Lung stereotactic body radiation therapy: personalized PTV margins according to tumor location and number of four-dimensional CT scans

Objectives To characterise the motion of pulmonary tumours during stereotactic body radiation therapy (SBRT) and to evaluate different margins when creating the planning target volume (PTV) on a single 4D CT scan (4DCT). Methods We conducted a retrospective single-site analysis on 30 patients undergoing lung SBRT. Two 4DCTs (4DCT1 and 4DCT2) were performed on all patients. First, motion was recorded for each 4DCT in anterior–posterior (AP), superior-inferior (SI) and rightleft (RL) directions. Then, we used 3 different margins (3,4 and 5 mm) to create the PTV, from the internal target volume (ITV) of 4DCT1 only (PTV D1 + 3, PTV D1 + 4, PTV D1 + 5). We compared, using the Dice coefficient, the volumes of these 3 PTVs, to the PTV actually used for the treatment (PTVttt). Finally, new treatment plans were calculated using only these 3 PTVs. We studied the ratio of the D2%, D50% and D98% between each new plan and the plan actually used for the treatment (D2% PTVttt, D50% PTVttt, D50% ITVttt D98% PTVttt). Results 30 lesions were studied. The greatest motion was observed in the SI axis (8.8 ± 6.6 [0.4–25.8] mm). The Dice index was higher when comparing PTVttt to PTV D1 + 4 mm (0.89 ± 0.04 [0.82–0.98]). Large differences were observed when comparing plans relative to PTVttt and PTV D1 + 3 for D98% PTVttt (0.85 ± 0.24 [0.19–1.00]). and also for D98% ITVttt (0.93 ± 0.12 [0.4–1.0]).D98% PTVttt (0.85 ± 0.24 [0.19–1.00], p value = 0.003) was statistically different when comparing plans relative to PTVttt and PTV D1 + 3. No stastistically differences were observed when comparing plans relative to PTVttt and PTV D1 + 4. A difference greater than 10% relative to D98% PTVttt was found for only in one UL lesion, located under the carina. Conclusion A single 4DCT appears feasible for upper lobe lesions located above the carina, using a 4-mm margin to generate the PTV. Advance in knowledge Propostion of a personalized SBRT treatment (number of 4DCT, margins) according to tumor location (above or under the carina).

Dosimetric Comparison of Intraoperative Radiotherapy and SRS for Liver Metastases

Purpose/ObjectivesTo perform a dosimetric comparison between kilovoltage intraoperative radiotherapy (IORT) and stereotactic radiosurgery (SRS) simulating both deep-inspiration breath-hold (DIBH) and free-breathing (FB) modalities for patients with liver metastases.Methods/MaterialsDiagnostic computed tomographies (CT) of patients carrying one or two lesions <4 cm and who underwent surgery were retrospectively screened and randomly selected for the study. For DIBH-SRS, a gross target volume (GTV) plus planning target volume (PTV) were delineated. For FB-SRS, a GTV plus an internal target volume (ITV) and PTV were defined. Accounting for the maximal GTV diameters, a modified GTV (GTV-IORT) was expanded circumferentially to simulate a resection cavity. The best suitable round-applicator size was thereafter selected. All treatment plans were calculated homogeneously to deliver 40 Gy. Doses delivered to organs at risk (OAR) and target volumes were compared for IORT vs. both SRS modalities.ResultsEight patients encompassing 10 lesions were included in the study. The mean liver volume was 2,050.97 cm3 (SD, 650.82), and the mean GTV volume was 12.23 cm3 (SD, 12.62). As for target structures, GTV-IORT [19.44 cm3 (SD, 17.26)] were significantly smaller than both PTV DIBH-SRS [30.74 cm3 (SD, 24.64), p = 0.002] and PTV FB-SRS [75.82 cm3 (SD, 45.65), p = 0.002]. The median applicator size was 3 cm (1.5–4.5), and the mean IORT simulated delivery time was 45.45 min (SD, 19.88). All constraints were met in all modalities. Liver V9.1 showed significantly smaller volumes with IORT [63.39 cm3 (SD, 35.67)] when compared to DIBH-SRS [150.12 cm3 (SD, 81.43), p = 0.002] or FB-SRS [306.13 cm3 (SD, 128.75), p = 0.002]. No other statistical or dosimetrically relevant difference was observed for stomach, spinal cord, or biliary tract. Mean IORT D90 was 85.3% (SD, 6.05), whereas D95 for DIBH-SRS and FB-SRS were 99.03% (SD, 1.71; p = 0.042) and 98.04% (SD, 3.46; p = 0.036), respectively.ConclusionKilovoltage IORT bears the potential as novel add-on treatment for resectable liver metastases, significantly reducing healthy liver exposure to radiation in comparison to SRS. Prospective clinical evidence is required to confirm this hypothesis.

Development of a Margin Determination Framework for Tumor-Tracking Radiation Therapy With Intraoperatively Implanted Fiducial Markers

PurposeTo develop an internal target volume (ITV) margin determination framework (or decision-supporting framework) for treating multiple lung metastases using CyberKnife Synchrony with intraoperatively implanted fiducial markers (IIFMs). The feasibility of using non-ideally implanted fiducial markers (a limited number and/or far from a target) for tracking-based lung stereotactic ablative radiotherapy (SABR) was investigated.MethodsIn the developed margin determination framework, an optimal set of IIFMs was determined to minimize a tracking uncertainty-specific ITV (ITVtracking) margin (margin required to cover target-to-marker motion discrepancy), i.e., minimize the motion discrepancies between gross tumor volume (GTV) and the selected set of fiducial markers (FMs). The developed margin determination framework was evaluated in 17 patients with lung metastases. To automatically calculate the respiratory motions of the FMs, a template matching-based FM tracking algorithm was developed, and GTV motion was manually measured. Furthermore, during-treatment motions of the selected FMs were analyzed using log files and compared with those calculated using 4D CTs.ResultsFor 41 of 42 lesions in 17 patients (97.6%), an optimal set of the IIFMs was successfully determined, requiring an ITVtracking margin less than 5 mm. The template matching-based FM tracking algorithm calculated the FM motions with a sub-millimeter accuracy compared with the manual measurements. The patient respiratory motions during treatment were, on average, significantly smaller than those measured at simulation for the patient cohort considered.ConclusionUse of the developed margin determination framework employing CyberKnife Synchrony with a limited number of IIFMs is feasible for lung SABR.

ACTIVE INVOLVEMENT OF RADIOLOGIST IN RADIATION ONCOLOGY PRACTICE

Advancements in Radiation Oncology from conventional to 3D conformal radiotherapy treatment demands expertise in many steps of radiation planning, the horizon of radiologist is now expanded by many folds and made radiologist as a integral part of the Radiation Oncology Department. A critical aspect of radiotherapy treatment planning (RTP) is determining how to deliver the required radiation dosage to cancer cells while minimising the exposure to normal tissue for which the prerequisite is identification and accurate delineation of tumour volume as well as normal structure resulted in an increase in the therapeutic ratio by reducing complication associated with normal tissue and allowing for higher target dosage and better local control. In modern radiotherapy CT images are the standard set of imaging modality required for the radiotherapy planning along with it many other modalities like MRI, PET or DSA are used by superimposing on original CT images in order to contour or delineate the structures defined by International Commission on Radiation Units and Measurements in Reports 50, 62 and 71 (ICRU) for radiotherapy planning which comprise of Gross tumour volume, clinical target volume, planning target volume, irradiated volume, Internal target volume and the normal structures as Organ at risk. It is self-evident that the contribution of a radiologist with a thorough knowledge of the development of these new modalities is critical for optimising the potential of these novel modes of radiation treatment delivery.

Improving interinstitutional and intertechnology consistency of pulmonary SBRT by dose prescription to the mean internal target volume dose

Purpose Dose, fractionation, normalization and the dose profile inside the target volume vary substantially in pulmonary stereotactic body radiotherapy (SBRT) between different institutions and SBRT technologies. Published planning studies have shown large variations of the mean dose in planning target volume (PTV) and gross tumor volume (GTV) or internal target volume (ITV) when dose prescription is performed to the PTV covering isodose. This planning study investigated whether dose prescription to the mean dose of the ITV improves consistency in pulmonary SBRT dose distributions. Materials and methods This was a multi-institutional planning study by the German Society of Radiation Oncology (DEGRO) working group Radiosurgery and Stereotactic Radiotherapy. CT images and structures of ITV, PTV and all relevant organs at risk (OAR) for two patients with early stage non-small cell lung cancer (NSCLC) were distributed to all participating institutions. Each institute created a treatment plan with the technique commonly used in the institute for lung SBRT. The specified dose fractionation was 3 × 21.5 Gy normalized to the mean ITV dose. Additional dose objectives for target volumes and OAR were provided. Results In all, 52 plans from 25 institutions were included in this analysis: 8 robotic radiosurgery (RRS), 34 intensity-modulated (MOD), and 10 3D-conformal (3D) radiation therapy plans. The distribution of the mean dose in the PTV did not differ significantly between the two patients (median 56.9 Gy vs 56.6 Gy). There was only a small difference between the techniques, with RRS having the lowest mean PTV dose with a median of 55.9 Gy followed by MOD plans with 56.7 Gy and 3D plans with 57.4 Gy having the highest. For the different organs at risk no significant difference between the techniques could be found. Conclusions This planning study pointed out that multiparameter dose prescription including normalization on the mean ITV dose in combination with detailed objectives for the PTV and ITV achieve consistent dose distributions for peripheral lung tumors in combination with an ITV concept between different delivery techniques and across institutions.

Lung Stereotactic Body Radiotherapy (SBRT) Using Spot-Scanning Proton Arc (SPArc) Therapy: A Feasibility Study

PurposeWe developed a 4D interplay effect model to quantitatively evaluate breathing-induced interplay effects and assess the feasibility of utilizing spot-scanning proton arc (SPArc) therapy for hypo-fractionated lung stereotactic body radiotherapy (SBRT). The model was then validated by retrospective application to clinical cases.Materials and MethodsA digital lung 4DCT phantoms was used to mimic targets in diameter of 3cm with breathing motion amplitudes: 5, 10, 15, and 20 mm, respectively. Two planning groups based on robust optimization were generated: (1) Two-field Intensity Modulated Proton Therapy (IMPT) plans and (2) SPArc plans via a partial arc. 5,000 cGy relative biological effectiveness (RBE) was prescribed to the internal target volume (ITV) in five fractions. To quantitatively assess the breathing induced interplay effect, the 4D dynamic dose was calculated by synchronizing the breathing pattern with the simulated proton machine delivery sequence, including IMPT, Volumetric repainting (IMPTvolumetric), iso-layered repainting (IMPTlayer) and SPArc. Ten lung patients’ 4DCT previously treated with VMAT SBRT, were used to validate the digital lung tumor model. Normal tissue complicated probability (NTCP) of chestwall toxicity was calculated.ResultTarget dose were degraded as the tumor motion amplitude increased. The 4D interplay effect phantom model indicated that motion mitigation effectiveness using SPArc was about five times of IMPTvolumetric or IMPTlayer using maximum MU/spot as 0.5 MU at 20 mm motion amplitude. The retrospective study showed that SPArc has an advantage in normal tissue sparing. The probability of chestwall’s toxicity were significantly improved from 40.2 ± 29.0% (VMAT) (p = 0.01) and 16.3 ± 12.0% (IMPT) (p = 0.01) to 10.1 ± 5.4% (SPArc). SPArc could play a significant role in the interplay effect mitigation with breathing-induced motion more than 20 mm, where the target D99 of 4D dynamic dose for patient #10 was improved from 4,514 ± 138 cGy [RBE] (IMPT) vs. 4,755 ± 129 cGy [RBE] (SPArc) (p = 0.01).ConclusionSPArc effectively mitigated the interplay effect for proton lung SBRT compared to IMPT with repainting and was associated with normal tissue sparing. This technology may make delivery of proton SBRT more technically feasible and less complex with fewer concerns over underdosing the target compared to other proton therapy techniques.