost the fluence from tangential angles to superficial parts of the PTV due to the buildup effect. This can result in significant regions of high dose upon a small movement in the patient's position. We analyzed different methods of reducing this effect in the TomoTherapy planning system, including using pretend bolus, clipping the PTV back from the skin surface, and combinations of both. No alternative technique was seen to be unconditionally superior to a 5 mm planning bolus. A clip distance of 3 mm gave acceptable results, but consideration should be given to the reduction in minimum dose or increase in maximum dose seen in clip distances just 1 mm less or more, respectively, when treating head and neck cases in TomoTherapy.PACS numbers: 87.55.de, 87.55.dh

Poster - Thur Eve - 57: Evaluation of laryngeal mucosal dose with conventional linac and TomoTherapy

The purpose of this study was to examine whether or not underdosing occurs in the mucosal layer during treatment of glottis cancer. A larynx phantom was produced and regions at risk of recurrence due to suspected underdosing were identified and wells drilled into the phantom for flush placement of TLDs. Seven interest points were chosen. CT simulation was completed prior to the wells being drilled, and again afterwards with the TLD locations indicated using BBs. Treatment plans created for this investigation included: 3DCRT using Elekta-XiO (n=9) and VMAT created using Elekta-Monaco (n=9), both delivered on an Elekta linac; standard TomoTherapy plan (n=11) and a directionally blocked TomoTherapy plan to approximate a 3D-conformal approach (n=5). Imaging dose during TomoTherapy deliveries was accounted for. The average TLD result at each interest point was compared to the planned value using a paired t-test. There was no significant difference between the planned and measured 3DCRT dose (268.9 vs. 267.0 cGy, respectively; p>0.05). Similarly, the planned and measured TomoTherapy treatment did not show any significant differences (271.7 vs 269.7 cGy; p>0.05). In the blocked TomoTherapy plan, significant overdosing was seen (274.5 vs 294.9 cGy; p<0.05) and underdosing was not seen in the VMAT treatment (303.5 vs 321.8 cGy; p>0.05). Further investigation is ongoing to ensure appropriate normalization of results and to investigate the overdosing noted with the blocked TomoTherapy plan. Results from this study suggest that significant underdosing does not occur in the conventional treatment of early glottic cancer using 6MV photons.

Sci-Fri PM: Dosimetry - 03: Delta4 diode absolute dose response for large and small target volume IMRT QA

The goal of this project was to quantify the over-response/under-response of the Delta4 diodes for Helical Tomotherapy plans on extreme target volume sizes. A custom Delta4 phantom quarter with a hole to insert an ionisation chamber (IC) close to the center of the phantom have been used to acquire simultaneous IC and diodes absolute dose measurements. Eight plans for different target volumes were created from 20cm to 1cm diameter. Diodes dose measurements in the target were compared with IC measurement, to quantify absolute dose accuracy. IC measurements show a good agreement with planned dose (±2%). Diode measurements demonstrate a good agreement with IC for regular target size of 5 and 10cm (0 to 1%). For larger targets, an over-response is observed for FW 25mm and 10mm (2 to 3%). for small target of 1cm diameter, a major under-response is observed for FW 25mm and 10mm (−8 and −36%). The over-response could to be due to the extra amount of scattered radiation and the opposite for under-response. Although this scatter hypothesis still has to be proven, early testing demonstrates an over-response of 40%/20% of the central diodes compare to IC when an open helical rotational beam is delivered 75mm/25mm away frommore » the center of the phantom. These results are in agreement with the real patient Delta4 DQA results at our center.« less

SU-E-T-83: Comparison of the Dosimetric Efficacy of Helical Dynamic Jaw Vs. Fixed Jaw of TomoTherapy HDA

Purpose: One of the advanced features of Tomotherapy HDA is that the jaws move dynamically at the cephalocaudal ends of the target to make shaper dose fall-off. The purpose of this project is to determine the dosimetric efficacy of volumetric dose shaping using dynamic jaw vs. fixed jaw. Methods: To evaluate the improvement offered by dynamic jaw collimation, five different cases were planned using both dynamic and fixed jaw collimation. Dose profiles and statistics for PTV and OARs were compared for each treatment site. The treatment plans were then delivered to a PTW 2D array and to a SunNuclear ArcCheck for volumetric dose comparison. The total delivered dose of one patient plan using dynamic jaw was reconstructed based on MVCTs of Tomotherapy and compared with the original plan by Tomotherapy Plan Adaptive. Plans using different jaw sizes and total treatment time from dynamic jaw vs. fixed jaw were dosimetrically evaluated. Results: Cephalocaudal dose profiles using dynamic jaw vs. fixed jaw show large differences but the lateral dose profiles are similar. A plan using dynamic jaw reduced mean doses to spinal cord(42.6%), kidney(42.7%), liver(60.4%) and bowel(51%) comparing to a plan with fixed jaw while the PTV dose coverage is similar. The largest dose discrepancy occurs when comparing plans with 5cm dynamic jaws vs. 5cm fixed jaws. The 2.5cm dynamic jaw produced the sharpest dose fall off. Plans using the 5cm dynamic Jaw can produce better yet similar results compared to the plan using 2.5cm fixed jaw. There were no significant differences between the reconstructed delivered doses vs. dose from the original plan. Conclusion: The use of dynamic jaws in Tomotherapy planning largely spare doses to OARs adjacent to the PTV. When this planning option is not available, planners should use < 2.5cm fixed jaw when critical OARs are located cephalocaudally to the PTV.