A two isocenter IMRT technique with a controlled junction dose for long volume targets

Background: The increasing patient load in radiotherapy centres demands selection of thetechnique that provides plans with optimal dosimetry in terms of target volume coverage, organs atrisk (OAR) sparing and a lesser treatment time. This study was designed to compare the two widelypractised conformal techniques, IMRT and VMAT in head and neck cancer patients in terms ofplanning target volume (PTV) coverage, OAR sparing and treatment delivery parameters. Materialsand methods: For ten postoperative head and neck cancer patients who had been treated by IMRTtechnique virtual VMAT plans were generated for study purposes. The dose prescribed to PTV was 60Gy in 30 fractions. The dose-volume parameters of PTV and OARs and the treatment deliveryparameters were compared amongst both the techniques. Statistical significance was calculatedusing paired ‘t’ test. Results: Both the plans were comparable in terms of dosimetry. The onlysignificant difference being better conformity in the IMRT plans. The dose to OARs was alsocomparable in both the techniques except for a significant reduction in the point dose to brainstemwith the IMRT technique. Given the treatment delivery parameters, there was a significant reductionin the treatment delivery time and monitor units with the VMAT technique compared to the IMRTtechnique. Conclusion: VMAT technique gave comparable plans to that of the IMRT technique interms of dosimetry but reduced the treatment time. It seems feasible in radiotherapy centres withincreased patient load.

Purpose: To investigate how diodes respond to various IMRT beam conditions and to evaluate the feasibility of using in‐vivo diode dosimetry for IMRT treatment delivery verification. Method and Materials: A series of dynamic MLC files were created to simulate IMRT fields. Diodes and a Farmer chamber were used to measure IMRT doses at dmax in a phantom. Dose dependence on field size (FS), sliding window MLC leaf gap width, SSD and energy was investigated. In‐vivo diode dosimetry was also used for patient IMRT treatment dose verification and compared with IMRT plan calculations. Results: Measured doses decreased with decreasing FS and MLC leaf gap. Diode readings agreed with ion chamber at dmax to within 3%. All measured data points were fitted to a straight line of “Measured Dose” vs. “%Primary Beam”, with a slope of 0.945, intercept 0.04 cGy/MU, and correlation coefficient 0.997. For a given FS and leaf gap, measured doses increased with decreasing SSD. SSD dependence for ion chamber agreed with Inverse Square Factor (ISF) to ∼1%. Diode SSD dependence deviated ISF by 1∼4%. Conclusion: In measuring IMRT dose at dmax in phantom, diode readings agreed with ion chamber to within a few percent. Diode correction factors for SSD, FS, leaf gap, and energy dependence in IMRT treatment fields are small. Our study has shown that diode dosimetry can be used for in‐vivo patient IMRT treatment delivery verification. In most cases, in‐vivo diode measurements and IMRT plan calculations should agree to within ±7%. It is important to select a low dose gradient region from IMRT plan for diode placement. Larger discrepancies between measurements and calculations usually can be attributed to high dose gradient, errors in patient setup and diode positioning, and SSD change.

The aim of the study is to formulate a dosimetry index that can be used for Quality assessment of IMRT treatment plans of prostate site. To compare different radiotherapy plans different indices are utilized. The present study will attempt to combine all the indices into one unified quality index which can define the quality of a treatment plan and serve as a quality indicator of treatment plan developed among similar and different modalities. This quality index will incorporate homogeneity index, conformity index, dose gradient and coverage index resulting in a direct comparison to an ideal plan. We developed another index in this study related to sparing of organs i.e. UDI (OARs) that incorporate the doses of OARs to compare which plan is optimum in terms of target coverage and OARs sparing. In comparison to this UDI (OARs) another work was done to analyze the dosimetry of overall plan including target coverage and organ at risk sparing i.e. critical organ scoring index (COSI). To validate UDI (Target), UDI (OARs) and COSI, 22 patients were studied which were planned on IMRT. We also define the ranking of the plan according to their target coverage and organ sparing quantities as ‘Excellent’, ‘Good’, ‘Average’ or ‘Poor’ plan. UDI index for the target and organ at risk is found to be a useful tool to quantify a treatment plan quality when volumes with different dose prescription are treated. Keywords: Treatment planning system, Conformity index, Homogeneity index, Dose coverage, Dose gradient, Critical organs DOI : 10.7176/CMR/12-1-04 Publication date: January 31 st 2020

Objective To assess the dosimetric impact on the target volumes and organs at risk (OARs) using simultaneous integrated boost (SIB) for the hypoxic regions of the pancreatic cancer patients treated with stereotactic body radiotherapy (SBRT), and to predict an optimal way of SIB. Methods The setup corrections guided by 100 sets of CBCT scans of 10 patients previously treated with SBRT were imported to the treatment planning system (TPS) to recalculate the dose to the target and OARs. Two tumor control probability (TCP) models were applied to calculate the TCP under various hypoxic situations. The correlations between the TCP and target dose were analyzed. Results Without setup corrections, the PTV and ITV were underdosed by 8.9% and 9.2% on average respectively relative to planed dose. With setup corrections, the mean dose to PTV and ITV coverage were 1.6% and 1.3% lower than planned respectively. The mean deviations of OAR dose were between -0.11 Gy and 0.26 Gy for all plans. The predictive values of Dmean on hypoxic regions were 31.4, 34.0 and 37.2 Gy (Niemierko model) or 31.6, 33.9 and 37.2 Gy (Poisson model) when the oxygen enhancement ratios (OERs) were 1, 1.5 and 3 respectively. Conclusions With CBCT setup corrections, the dosimetric impacts of setup errors on the target and OARs can be neglected. Significant deviations of TCP calculation were observed without accounting for tumor hypoxia. To counteract the impacts of hypoxia, the mean dose to the hypoxic regions should be at least 1.24 times of prescribed dose. Key words: Simultaneous integrated boost; Setup error; TCP

Comparison of Treatment Plan Robustness Between Dynamic Trajectory Radiotherapy and VMAT

Dosimetric Implication of Neural Planning Organ at Risk Volumes in Stereotactic BodyRadiation Therapy of the Spine

Post-mastectomy adjuvant internal mammary lymph node irradiation — benefit and cardiac toxicity: an Indian experience

Objective To evaluate the effect of setup errors upon the target area and the organs at risk (OAR) during radiotherapy for prostate cancer. Methods Twelve prostate cancer patients receiving treatment in the recent 1 year were randomly recruited in this study. The position of each patient was verified by using cone beam CT (CBCT) for 6-10 times during the treatment. In treatment planning system (TPS), the isocenter position was moved along the setup errors with averaging error value (Plan_A) and each CBCT value (Plan_F). The dose distribution was recalculated without changing the beam setting, weight factors and monitor units (MUs). The dose difference was statistically compared between the simulation and original plans (Plan_O). Results For clinical target volume (CTV) D95, there was a significant difference between Plan_A and Plan_O (P=0.008), whereas no significant difference was observed between Plan_F and Plan_O. There were significant differences between Plan_F and Plan_O, Plan_A and Plan_O (P=0.004, and 0.041) for the planned target volume (PTV) D95.For OAR, rectal V60, Dmax, left femoral V20, Dmax and right femoral Dmax significantly differed between Plan_F and Plan_O (P=0.026, 0.015, 0.041, 0.049, 0.003). However, only left femoral Dmax significantly differed between Plan_A and Plan_O (P=0.045). The movement in the superior-inferior (SI) direction was significantly correlated with the changes in the rectal V40, V50 and V60 and PTV D95(r=-0.785, -0.887, -0.833, 0.682). The movement in the anterior-posterior (AP) direction was significantly associated with the variations in the bladder V20, V30, V40, V50 and V60(r=-0.945, -0.823, -0.853, -0.818, -0.774). The evaluation indexes of all normal tissues in the re-plan could meet the clinical requirements. However, the volume of target prescription volume had different levels of deficit, and the deficit of Plan_F was greater than that of Plan_A. Conclusions The simulation results of averaging into the TPS underestimates the effect of daily setup errors on the dose distribution. The effect of setup errors on the dose distribution in target area is greater than that of normal tissues. Y-direction errors are more likely to cause the variations of the rectal and PTV dose, and the errors in the z-direction are inclined to cause the changes in the bladder dose. Key words: Prostate neoplasm/radiotherapy; Setup error; Dose distribution

BackgroundImmobilization devices are essential for maintaining accurate and repeatable patient positioning in radiotherapy. This study aimed to evaluate the setup errors and dosimetric deviation induced by the deformation of immobilization devices in thoracic cancer radiotherapy using CT‐linac.Materials and methodsA retrospective analysis was conducted on 40 thoracic cancer patients who underwent radiotherapy, using vacuum cushion (VC) and thermoplastic mask (TM) for immobilization. A total of 206 weekly Fan‐beam CT (FBCT) images (4–7 per patient) were analyzed to manually delineate immobilization devices and assess their geometric deformations against setup errors. Dosimetric deviations between the clinical plan (CT‐plan) and the delivered plan (FBCT‐plan) were compared for planning target volume (PTV) and organs at risk (OARs). Correlations between dose variations and setup errors were analyzed in lateral (LAT), longitudinal (LNG), and vertical (VRT) axes.ResultsThe conformity of the VC (Nup) and TM (Ndown) with the patient in simulation CT exhibited moderate to strong correlations with VRT setup errors (Ndown: r = −0.484, p < 0.01;Nup: r = −0.697, p < 0.01). However, intra‐fraction deformation of immobilization devices (in FBCT) showed no significant correlation with setup errors. In the dosimetric analysis of OARs, lung dose parameters (Dmean, V5, V20) and heart Dmean exhibited a consistent absolute difference with increasing setup errors. Dose variation decreased significantly when errors exceeded 5 mm, particularly in the VRT direction for most PTV indices, with the exception of CI and HI. Spinal cord Dmax variation correlated significantly with setup accuracy along the LNG axis, but not along other axes.ConclusionThe conformity of immobilization devices in simulation CT exhibits a stronger correlation with setup accuracy than the deformation of these devices in intra‐fraction FBCT. FBCT is recommended for improving treatment precision through dosimetric assessment and planning adjustments.

Background and ObjectiveIn stereotactic body radiotherapy (SBRT) for lung cancer, the choice of volumetric modulated arc therapy (VMAT) optimization strategy is critical for achieving optimal target dose coverage while minimizing exposure to normal tissues. This study aims to compare the dosimetric performance and plan complexity of two VMAT optimization strategies in Monaco: single-beam dual-arc (1B2A) versus dual-beam single-arc (2B1A).MethodsA retrospective analysis was conducted on 50 lung cancer patients treated with SBRT (prescription dose: 50 Gy in 5 fractions). Two VMAT plans were re-optimized using the Monaco treatment planning system: the 1B2A plan (single-beam dual-arc, collimator angle 10°) and the 2B1A plan (dual-beam single-arc, collimator angles 10° and 350°). Dosimetric parameters, including target dose coverage, conformity index (CI), and gradient index (GI), were evaluated for the internal target volume (ITV) and planning target volume (PTV). Dose metrics for organs at risk (OARs) were also analyzed. Plan complexity was assessed based on monitor units (MU), number of control points, complexity index, and integral dose to normal tissues.ResultsSignificant dosimetric differences were observed between the two strategies. When normalized to ensure the prescribed 50 Gy isodose line covers 95% of the PTV volume, the high-dose parameters (D1%, D50%, Dmean) of the ITV and PTV were significantly lower in the 1B2A group compared to the 2B1A group (p < 0.001), indicating superior dose distribution with the 2B1A approach. Although the 1B2A plan exhibited marginally better CI, GI, and low-dose lung sparing (V5–V30), these differences were minimal and clinically insignicant. No substantial dierences were found in the dose sparing of other OARs, including the spinal cord, heart, and ribs. Additionally, the 1B2A plan required signicantly higher MU (+15.5%, p < 0.001) and had greater plan complexity (+9.47%, p < 0.001), suggesting lower treatment efficiency.ConclusionsIn peripheral lung cancer SBRT, the dual-beam single-arc (2B1A) strategy offers superior target dose distribution and treatment efficiency, making it a preferable optimization approach.

Objective To evaluate the dosage effect of position numbers in intensity-modulated radiotherapy plan (IMRT) for breast cancer. Method A 56 year old patient with breast cancer was selected and treated by IMRT in this study. According to the images of the position verification, the target dosage and organ at risk (OAR) were calculated and verified in detail. Results When the position number was 5, the doses of the target and OARs were CTV 48.8Gy, PTV 48.2Gy, spinal cord 27.5Gy, lung 5.5Gy with V 20 10 <33.7%, femoral head 38.2Gy. Although all the doses met the prescription of radiotherapeutic physicist, however, the doses of OAR closed to the dosage limitation. In order the decrease the doses of OAR, the number of position verification increased to 7. Therefore the doses of OAR decreased significantly. Conclusion The positioning error can be decreased by increasing the number of position verification.

BackgroundStereotactic radiotherapy (SRT) for kidney cancer, particularly when tumours are situated near critical organs-at-risk (OARs), presents significant challenges in achieving optimal dose delivery. MR-guided online adaptive radiotherapy (MRgoART) offers a promising solution by allowing real-time anatomical modification and plan reoptimisation. However, the ideal strategy for prescription dose selection and reoptimisation remains unclear.MethodsThis single-centre planning study evaluated kidney tumours located within 1 cm of gastrointestinal OARs. In Step 1, prescription doses for MRgoART were compared: the target dose (26 Gy) versus the planned dose (adjusted during pre-treatment planning to satisfy OAR constraints). In Step 2, two optimisation strategies were assessed: (1) covering 99% of the planning target volume (PTV) with the prescription dose (99%_xGy_Plan) and (2) delivering the full target dose with acceptable partial PTV coverage accepting dose heterogeneity (26Gy_x%_Plan), both respecting OAR constraints. Dose-volume parameters and blinded expert preferences were evaluated.ResultsOf 22 patients assessed, 14 patients with 18 tumours met the inclusion criteria. Among these, 36 MRgoART plans for 12 tumours were analysed in Step 1. Reoptimisation using the target dose resulted in significantly higher mean tumour doses, improved dose gradients, and PTV coverage metrics compared to reoptimisation based on the planned dose. In Step 2, 54 plans were assessed. Although the 26Gy_x%_Plan demonstrated superior mean tumour and PTV dose, it exhibited lower conformity. Radiation oncologists preferred the 26Gy_x%_Plan in 48% of cases, following 26% deemed almost equal, indicating its clinical advantage.ConclusionFor kidney tumours adjacent to gastrointestinal OARs, MRgoART planning should favour reoptimisation based on the target dose. A high-dose strategy with partial PTV coverage (26Gy_x%_Plan) was generally preferred by radiation oncologists, balancing therapeutic effectiveness with OAR protection.

PurposeLattice radiotherapy (LRT), a form of spatially fractionated radiation therapy (SFRT), has emerged as a promising approach for treating massive tumors. By delivering high‐dose regions within the tumor while sparing surrounding healthy tissue, LRT offers distinct advantages over conventional radiotherapy. Recent advancements in treatment planning systems (TPS), particularly the integration of intelligent optimization engines (IOEs) with automated planning capabilities, have the potential to further refine and expand the clinical utility of LRT. This study aimed to comparatively evaluate the planning quality and clinical feasibility of lattice SFRT treatment plans generated using the Ethos planning system, equipped with an IOE and O‐ring linear accelerator, versus the Eclipse planning system paired with a conventional C‐arm TrueBeam linac, in patients with stage III non‐small cell lung cancer (NSCLC).MethodsTwenty retrospective stage III NSCLC cases (GTV > 200 cc) with available PET‐CT imaging were selected. A total of 40 plans (20 Eclipse, 20 Ethos) were compared, incorporating lattice spheres (1 cm diameter, 2 cm spacing between spheres) placed in the tumor, FDG‐PET/CT‐informed intratumoral heterogeneity, prioritizing viable perinecrotic subregions while avoiding critical OARs. Plans aimed to deliver 15 Gy to lattice spheres, limit Valley (PTV minus spheres) doses to 2 Gy, and restrict doses to organs at risk (OARs) to ≤ 3 Gy. Dose conformity, OAR sparing, dose gradient parameters (PEDR, PVDR), planning time, and deliverability, which was evaluated using ArcCheck, EPID gamma analysis, and MLC log‐file verification.ResultsEthos demonstrated statistically significant improvements compared to Eclipse in lattice sphere mean dose (17.2 Gy vs. 15.83 Gy, p < 0.001), V15 Gy coverage (98.2 % vs. 91.74 %, p < 0.001), and dose gradient metrics (PEDR: 6.42 vs. 5.80; PVDR: 3.70 vs. 3.29; both p < 0.001, and VPDR: 0.131 vs. 0.135; PVDRDVH: 7.62 vs. 7.41). For the valley target, Ethos plans demonstrated a lower mean dose (Dmean: 4.72 Gy vs. 4.91 Gy, p = 0.064), although not statistically significant, and achieved significantly improved dose gradient at V7.5 Gy (14.5% vs. 16.35%, p = 0.019), V5Gy (30.77% vs. 34.84%, p = 0.006), and V2Gy (99.77% vs. 97.79%, p < 0.001) compared to Eclipse. Ethos achieved significantly better OAR sparing, particularly for the bronchial tree, heart, spinal cord, esophagus, and great vessels (all p < 0.01). Furthermore, Ethos substantially reduced planning time (36.55 vs. 95.96 min, p < 0.001). Both planning systems achieved high gamma passing rates (> 95%), confirming the accuracy and deliverability of the treatment plans.ConclusionThe Ethos automated treatment planning demonstrated superior lattice dose conformity, enhanced OAR sparing, and significantly faster optimization compared to Eclipse. This automated optimization capability highlights the potential of Ethos for efficient and effective lattice radiotherapy in managing massive NSCLC tumors.

Impact of Immobilization on Intrafraction Motion for Spine Stereotactic Body Radiotherapy Using Cone Beam Computed Tomography

Purpose: Automatic treatment planning can be used to determine achievable dose values before optimization. Reverse auto planning instead looks to find the highest target dose while still meeting critical structure objectives. A tool incorporating reverse auto planning is proposed for prediction of planning difficulty. Methods: A SQL database of 53 pancreas stereotactic body radiotherapy patients is populated with dose and structure information. Overlap volume histograms (OVH) are generated for each organ at risk (OAR) and target pair. For each structure, the lowest achievable target dose, which was calculated by selecting the lowest dose from all patients with a smaller distance to overlap, was queried together with maximum target dose. The lowest target dose from the list of maximum target dose per structure represents the highest achieved dose from the database population. Additional scaling can be used which scales the maximum achievable target dose per structure to the limiting dose of the OAR. Results: For patients analyzed using this tool, the predicted maximum target dose serves as a predictor of planning difficulty. If the predicted maximum target dose is high, plans which meet OAR objectives are easier to generate. If the predicted maximum target dose is low, plans are more difficult and planning objectives may not be achievable. Conclusion: A tool for predicting the maximum achievable target dose is developed. This tool estimates plan difficulty and achievability prior to planning and can be used to determine if dose escalation may be appropriate or if alternate treatment strategies should be used due to difficulty in achieving desired goals. Support by Philips Healthcare: Philips Radiation Oncology Services