A Dose Falloff Gradient Study in RapidArc Planning of Lung Stereotactic Body Radiation Therapy

Introduction: Radiation Therapy Oncology Group (RTOG) report #0813 and 0915 recommends using D2cm and R50% as plan quality metrics for evaluation of normal tissue sparing in stereotactic body radiation therapy (SBRT) of lung lesion. This study introduces dose falloff gradient (DFG) as a tool for analyzing the dose beyond the planning target volume (PTV) extending into normal tissue structures. In ascertaining the impact of PTV size and SBRT planning techniques in DFG, this study questions the independence of the RTOG recommended metrics. Materials and Methods: In this retrospective study, 41 RapidArc lung SBRT plans with 2 or 3 complete or partial arcs were analyzed. PTV volumes ranged between 5.3 and 113 cm3 and their geographic locations were distributed in both lungs. 6MV, 6 MV-FFF, 10 MV, or 10 MV-FFF energies were used. RTOG-0915 metrics conformity index, homogeneity index, D2cm, R50%, and HDloc were evaluated. DFG was computed from the mean and maximum dose in seven concentric 5 mm wide rings outside the PTV. DFG was investigated against the volume of normal lung irradiated by 50% isodose volume. Treatment plans with alternate energy and couch rotations were generated. Results: The dose falloff beyond PTV was modeled using a double exponential fit and evaluated for relationship with intermediate lung dose. Photon energy and beam configuration had a minimal impact on the dose falloff outside. The product of normalized D2cm and R50% was estimated to have a slowly varying value. Conclusions: Dose falloff outside PTV has been studied as a function of radial distance and ascertained by intermediate dose to normal lung. DFG can serve as a complementary plan quality metric

Dosimetric and radiobiological comparison for quality assurance of IMRT and VMAT plans

INTRODUCTION: The gamma analysis used for quality assurance of a complex radiotherapy plan examines the dosimetric equivalence between planned and measured dose distributions within some tolerance. This study explores whether the dosimetric difference is correlated with any radiobiological difference between delivered and planned dose. METHODS: VMAT or IMRT plans optimized for 14 cancer patients were calculated and delivered to a QA device. Measured dose was compared against planned dose using 2-D gamma analysis. Dose volume histograms (for various patient structures) obtained by interpolating measured data were compared against the planned ones using a 3-D gamma analysis. Dose volume histograms were used in the Poisson model to calculate tumor control probability for the treatment targets and in the Sigmoid dose-response model to calculate normal tissue complication probability for the organs at risk. RESULTS: Differences in measured and planned dosimetric data for the patient plans passing at ≥94.9% rate at 3%/3 mm criteria are not statistically significant. Average ± standard deviation tumor control probabilities based on measured and planned data are 65.8±4.0% and 67.8±4.1% for head and neck, and 71.9±2.7% and 73.3±3.1% for lung plans, respectively. The differences in tumor control probabilities obtained from measured and planned dose are statistically insignificant. However, the differences in normal tissue complication probabilities for larynx, lungs-GTV, heart, and cord are statistically significant for the patient plans meeting ≥94.9% passing criterion at 3%/3 mm. CONCLUSION: A ≥90% gamma passing criterion at 3%/3 mm cannot assure the radiobiological equivalence between planned and delivered dose. These results agree with the published literature demonstrating the inadequacy of the criterion for dosimetric QA and suggest for a tighter tolerance

Impact of jaw position on sparing organs at risk in 3-dimensional conformal radiation therapy of pancreatic cancer

Purpose: The objective of this work is to investigate the impact of collimator jaw position on dose to organs at risk (OARs) during a 3-dimensional conformal radiotherapy (3DCRT) of pancreatic cancer and postulate a method to minimize OAR dose by proper positioning of the jaws.Methods: Clinically delivered 3DCRT treatment plans for 10 patients optimized with multiple static beams using multileaf collimator (MLC) leaves conformed to a block margin around target, and collimator jaws aligned with outer extent of the block margin were selected. Subsequent plans were generated by displacing the collimator jaws outward in lateral, superior-inferior or both directions by 1 and 2 cm without altering the MLC position. Computed dose to OARs and target with unaltered dose normalization were compared against the corresponding dose obtained from the original plans.Results: Outward displacement of the collimator jaws by 1 cm in lateral and/or superior-inferior direction resulted in a significant increase in mean dose to the studied OARs. The increase was found to be proportional to the outward displacement of the jaws. The increase in maximum dose to spinal cord was significant in a few patients while it was insignificant for all other OARs.Conclusion: Collimator jaws aligned with outer extent of a block margin minimize dose to OARs. Any gap between the block margin and the collimator jaws can lead to an inadvertent delivery of higher dose to the OARs. Hence, the use of an optimal jaw position during treatment planning becomes important to all patient plans

Gonadal-sparing total body irradiation with the use of helical tomotherapy for nonmalignant indications

Background: The aim was to demonstrate the feasibility and technique of gonadal sparing total body irradiation (TBI) with helical tomotherapy. Total body irradiation is a common part of the conditioning regimen prior to allogeneic stem cell transplantation. Shielding or dose-reduction to the gonads is often desired to preserve fertility, particularly in young patients undergoing transplant for non-malignant indications. Helical tomotherapy (HT) has been shown to be superior to traditional TBI delivery for organ at risk (OAR) doses and dose homogeneity. Materials and methods: We present two representative cases (one male and one female) to illustrate the feasibility of this technique, each of whom received 3Gy in a single fraction prior to allogeneic stem cell transplant for benign indications. The planning target volume (PTV) included the whole body with a subtraction of OARs including the lungs, heart, and brain (each contracted by 1cm) as well as the gonads (testicles expanded by 5 cm and ovaries expanded by 0.5 cm). Results: For the male patient we achieved a homogeneity index of 1.35 with a maximum and median planned dose to the testes of 0.53 Gy and 0.35 Gy, respectively. In-vivo dosimetry demonstrated an actual received dose of 0.48 Gy. For the female patient we achieved a homogeneity index of 1.13 with a maximum and median planned dose to the ovaries of 1.66 Gy and 0.86 Gy, respectively. Conclusion: Gonadal sparing TBI is feasible and deliverable using HT in patients with non-malignant diseases requiring TBI as part of a pre-stem cell transplant conditioning regimen

Quantification of the skin sparing effect achievable with high-energy photon beams when carbon fiber tables are used

To quantify the skin doses resulting from the use of carbon fiber couches (CFCs) for patient support. BrainLab’s CFC was evaluated for five prostate patients and five lung patients. For each patient PTV, organs at risk (OARs), and a 0.3 cm thick skin contour on the patient’s posterior surface were outlined. Two sets of IMRT plans, each consisting of 4, 5, 7, and 9 beams, were generated per patient. The sets were identical with the exception that in the first set only 6 MV energy was used, while in the second set (mixed energy) the photon energy of the beams traversing the CFC was 18 MV. The plans for each patient were normalized to deliver the same dose to 95% of the PTV. The CFC skin dose was evaluated by the maximum dose received by 1% ( D 1%) of the skin volume. Paired one-tailed t-tests were used to establish the statistical significance. The mixed energy plans resulted in D 1% increase from 18% to more than 23% as the number of beams in the plan was decreased from 9 to 4. Skin doses as high as ∼70% of the prescription dose were found even in 9-beam mixed energy plans. Therefore mixed energy plans may be more beneficial for patients treated with higher fractional doses

High dose bystander effects in spatially fractionated radiation therapy

Traditional radiotherapy of bulky tumors has certain limitations. Spatially fractionated radiation therapy (GRID) and intensity modulated radiotherapy (IMRT) are examples of advanced modulated beam therapies that help in significant reductions in normal tissue damage. GRID refers to the delivery of a single high dose of radiation to a large treatment area that is divided into several smaller fields, while IMRT allows improved dose conformity to the tumor target compared to conventional three-dimensional conformal radiotherapy. In this review, we consider spatially fractionated radiotherapy approaches focusing on GRID and IMRT, and present complementary evidence from different studies which support the role of radiation induced signaling effects in the overall radiobiological rationale for these treatments

Practice Patterns of Pediatric Total Body Irradiation Techniques: A Children\u27s Oncology Group Survey

PURPOSE: The aim of this study was to examine current practice patterns in pediatric total body irradiation (TBI) techniques among COG member institutions. METHODS AND MATERIALS: Between November 2019 and February 2020, a questionnaire containing 52 questions related to the technical aspects of TBI was sent to medical physicists at 152 COG institutions. The questions were designed to obtain technical information on commonly used TBI treatment techniques. Another set of 9 questions related to the clinical management of patients undergoing TBI was sent to 152 COG member radiation oncologists at the same institutions. RESULTS: Twelve institutions were excluded because TBI was not performed in their institutions. A total of 88 physicists from 88 institutions (63% response rate) and 96 radiation oncologists from 96 institutions (69% response rate) responded. The anterior-posterior/posterior-anterior (AP/PA) technique was the most common technique reported (49 institutions [56%]); 44 institutions (50%) used the lateral technique, and 14 (16%) used volumetric modulated arc therapy or tomotherapy. Midplane dose rates of 6 to 15 cGy/min were most commonly used. The most common specification for lung dose was the midlung dose for both AP/PA techniques (71%) and lateral techniques (63%). Almost all physician responders agreed with the need to refine current TBI techniques, and 79% supported the investigation of new TBI techniques to further lower the lung dose. CONCLUSIONS: There was no consistency in the practice patterns, methods for dose measurement, and reporting of TBI doses among COG institutions. The lack of standardization precludes meaningful correlation between TBI doses and clinical outcomes including disease control and normal tissue toxicity. The COG radiation oncology discipline is currently undertaking several steps to standardize the practice and dose reporting of pediatric TBI using detailed questionnaires and phantom-based credentialing for all COG centers

A dose falloff gradient study in RapidArc planning of lung stereotactic body radiation therapy

Introduction: Radiation Therapy Oncology Group (RTOG) report #0813 and 0915 recommends using D2cmand R50%as plan quality metrics for evaluation of normal tissue sparing in stereotactic body radiation therapy (SBRT) of lung lesion. This study introduces dose falloff gradient (DFG) as a tool for analyzing the dose beyond the planning target volume (PTV) extending into normal tissue structures. In ascertaining the impact of PTV size and SBRT planning techniques in DFG, this study questions the independence of the RTOG recommended metrics. Materials and Methods: In this retrospective study, 41 RapidArc lung SBRT plans with 2 or 3 complete or partial arcs were analyzed. PTV volumes ranged between 5.3 and 113 cm3 and their geographic locations were distributed in both lungs. 6MV, 6 MV-FFF, 10 MV, or 10 MV-FFF energies were used. RTOG-0915 metrics conformity index, homogeneity index, D2cm, R50%, and HDlocwere evaluated. DFG was computed from the mean and maximum dose in seven concentric 5 mm wide rings outside the PTV. DFG was investigated against the volume of normal lung irradiated by 50% isodose volume. Treatment plans with alternate energy and couch rotations were generated. Results: The dose falloff beyond PTV was modeled using a double exponential fit and evaluated for relationship with intermediate lung dose. Photon energy and beam configuration had a minimal impact on the dose falloff outside. The product of normalized D2cmand R50%was estimated to have a slowly varying value. Conclusions: Dose falloff outside PTV has been studied as a function of radial distance and ascertained by intermediate dose to normal lung. DFG can serve as a complementary plan quality metric

Lung dose for minimally moving thoracic lesions treated with respiration gating

To evaluate incidental doses to benign lung tissue for patients with minimally moving lung lesions treated with respiratory gating. Seventeen lung patient plans were studied retrospectively. Tumor motion was less than 5 mm in all cases. For each patient, mid-ventilation (MidVen) and mid-inhalation (MidInh) breathing phases were reconstructed. The MidInh phase was centered on the end-of-inhale (EOI) phase within a 30% gating window. Planning target volumes, heart, and spinal cord were delineated on the MidVen phase and transferred to the MidInh phase. Lungs were contoured separately on each phase. Intensity-modulated radiotherapy plans were generated on the MidVen phases. The plans were transferred to the MidInh phase, and doses were recomputed. The evaluation metric was based on dose indices, volume indices, generalized equivalent uniform doses, and mass indices for targets and critical structures. Statistical tests were used to establish the significance of the differences between the reference (MidVen) and compared (MidInh) dose distributions. Statistical tests demonstrated that the indices evaluated for targets, cord, and heart differed by within 2.3%. The index differences in the lungs, however, are in excess of 6%, indicating the potentially achievable lung sparing and/or dose escalation. Respiratory gating is a clinical option for patients with minimally moving lung lesions treated at EOI. Gating will be more beneficial for larger tumors, since dose escalation in those cases will result in a larger increase in the tumor control probability

History and current perspectives on the biological effects of high-dose spatial fractionation and high dose-rate approaches: GRID, Microbeam & FLASH radiotherapy

The effects of various forms of ionising radiation are known to be mediated by interactions with cellular and molecular targets in irradiated and in some cases non-targeted tissue volumes. Despite major advances in advanced conformal delivery techniques, the probability of normal tissue complication (NTCP) remains the major dose-limiting factor in escalating total dose delivered during treatment. Potential strategies that have shown promise as novel delivery methods in achieving effective tumour control whilst sparing organs at risk involve the modulation of critical dose delivery parameters. This has led to the development of techniques using high dose spatial fractionation (GRID) and ultra-high dose rate (FLASH) which have translated to the clinic. The current review discusses the historical development and biological basis of GRID, microbeam and FLASH radiotherapy as advanced delivery modalities that have major potential for widespread implementation in the clinic in future years