Germ Cell Tumor of the Testis: Lethal Subtypes of a Curable Cancer.

Germ cell tumor of the testis (GCT) is a curable cancer even when it is widely metastatic; however, outcomes can differ based on tumor histology. Chemo-resistance in certain phenotypes, such as teratoma and yolk sac tumor, contributes to poor clinical outcomes in some patients with GCT. Despite this resistance to S-YSTemic therapy, many of these tumor subtypes remain amenable to surgical resection and possible cure. In this study, we report on a series of seven patients highlighting two chemo-resistant subtypes of nonseminomatous germ cell tumor (NSGCT), sarcomatoid yolk sac tumor (S-YST), and epithelioid trophoblastic tumor (ETT) for which early resection rather than additional salvage chemotherapy or high-dose intense chemotherapy might provide a superior clinical outcome and enhance cure rate.

Stem Cell Theory of Cancer: Clinical Implications for Cellular Metabolism and Anti-Cancer Metabolomics.

Although Otto Warburg may be right about the role of glycolysis versus OXPHOS in cancer metabolism, it remains unclear whether an altered metabolism is causative or correlative and is the main driver or a mere passenger in the pathogenesis of cancer. Currently, most of our successful treatments are designed to eliminate non-cancer stem cells (non-CSCs) such as differentiated cancer cells. When the treatments also happen to control CSCs or the stem-ness niche, it is often unintended, unexpected, or undetected for lack of a pertinent theory about the origin of cancer that clarifies whether cancer is a metabolic, genetic, or stem cell disease. Perhaps cellular context matters. After all, metabolic activity may be different in different cell types and their respective microenvironments-whether it is in a normal progenitor stem cell vs. progeny differentiated cell and whether it is in a malignant CSC vs. non-CSC. In this perspective, we re-examine different types of cellular metabolism, e.g., glycolytic vs. mitochondrial, of glucose, glutamine, arginine, and fatty acids in CSCs and non-CSCs. We revisit the Warburg effect, an obesity epidemic, the aspartame story, and a ketogenic diet. We propose that a pertinent scientific theory about the origin of cancer and of cancer metabolism influences the direction of cancer research as well as the design of drug versus therapy development in cancer care.

A Prospective Study Measuring Resident and Faculty Contour Concordance: A Potential Tool for Quantitative Assessment of Residents' Performance in Contouring and Target Delineation in Radiation Oncology Residency.

PURPOSE/OBJECTIVE(S): Accurate target delineation (ie, contouring) is essential for radiation treatment planning and radiotherapy efficacy. As a result, improving the quality of target delineation is an important goal in the education of radiation oncology residents. The purpose of this study was to track the concordance of radiation oncology residents' contours with those of faculty physicians over the course of 1 year to assess for patterns. MATERIALS/METHODS: Residents in postgraduate year (PGY) levels 2 to 4 were asked to contour target volumes that were then compared to the finalized, faculty physician-approved contours. Concordance between resident and faculty physician contours was determined by calculating the Jaccard concordance index (JCI), ranging from 0, meaning no agreement, to 1, meaning complete agreement. Multivariate mixed-effect models were used to assess the association of JCI to the fixed effect of PGY level and its interactions with cancer type and other baseline characteristics. Post hoc means of JCI were compared between PGY levels after accounting for multiple comparisons using Tukey's method. RESULTS: In total, 958 structures from 314 patients collected during the 2020-2021 academic year were studied. The mean JCI was 0.77, 0.75, and 0.61 for the PGY-4, PGY-3, and PGY-2 levels, respectively. The JCI score for PGY-2 was found to be lower than those for PGY-3 and PGY-4, respectively (all P < .001). No statistically significant difference of JCI score was found between the PGY-3 and PGY-4 levels. The average JCI score was lowest (0.51) for primary head and/or neck cancers, and it was highest (0.80) for gynecologic cancers. CONCLUSIONS: Tracking and comparing the concordance of resident contours with faculty physician contours is an intriguing method of assessing resident performance in contouring and target delineation and could potentially serve as a quantitative metric, which is lacking currently, in radiation oncology resident evaluation. However, additional study is necessary before this technique can be incorporated into residency assessments.

Stem Cell Origin of Cancer: Clinical Implications for Cancer Immunity and Immunotherapy.

A simple way to understand the immune system is to separate the self from non-self. If it is self, the immune system tolerates and spares. If it is non-self, the immune system attacks and destroys. Consequently, if cancer has a stem cell origin and is a stem cell disease, we have a serious problem and a major dilemma with immunotherapy. Because many refractory cancers are more self than non-self, immunotherapy may become an uphill battle and pyrrhic victory in cancer care. In this article, we elucidate cancer immunity. We demonstrate for whom, with what, as well as when and how to apply immunotherapy in cancer care. We illustrate that a stem cell theory of cancer affects our perspectives and narratives of cancer. Without a pertinent theory about cancer's origin and nature, we may unwittingly perform misdirected cancer research and prescribe misguided cancer treatments. In the ongoing saga of immunotherapy, we are at a critical juncture. Because of the allure and promises of immunotherapy, we will be treating more patients not immediately threatened by their cancer. They may have more to lose than to gain, if we have a misconception and if we are on a wrong mission with immunotherapy. According to the stem cell theory of cancer, we should be careful with immunotherapy. When we do not know or realize that cancer originates from a stem cell and has stem-ness capabilities, we may cause more harm than good in some patients and fail to separate the truth from the myth about immunotherapy in cancer care.

Cleaning the dose falloff with low modulation in SBRT lung plans.

Purpose.This dosimetric study is intended to lower the modulation factor in lung SBRT plans generated in the Eclipse TPS that could replace highly modulated plans that are prone to the interplay effect.Materials and methods.Twenty clinical lung SBRT plans with high modulation factors (≥4) were replanned in Varian Eclipse TPS version 15.5 utilizing 2 mm craniocaudal and 1 mm axial block margins followed by light optimization in order to reduce modulation. A unique plan optimization methodology, which utilizes a novel shell structure (OptiForR50) for R50%optimization in addition to five consecutive concentric 5 mm shells, was utilized to control dose falloff according to RTOG 0813 and 0915 recommendations. The prescription varied from 34-54 Gy in 1-4 fractions, and the dose objectives were PTV D95%= Rx, PTV Dmax< 140% of Rx, and minimizing the modulation factor. Plan evaluation metrics included modulation factor, CIRTOG, homogeneity index (HI), R50%, D2cm, V105%, and lung V8-12.8Gy(Timmerman Constraint). A random-intercept linear mixed effects model was used with a p ≤ 0.05 threshold to test for statistical significance.Results.The retrospectively generated plans had significantly lower modulation factors (3.65 ± 0.35 versus 4.59 ± 0.54; p < 0.001), lower CIRTOG(0.97 ± 0.02 versus 1.02 ± 0.06; p = 0.001), higher HI (1.35 ± 0.06 versus 1.14 ± 0.04; p < 0.001), lower R50%(4.09 ± 0.45 versus 4.56 ± 0.56; p < 0.001), and lower lungs V8-12.8Gy(Timmerman) (4.61% ± 3.18% versus 4.92% ± 3.37%; p < 0.001). The high dose spillage V105%was borderline significantly lower (0.44% ± 0.49% versus 1.10% ± 1.64%; p = 0.051). The D2cmwas not statistically different (46.06% ± 4.01% versus 46.19% ± 2.80%; p = 0.835).Conclusion.Lung SBRT plans with significantly lower modulation factors can be generated that meet the RTOG constraints, using our planning strategy.

Stem Cell Origin of Cancer: Implications of Oncogenesis Recapitulating Embryogenesis in Cancer Care.

From this perspective, we wonder about the clinical implications of oncology recapturing ontogeny in the contexts of neoantigens, tumor biomarkers, and cancer targets. We ponder about the biological ramifications of finding remnants of mini-organs and residuals of tiny embryos in some tumors. We reminisce about classical experiments showing that the embryonic microenvironment possesses antitumorigenic properties. Ironically, a stem-ness niche-in the wrong place at the wrong time-is also an onco-niche. We marvel at the paradox of TGF-beta both as a tumor suppressor and a tumor promoter. We query about the dualism of EMT as a stem-ness trait engaged in both normal development and abnormal disease states, including various cancers. It is uncanny that during fetal development, proto-oncogenes wax, while tumor-suppressor genes wane. Similarly, during cancer development, proto-oncogenes awaken, while tumor-suppressor genes slumber. Importantly, targeting stem-like pathways has therapeutic implications because stem-ness may be the true driver, if not engine, of the malignant process. Furthermore, anti-stem-like activity elicits anti-cancer effects for a variety of cancers because stem-ness features may be a universal property of cancer. When a fetus survives and thrives despite immune surveillance and all the restraints of nature and the constraints of its niche, it is a perfect baby. Similarly, when a neoplasm survives and thrives in an otherwise healthy and immune-competent host, is it a perfect tumor? Therefore, a pertinent narrative of cancer depends on a proper perspective of cancer. If malignant cells are derived from stem cells, and both cells are intrinsically RB1 negative and TP53 null, do the absence of RB1 and loss of TP53 really matter in this whole narrative and an entirely different perspective of cancer?

Facilitators and barriers to employment for rural women cancer survivors.

PURPOSE: Limited research exists on the employment experiences of rural women cancer survivors, yet this population may face unique barriers to employment following a cancer diagnosis. This study aims to identify facilitators and barriers to employment for rural women cancer survivors. METHODS: We used a qualitative descriptive design to examine facilitators and barriers to employment for rural women cancer survivors. We conducted interviews with 33 rural women with cancer histories. RESULTS: Facilitators of employment included paid time off, flexible work arrangements, and supportive workplace social networks, while barriers to employment included compromised immunity, long-term treatment effects, stigma and discrimination, and limited rural job markets. Rural women with secure employment histories generally experienced facilitators of employment, while rural women with insecure (e.g., temporary, informal, non-standard) employment histories generally faced barriers to retaining jobs and finding employment. CONCLUSIONS: Formal and informal workplace support helped rural women retain their jobs during and following cancer treatment, especially those with secure employment. However, women with insecure employment histories generally faced multiple barriers to retaining and finding employment. More inclusive policies to support workers facing disabling illnesses, such as paid medical leave, are needed to ensure cancer survivors can maintain employment and/or financial security during and following their cancer treatment. IMPLICATIONS FOR CANCER SURVIVORS: Cancer survivors with secure employment may benefit from formal and informal workplace support in retaining their employment. Those with insecure employment histories may benefit from access to job placement services and inclusive policies protecting employment for all workers experiencing disabling illness.

The Role of Radiotherapy Among Patients With Prostate Ductal Adenocarcinoma.

INTRODUCTION: The aim of this study was to perform a Surveillance, Epidemiology, and End Results (SEER) analysis on the effect of radiotherapy (RT) on survival among patients with prostate ductal adenocarcinoma (DA), a rare variant of prostate cancer. PATIENTS AND METHODS: Cases of T1 to 4 N0 M0 prostate DA diagnosed between 2004 and 2013 were extracted from SEER. The association between categorical variables and radiation therapy was assessed for statistical significance using the χ2 test or Fisher exact test. Difference in continuous variables across the RT groups was assessed for statistical significance using the 2-sample t test or non-parametric test. The distribution of overall survival (OS) and disease-specific survival (DSS) between the RT groups was assessed using the Kaplan-Meier method and the log rank test and after propensity matching. The association between hazards of death (HR) and covariates was examined using Cox proportional hazards model. A 2-sided P-value of .05 was used to determine statistical significance. RESULTS: A total of 205 patients met inclusion criteria. On univariate analysis, RT was associated with significant improvement in OS and DSS. On multivariate Cox regression, RT significantly decreased risk of death for both OS and DSS (HR, 0.516; 95% confidence interval [CI], 0.273-0.978 and HR, 0.232; 95% CI, 0.082-0.658, respectively). After propensity score matching, RT demonstrated a persistent improvement in both OS and DSS. CONCLUSIONS: RT decreased risk of death for both OS and DSS in patients with node-negative, nonmetastatic prostate DA on multivariable analysis. RT was also associated with improved OS and DSS after propensity matching.

Brachytherapy vs external beam therapy among NSCLC patients undergoing limited surgical resection.

PURPOSE: To compare brachytherapy to external beam radiation therapy (EBRT) with respect to overall survival (OS) and disease-specific survival (DSS) among NSCLC patients undergoing limited surgical resection. METHODS: All cases of T1-4 N0 M0 NSCLC undergoing limited resection and either brachytherapy or EBRT diagnosed between 2004 and 2014 were extracted from the Surveillance, Epidemiology, and End Results database. Chi-square test and Fisher's exact analysis were used to analyze categorical variables while Student's t-test was used to analyze continuous variables. Univariate analysis to assess for differences in survival with respect to covariates was performed with the log-rank test. Multivariable analysis was performed with Cox proportional hazards regression models among the entire cohort and after sub-stratification by T stage. RESULTS: Among 543 patients, 471 underwent EBRT and 72 underwent brachytherapy. Brachytherapy demonstrated improved OS and DSS on univariate analysis as compared to EBRT (p < 0.05). Cox regression also demonstrated improved OS and DSS with brachytherapy (HR 0.604; 95% CI [0.380; 0.961] and HR 0.524; 95% CI [0.303; 0.908], respectively). Sub-cohort analysis demonstrated significant improvement in survival only among patients with T1 disease with similar survival between brachytherapy and EBRT among higher stage disease. CONCLUSIONS: Patients undergoing brachytherapy for T1-T4, N0, M0 NSCLC demonstrated at least similar survival as compared to those undergoing EBRT among patients undergoing limited resection. Improved survival was demonstrated among patients with T1 disease.

The effect of post mastectomy radiation therapy on survival in breast cancer patients with N1mic disease.

BACKGROUND: The role of post mastectomy radiation therapy (PMRT) in patients with N1mic breast cancer has not been well defined. A retrospective analysis was performed using the SEER database to evaluate the impact of PMRT on survival in patients with N1mic breast cancer. MATERIALS AND METHODS: Women with T1-T2, N1mic, M0 breast cancer who had undergone mastectomy were analyzed. Descriptive statistics were calculated for all variables. Univariate analysis to assess for differences in survival with respect to covariates was performed using the log rank test while multivariate analysis was performed with Cox proportional hazards regression. Sub-cohort analysis with propensity score matching was used to assess differences in survival among patients undergoing PMRT vs no PMRT. Comparisons were considered statistically significant at P < 0.05. RESULTS: Among 5878 patients, 1202 (20%) underwent PMRT. On univariate analysis, PMRT was a significant predictor of CSS, but not OS. There was no difference in either OS or CSS between the PMRT vs no PMRT groups on multivariate Cox regression analysis and after propensity score matching. CONCLUSIONS: Among patients with T1-T2, N1mic, M0 breast IDC from the SEER database, there was no difference in either OS or CSS among patients who underwent PMRT vs no PMRT. These results suggest that PMRT does not impact survival among breast cancer patients with N1mic disease. However, additional prospective studies with longer follow up are necessary for further evaluation.

The role of poly(ADP-ribose) polymerase inhibitors in the treatment of cancer and methods to overcome resistance: a review.

Poly(ADP-ribose) polymerase (PARP) inhibitors represent one of the successful novel approaches to targeted cancer treatment. Indeed, the US Food and Drug Administration (FDA) has recently approved PARP inhibitors for the treatment of breast and ovarian cancers. Despite the proven efficacy of these agents, certain challenges remain with their use. Among the most important are primary and secondary resistance. Here, we review the mechanism of action of PARP inhibitors and their ability to exploit certain inherent deficiencies among malignant cells to improve cell killing, with a focus on deficiencies in homologous recombination among cells with BRCA1 and BRCA2 mutations. Moreover, we discuss the different mechanisms of resistance including development of secondary resistance and strategies to overcome them. Finally, we discuss the limitations of novel therapeutic interventions and possible future studies to exploit biochemical pathways in order to improve therapeutic efficacy of PARP inhibitors.

The epidemiology and role of surgery in the treatment of urethral clear cell carcinoma.

PURPOSE: To perform a retrospective analysis of the epidemiology and role of surgery on survival in patients with urethral clear cell carcinoma (UCCC) using the National Cancer Institute's Surveillance, Epidemiology, and End Results (SEER) database. METHODS: UCCC cases diagnosed from January 1, 1973, to December 31, 2014, were extracted from SEER. Descriptive statistics were calculated for all variables. Univariate analysis to assess for differences in survival with respect to covariates was performed using the log-rank test. Multivariate analysis was performed with Cox proportional hazards regression models to determine the predictive performance of covariates with respect to overall survival (OS) and disease-specific survival (DSS), reported as hazard ratio (HR) with 95% CIs. Comparisons were considered statistically significant at P < 0.05. RESULTS: Sixty-one cases were extracted for analysis. Mean age ± SD was 63.0 ± 13.9 years. Fifty (82%), 18 (29.5%), and 14 (23.0%) patients underwent surgery, radiation, and chemotherapy, respectively. On univariate analysis, the following covariates were associated with both OS and DSS: age, stage, and surgery (all P < 0.001). On multivariate analysis, surgery was a predictor for improved OS and DSS (HR, 0.178; 95% CI [0.068; 0.464]) and HR, 0.166; 95% CI [0.057; 0.484], respectively). Neither radiation nor chemotherapy was significantly associated with OS or DSS. CONCLUSION: Surgery was associated with improved OS and DSS in patients with UCCC. While neither radiation nor chemotherapy was significantly associated with survival, additional studies are necessary to determine how these therapeutic interventions may impact prognosis.

The Role of Immunotherapy and Radiation Therapy in Tumor Chemosensitivity in Advanced Head and Neck Cancer.

BACKGROUND Cancer is the second leading cause of death internationally, resulting in millions of deaths each year. While treatment in the past has heavily relied on surgery and radiotherapy, chemotherapy and immunotherapy are being increasingly utilized depending on disease presentation. CASE REPORT A 56-year-old male presented to the Emergency Department with a 3-week history of a rapidly enlarging left supraclavicular neck mass. Computed tomography scan revealed a 12×13 cm mass extending from the angle of the mandible to the supraclavicular area. A biopsy confirmed advanced stage squamous cell carcinoma of the head and neck. The patient was started on a chemotherapy regimen of docetaxel, cisplatin, and 5-fluorouracil (TCF). The tumor progressed through chemotherapy, which was switched to cetuximab; however, this therapy was discontinued after an anaphylactic reaction. Palliative radiation treatment was begun along with pembrolizumab. Pembrolizumab was continued, and after 9 cycles, the patient's cancer was almost in complete remission. Three months later, disease progression was once again noted with pembrolizumab treatment, which was subsequently discontinued. The patient was started on paclitaxel and carboplatin chemotherapy regimen as a last resort, despite failure of prior TCF treatment, and the patient responded, this time with complete remission in 4 months. CONCLUSIONS This case demonstrates a unique outcome in which a patient who previously was resistant to chemotherapy, later responded to chemotherapy after a trial of radiation therapy and immunotherapy. Immunotherapy may have a synergistic effect with radiation therapy and play a role in tumor sensitivity to chemotherapy in head and neck cancer treatment.

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.

Robust optimization in lung treatment plans accounting for geometric uncertainty.

Robust optimization generates scenario-based plans by a minimax optimization method to find optimal scenario for the trade-off between target coverage robustness and organ-at-risk (OAR) sparing. In this study, 20 lung cancer patients with tumors located at various anatomical regions within the lungs were selected and robust optimization photon treatment plans including intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) plans were generated. The plan robustness was analyzed using perturbed doses with setup error boundary of ±3 mm in anterior/posterior (AP), ±3 mm in left/right (LR), and ±5 mm in inferior/superior (IS) directions from isocenter. Perturbed doses for D99 , D98 , and D95 were computed from six shifted isocenter plans to evaluate plan robustness. Dosimetric study was performed to compare the internal target volume-based robust optimization plans (ITV-IMRT and ITV-VMAT) and conventional PTV margin-based plans (PTV-IMRT and PTV-VMAT). The dosimetric comparison parameters were: ITV target mean dose (Dmean ), R95 (D95 /Dprescription ), Paddick's conformity index (CI), homogeneity index (HI), monitor unit (MU), and OAR doses including lung (Dmean , V20 Gy and V15 Gy ), chest wall, heart, esophagus, and maximum cord doses. A comparison of optimization results showed the robust optimization plan had better ITV dose coverage, better CI, worse HI, and lower OAR doses than conventional PTV margin-based plans. Plan robustness evaluation showed that the perturbed doses of D99 , D98 , and D95 were all satisfied at least 99% of the ITV to received 95% of prescription doses. It was also observed that PTV margin-based plans had higher MU than robust optimization plans. The results also showed robust optimization can generate plans that offer increased OAR sparing, especially for normal lungs and OARs near or abutting the target. Weak correlation was found between normal lung dose and target size, and no other correlation was observed in this study.

Technical Note: A planning technique to lower normal tissue toxicity in lung SBRT plans based on two likely dependent RTOG metrics.

PURPOSE: Intermediate- and low-dose falloff in stereotactic body radiotherapy (SBRT) of lung tumor is known to relate to normal tissue toxicity. The purpose is twofold to analyze the relation between RTOG parameters (namely, R50%, D2cm) in lung SBRT plans and to explore planning methods that correlate with higher than acceptable dose to normal tissue. METHODS: RTOG recommended target dose coverage, conformity index, homogeneity index, R50%, and D2cm were evaluated retrospectively in 105 lung tumor SBRT plans. Deviations in R50% and D2cm were correlated with parameters including prescription dose, tumor location, number of beams or arcs, beam configuration (coplanar or noncoplanar), type of treatment plan (3D-CRT, IMRT or volumetric arc therapy), and shortest distance to the chest wall. RESULT: All plans met the target coverage, conformity index, homogeneity index, and critical organ dose tolerance objectives. Dose falloff product (DFP) of R50% and D2cm has a small variance and small dependence on PTV. Low correlation between DFP and PTV suggests that R50% and D2cm are not independent. Coplanar beam placement was found to be prevalent among plans with large deviations in R50%, D2cm. CONCLUSION: This study questions the independence of the two RTOG recommended metrics, R50% and D2cm in lung SBRT plans, and suggests that noncoplanar beams may provide better normal tissue sparing by reducing the intermediate dose falloff.

Therapeutic benefits in grid irradiation on Tomotherapy for bulky, radiation-resistant tumors.

INTRODUCTION: Spatially fractionated radiation therapy (SFRT or grid therapy) has proven to be effective in management of bulky tumors. The aim of this project is to study the therapeutic ratio (TR) of helical Tomotherapy (HT)-based grid therapy using linear-quadratic cell survival model. MATERIAL AND METHODS: HT-based grid (or HT-GRID) plan was generated using a patient-specific virtual grid pattern of high-dose cylindrical regions using MLCs. TR was defined as the ratio of normal tissue surviving fraction (SF) under HT-GRID irradiation to an open debulking field of an equivalent dose that result in the same tumor cell SF. TR was estimated from DVH data on ten HT-GRID patient plans with deep seated, bulky tumor. Dependence of the TR values on radiosensitivity of the tumor cells and prescription dose was analyzed. RESULTS: The mean ± standard deviation (SD) of TR was 4.0 ± 0.7 (range: 3.1-5.5) for the 10 patients with single fraction maximum dose of 20 Gy to GTV assuming a tumor cell SF at 2 Gy (SF2t) value of 0·5. In addition, the mean ± SD of TR values for SF2t values of 0.3 and 0.7 were found to be 1 ± 0.1 and 18.0 ± 5.1, respectively. Reducing the prescription dose to 15 and 10 Gy lowered the respective TR values to 2.0 ± 0.2 and 1.2 ± 0.04 for a SF2t value of 0.5. CONCLUSION: HT-GRID therapy demonstrates a significant therapeutic advantage over uniform dose from an open field irradiation for the same tumor cell kill. TR increases with the radioresistance of the tumor cells and with prescription dose.

A comparative study to evaluate the efficacy of on board imaging with cone beam CT using target registration in patients with lung tumors undergoing stereotactic body radiation therapy and comparison with ExacTrac using skeletal registration on Novalis Tx.

BACKGROUND: Stereotactic body radiation therapy is an advanced technique, which delivers ablative doses to lung lesions. Target verification is done either by orthogonal x-rays or cone beam CT. This study was undertaken to compare these two verification methods. AIM: To evaluate the efficacy of ExacTrac and Cone Beam Computed Tomography (CBCT) for target repositioning while delivering Stereotactic Body Radiation Therapy (SBRT) for lung lesions and derive the population-based margin. MATERIALS AND METHODS: All patients who had undergone SBRT for lung lesions from February to September 2009 were involved. Patients were immobilized using the BodyFix double vacuum immobilization system, indexed to the computed tomography (CT) simulator and treatment machine. Four-dimensional (3-D) scan was done to generate internal target volume (ITV) and a free breathing CT scan for planning was done on the BrainLab iPlan 4.1 software. During treatment, patient's position was verified using ExacTrac and CBCT. The resulting vertical, lateral, and longitudinal shifts were noted. The random and systematic error were calculated and the margin recipe derived using the Van Herk formula. RESULTS: Sixteen patients had undergone SBRT for lung tumors from February to September 2009. Data from eight patients who had undergone 34 sessions of SBRT was analyzed. The systematic error for lateral, longitudinal, and vertical shifts for ExacTrac and CBCT were 3.68, 4.27, 3.5 mm and 0.53, 0.38, 0.70 mm, respectively. The random error were 1.10, 1.51, 1.96 mm and 0.32, 0.81, 0.59 mm. The lateral, longitudinal and vertical Van Herk margin recipe for ExacTrac were 9.98, 11.72, 10.18 mm, respectively, and for CBCT was 2.17, 1.53,1.55 mm. CONCLUSIONS: The systematic and random errors for CBCT were significantly lesser as compared to the errors with Exactrac.