M. Josipovic

ARTIFACTS IN CONVENTIONAL COMPUTED TOMOGRAPHY (CT) AND FREE BREATHING FOUR-DIMENSIONAL CT INDUCE UNCERTAINTY IN GROSS TUMOR VOLUME DETERMINATION

PURPOSE Artifacts impacting the imaged tumor volume can be seen in conventional three-dimensional CT (3DCT) scans for planning of lung cancer radiotherapy but can be reduced with the use of respiration-correlated imaging, i.e., 4DCT or breathhold CT (BHCT) scans. The aim of this study was to compare delineated gross tumor volume (GTV) sizes in 3DCT, 4DCT, and BHCT scans of patients with lung tumors. METHODS AND MATERIALS A total of 36 patients with 46 tumors referred for stereotactic radiotherapy of lung tumors were included. All patients underwent positron emission tomography (PET)/CT, 4DCT, and BHCT scans. GTVs in all CT scans of individual patients were delineated during one session by a single physician to minimize systematic delineation uncertainty. The GTV size from the BHCT was considered the closest to true tumor volume and was chosen as the reference. The reference GTV size was compared to GTV sizes in 3DCT, at midventilation (MidV), at end-inspiration (Insp), and at end-expiration (Exp) bins from the 4DCT scan. RESULTS The median BHCT GTV size was 4.9 cm(3) (0.1-53.3 cm(3)). Median deviation between 3DCT and BHCT GTV size was 0.3 cm(3) (-3.3 to 30.0 cm(3)), between MidV and BHCT size was 0.2 cm(3) (-5.7 to 19.7 cm(3)), between Insp and BHCT size was 0.3 cm(3) (-4.7 to 24.8 cm(3)), and between Exp and BHCT size was 0.3 cm(3) (-4.8 to 25.5 cm(3)). The 3DCT, MidV, Insp, and Exp median GTV sizes were all significantly larger than the BHCT median GTV size. CONCLUSIONS In the present study, the choice of CT method significantly influenced the delineated GTV size, on average, leading to an increase in GTV size compared to the reference BHCT. The uncertainty caused by artifacts is estimated to be in the same magnitude as delineation uncertainty and should be considered in the design of margins for radiotherapy.

IMRT in a Pregnant Patient: How to Reduce the Fetal Dose?

The purpose of our study was to find a solution for fetal dose reduction during head-and-neck intensity modulated radiation therapy (IMRT) of a pregnant patient. The first step was optimization of the IMRT treatment plan with as few monitor units (MUs) as possible, while maintaining an acceptable dose distribution. The peripheral dose originating from the final IMRT plan was measured at distances reaching from the most proximal to the most distal fetal position, along the accelerators longitudinal axis, using an anthropomorphic phantom extended with water-equivalent plastic. The measured peripheral dose was divided into leakage, and internal and collimator scatter, to find the degree to which each component influences the peripheral dose to build an appropriate shield. Collimator scatter was the greatest contributor to the peripheral dose throughout the range of the growing fetus. A shield was built and placed beneath the accelerator head, extending caudally from the field edge, to function as an extra collimator jaw. This shield reduced the fetal dose by a factor of 3.5. The peripheral dose components were also measured for simple rectangular fields and also here the collimator scatter was the greatest contributor to the peripheral dose. Therefore, the shielding used for the IMRT treatment of our patient could also be used when shielding in conventional radiotherapy. It is important for a radiation therapy department to be prepared for treatment of a pregnant patient to shield the fetus efficiently.

Percutaneously implanted markers in peripheral lung tumours: Report of complications

Lung tumours move during breathing, and tumour motion of more than 3 cm has been seen for tumours located near the diaphragmatic domes [1]. Breath-ing-adapted radiotherapy such as respiratory beam gating or tumour tracking, rely on the ability to determine and predict the breathing-related tumour motion based on an external or internal surrogate for tumour motion [2]. The prediction of the correlation between the tumour and the surrogate positions must be verifi ed throughout the treatment; the veri-fi cation can be performed with repeated kV-imaging of the tumour. However, not all lung tumours are well-defi ned on kV-images and therefore radio-opaque markers implanted in or close to the tumour have been used as a surrogate for tumour position. Markers can be implanted percutaneously guided by fl uoroscopy or computed tomography (CT) or trans-bronchially inserted in nearby small bronchi. The advantage of percutaneous implantation is the pos-sibility to implant the marker directly into the tumour assuring a good representation of tumour motion, but potentially at the cost of morbidity due to the risk of pneumothorax. We report the results of a small pilot study examining the feasibility of CT-guided percutaneous implantation of fi ducial markers.

Impact of beam angle choice on pencil beam scanning breath-hold proton therapy for lung lesions.

Abstract Introduction: The breath-hold technique inter alia has been suggested to mitigate the detrimental effect of motion on pencil beam scanned (PBS) proton therapy dose distributions. The aim of this study was to evaluate the robustness of incident proton beam angles to day-to-day anatomical variations in breath-hold. Materials and methods: Single field PBS plans at five degrees increments in the transversal plane were made and water-equivalent path lengths (WEPLs) were derived on the planning breath-hold CT (BHCT) for 30 patients diagnosed with locally-advanced non-small cell lung cancer (NSCLC), early stage NSCLC or lung metastasis. Our treatment planning system was subsequently used to recalculate the plans and derive WEPL on a BHCT scan acquired at the end of the treatment. Changes to the V95%, D95 and mean target dose were evaluated. Results: The difference in WEPL as a function of the beam angle was highly patient specific, with a median of 3.3 mm (range: 0.0–41.1 mm). Slightly larger WEPL differences were located around the lateral or lateral anterior/posterior beam angles. Linear models revealed that changes in dose were associated to the changes in WEPL and the tumor baseline shift (p < 0.05). Conclusions: WEPL changes and tumor baseline shift can serve as reasonable surrogates for dosimetric uncertainty of the target coverage and are well-suited for routine evaluation of plan robustness. The two lateral beam angles are not recommended to use for PBS proton therapy of lung cancer patients treated in breath-hold, due to the poor robustness for several of the patients evaluated.

OC-0162: Liquid fiducial markers' performance in non small cell lung cancer during radiotherapy

Conclusion: Within the limitations of a retrospective study, our results show that the growth and shift of brain metastasis over time can be significant and may vary over patient groups. Given the typical steep dose gradient in SRS treatments (>10%/mm), tumour growths and shifts may have a significant impact on the tumour dose. Therefore, this phenomenon must be considered if the workup and treatment of SRS for brain metastasis is encompassing multiple days.

Deep inspiration breath-hold volumetric modulated arc radiotherapy decreases dose to mediastinal structures in locally advanced lung cancer

Concomitant radio-chemotherapy is the standard treatment for locally advanced non-small cell lung cancer (NSCLC), with a five-year survival of only approximately 15–20% [1]. Several clinical studies have indicated a positive dose-response relationship with an improved survival with higher dose [2,3]. However, evidence of mediastinal toxicity is accumulating: The phase III dose escalation trial RTOG 0617 [4] showed that heart dose parameters were correlated to death in a multivariate analysis and in a dose escalation trial by Cannon et al. [5] serious late toxicity from central and perihilar structures was encountered. In breast cancer radiotherapy a large population-based case-control study showed an increased risk of major cardiac events with increasing heart dose [6]. During deep inspiration breath-hold (DIBH), the lungs are inflated and the heart displaced caudally and these anatomical changes can be exploited to achieve a lower radiation dose to the intra-thoracic normal tissue. Several reports have shown that DIBH radiotherapy (RT) is a feasible alternative and has the potential of decreasing the lung dose in conventional three-dimensional conformal radiotherapy (3D-CRT) [7–9] and also with volumetric modulated arc therapy (VMAT) [10]. Furthermore, DIBH RT does not acquire a lot of time [11] or economical resources for implementation. We previously reported preliminary results from the first 10 patients included in this study and found a statistically significant reduction of mean lung dose with DIBH 3D-CRT as well as with VMAT in patients with locally advanced NSCLC [12]. The combination of DIBH and VMAT reduced these parameters further and DIBH mitigated the low dose bath induced by VMAT. Most previous studies have focused on dose to the lungs and none of them have reported on dose to bronchi and trachea. In this final report, we examine if DIBH-VMAT holds potential to decrease doses to mediastinal organs at risk (OAR) compared to free breathing (FB) VMAT.