Intensity-modulated Total Body Irradiation (TBI) with TomoDirect™.

BACKGROUND: The new TomoDirect™ modality offers a non-rotational option with discrete beam angles. We have investigated this mode for TBI with the intention to test the feasibility and to establish it as a clinical routine method. Special foci were directed onto treatment planning, dosimetric accuracy and practical aspects. PATIENTS AND METHODS: TBI plans were calculated with TomoDirect™ for a Rando™ phantom and all patients with an intended fractionated total body irradiation between November 2013 and May 2014 (n = 8). Finally, four of these patients were irradiated with TomoDirect™. Additionally we studied variations in the modulation factor, pitch, field width of Y-jaws and dose grid during optimization. Dose measurements were performed using thermoluminescent rods in the Rando™ phantom, with the Delta4® and with ionization chambers in a solid water phantom. RESULTS: For all eight calculated plans with a prescribed dose of 12 Gy Dmean was 12.09-12.33 Gy (12,25 ± 0.08 Gy), D98 11.2-11.6 Gy (11.45 ± 0.12 Gy) and D2 12.6-13.1 Gy (12.94 ± 0.13 Gy). Dmean of inner lungs was 8.73 ± 0.22 Gy on the left side and 8.69 ± 0.27 Gy on the right side. When single planning parameters are varied with otherwise constant parameters, the modulation factor showed the greatest impact on dose homogeneity and treatment time. The impact of the pitch was marginally, and almost equal homogeneity can be obtained with field width of Y-jaws 5 cm and 2.5 cm. Measurements with thermoluminescent rods (n = 25) in the Rando™ phantom showed a mean dose deviation between measured and calculated dose of 0.66 ± 2.26%. 18 of 25 TLDs had a deviation below 3%, seven of 25 TLDs between 3% and 5%. CONCLUSION: TBI with TomoDirect™ allows a superior homogeneity compared to conventional methods, where lung blocks are widely accepted. The treatment is performed only in supine position and is robust and comfortable for the patient. TomoDirect™ allows the implementation of organ-specific dose prescriptions. So the discussion about the balance between the need for aggressive treatment and limited toxicity can be renewed with the new potentials of TomoDirect™ - for children as well as for adults - and possibly yield a better clinical outcome in the future.

Neural stem cell sparing by linac based intensity modulated stereotactic radiotherapy in intracranial tumors.

BACKGROUND: Neurocognitive decline observed after radiotherapy (RT) for brain tumors in long time survivors is attributed to radiation exposure of the hippocampus and the subventricular zone (SVZ). The potential of sparing capabilities for both structures by optimized intensity modulated stereotactic radiotherapy (IMSRT) is investigated. METHODS: Brain tumors were irradiated by stereotactic 3D conformal RT or IMSRT using m3 collimator optimized for PTV and for sparing of the conventional OARs (lens, retina, optic nerve, chiasm, cochlea, brain stem and the medulla oblongata). Retrospectively both hippocampi and SVZ were added to the list of OAR and their dose volume histograms were compared to those from two newly generated IMSRT plans using 7 or 14 beamlets (IMSRT-7, IMSRT-14) dedicated for optimized additional sparing of these structures. Conventional OAR constraints were kept constant. Impact of plan complexity and planning target volume (PTV) topography on sparing of both hippocampi and SVZ, conformity index (CI), the homogeneity index (HI) and quality of coverage (QoC) were analyzed. Limits of agreement were used to compare sparing of stem cell niches with either IMSRT-7 or IMSRT-14. The influence of treatment technique related to the topography ratio between PTV and OARs, realized in group A-D, was assessed by a mixed model. RESULTS: In 47 patients CI (p ≤ 0.003) and HI (p < 0.001) improved by IMSRT-7, IMSRT-14, QoC remained stable (p ≥ 0.50) indicating no compromise in radiotherapy. 90% of normal brain was exposed to a significantly higher dose using IMSRT. IMSRT-7 plans resulted in significantly lower biologically effective doses at all four neural stem cell structures, while contralateral neural stem cells are better spared compared to ipsilateral. A further increase of the number of beamlets (IMSRT-14) did not improve sparing significantly, so IMSRT-7 and IMSRT-14 can be used interchangeable. Patients with tumors contacting neither the subventricular zone nor the cortex benefit most from IMSRT (p < 0.001). CONCLUSION: The feasibility of neural stem cell niches sparing with sophisticated linac based inverse IMSRT with 7 beamlets in an unselected cohort of intracranial tumors in relation to topographic situation has been demonstrated. Clinical relevance testing neurotoxicity remains to be demonstrated.

Dosimetric quality assurance for intensity-modulated radiotherapy feasibility study for a filmless approach.

PURPOSE: Test and comparison of various 2-D real-time detectors for dosimetric quality assurance (QA) of intensity-modulated radiotherapy (IMRT) with the vision to replace radiographic films for 2-D dosimetry. MATERIAL AND METHODS: All IMRT treatment plans were created with the Konrad software (Siemens OCS). The final dose calculation was also carried out in Konrad. A Mevatron Primus (Siemens OCS) linear accelerator which provides 6-MV and 15-MV highenergy photon beams was used for the delivery of segmented multileaf-modulated IMRT. Three different 2-D detectors, each based on a different physical (interaction) principle, were tested for the field-related IMRT verification: (1) the MapCheck diode system (Sun Nuclear), (2) the I'mRT QA scintillation detector (Scanditronix/Wellhöfer), and the Seven29 ionization chamber array (PTW). The performance of these detector arrays was evaluated against IMRT dose distributions created and calculated with Konrad and the results obtained were compared with film measurements performed with radiographic films (EDR2, Kodak). Additionally, measurements were performed with point detectors, such as diamond, diodes (PTW) and ionization chambers (PTW, Scanditronics/ Wellhöfer) and radiochromic films (GafChromic film MD55, ISP). RESULTS: The results obtained with all three 2-D detector systems were in good agreement with calculations performed with the treatment-planning system and with the standard dosimetric tools, i.e., films or various point dose detectors. It could be shown that all three systems offer dosimetric characteristics required for performing field-related IMRT QA with relative dose measurements. The accuracy of the 2-D detectors was mostly +/- 3% normalized to dose maximum for a wide dynamic range. The maximum deviations did not exceed +/- 5% even in regions with a steep dose gradient. The main differences between the detector systems were the spatial resolution, the maximal field size, and the ability to perform absolute dosimetric measurements. CONCLUSION: Commercial 2-D detectors have the potential to replace films as an "area detector" for field-related verification of IMRT. The on-line information provided by the respective systems can even improve the efficiency of the QA procedures.