Online daily assessment of dose change in head and neck radiotherapy without dose-recalculation.

BACKGROUND: Head and neck cancers are commonly treated with radiation therapy, but due to possible volume changes, plan adaptation may be required during the course of treatment. Currently, plan adaptations consume significant clinical resources. Existing methods to evaluate the need for plan adaptation requires deformable image registration (DIR) to a new CT simulation or daily cone beam CT (CBCT) images and the recalculation of the dose distribution. In this study, we explore a tool to assist the decision for plan adaptation using a CBCT without re-computation of dose, allowing for rapid online assessment. METHODS: This study involved 18 head and neck cancer patients treated with CBCT image guidance who had their treatment plan modified based on a new CT simulation (ReCT). Dose changes were estimated using different methods and compared to the current gold standard of using DIR between the planning CT scan (PCT) and ReCT with recomputed dose. The first and second methods used DIR between the PCT and daily CBCT with the planned dose or recalculated dose from the ReCT respectively, with the dose transferred to the CBCT using rigid registration. The necessity of plan adaptation was assessed by the change in dose to 95% of the planning target volume (D95) and mean dose to the parotids. RESULTS: The treatment plans were adapted clinically for all 18 patients but only 7 actually needed an adaptation yielding 11 unnecessary adaptations. Applying a method using the daily CBCT with the planned dose distribution would have yielded only four unnecessary adaptations and no missed adaptations: a significant improvement from that done clinically. CONCLUSION: Using the DIR between the planning CT and daily CBCT can flag cases for plan adaptation before every fraction while not requiring a new re-planning CT scan and dose recalculation.

A multivariable model to predict survival for patients with hepatic carcinoma or liver metastasis receiving radiotherapy.

AIM: New parameters that correlate with overall survival were identified in patients with liver lesions treated with radiation therapy. METHODS: Pretreatment information and parameters of radiation treatment plans for 129 metastatic and 66 hepatocellular carcinoma liver cancer patients were analyzed. Study end points included overall survival collected from patient charts and electronic records. RESULTS: Two practical nomograms were constructed for primary hepatocellular carcinoma and liver metastasis patients. For patients with a Child-Pugh A, radiation dose escalation provided a significant survival benefit. However, for those with Child-Pugh B or C, increasing dose does not impact on survival. CONCLUSION: The developed models can potentially guide dose selection and provide prognostic information but still require external validation.

Assessment of function and quality of life in a phase II multi-institutional clinical trial of fractionated simultaneous in-field boost radiotherapy for patients with 1-3 metastases.

We examined functional outcomes and quality of life of whole brain radiotherapy (WBRT) with integrated fractionated stereotactic radiotherapy boost (FSRT) for brain metastases treatment. Eighty seven people with 1-3 brain metastases (54/87 lung primary, 42/87 single brain metastases) were enrolled on this Phase II trial of WBRT (30 Gy/10) + simultaneous FSRT, (60 Gy/10). Median overall follow-up and survival was 5.4 months, 6 month actuarial intra-lesional control was 78 %; only 1 patient exhibited grade 4 toxicity (worsened seizures); most treatment related toxicity was grade 1 or 2; 2/87 patients demonstrated asymptomatic radiation necrosis on follow-up imaging. Mean (Min-Max) baseline KPS, Mini Mental Status Exam (MMSE) and FACT-BR quality of life were 83 (70-100), 28 (21-30) and 143 (98-153). Lower baseline MMSE (but not KPS or FACT-Br) was associated with worse survival after adjusting for age, number of metastases, primary and extra-cranial disease status. Crude rates of deterioration (>10 points decrease from baseline for KPS and FACT-Br, MMSE fall to <27) ranged from 26 to 38 % for KPS, 32-59 % for FACT-Br and 0-16 % for MMSE depending on the time-point assessed with higher rates generally noted at earlier time points (≤6 months post-treatment). Using a linear mixed models analysis, significant declines from baseline were noted for KPS and FACT-Br (largest effects at 6 weeks to 3 months) with no significant change in MMSE. The effects on function and quality of life of this integrated treatment of WBRT + simultaneous FSRT were similar to other published series combining WBRT + radiosurgery.

Survival prediction in high-grade gliomas using CT perfusion imaging.

Patients with high-grade gliomas usually have heterogeneous response to surgery and chemoirradiation. The objectives of this study were (1) to evaluate serial changes in tumor volume and perfusion imaging parameters and (2) to determine the value of these data in predicting overall survival (OS). Twenty-nine patients with World Health Organization grades III and IV gliomas underwent magnetic resonance (MR) and computed tomography (CT) perfusion examinations before surgery, and 1, 3, 6, 9, and 12 months after radiotherapy. Serial measurements of tumor volumes and perfusion parameters were evaluated by receiver operating characteristic analysis, Cox proportional hazards regression, and Kaplan-Meier survival analysis to determine their values in predicting OS. Higher trends in blood flow (BF), blood volume (BV), and permeability-surface area product in the contrast-enhancing lesions (CEL) and the non-enhancing lesions (NEL) were found in patients with OS < 18 months compared to those with OS ≥ 18 months, and these values were significant at selected time points (P < 0.05). Only CT perfusion parameters yielded sensitivities and specificities of ≥ 70% in predicting 18 and 24 months OS. Pre-surgery BF in the NEL and BV in the CEL and NEL 3 months after radiotherapy had sensitivities and specificities >80% in predicting 24 months OS in patients with grade IV gliomas. Our study indicated that CT perfusion parameters were predictive of survival and could be useful in assessing early response and in selecting adjuvant treatment to prolong survival if verified in a larger cohort of patients.

Comparative analysis of image guidance in two institutions for prostate cancer patients.

AIM/BACKGROUND: The analysis of systematic and random errors obtained from the pooled data on inter-fraction prostate motion during radiation therapy in two institutions. MATERIALS AND METHODS: Data of 6085 observations for 216 prostate cancer patients treated on tomotherapy units in two institutions of position correction shifts obtained by co-registration of planning and daily CT studies were investigated. Three independent variables: patient position (supine or prone), target (prostate or prostate bed), and imaging mode (normal or coarse) were analyzed. Systematic and random errors were evaluated and used to calculate the margins for different options of referencing based on the position corrections observed with one, three, or five imaging sessions. RESULTS: Statistical analysis showed that only the difference between normal and coarse modes of imaging was significant, which allowed to merge the supine and prone position sub-groups as well as the prostate and prostate bed patients. In the normal and coarse imaging groups, the margins calculated using systematic and random errors in the medio-lateral and cranio-caudal directions (5.5 mm and 4.5 mm, respectively) were similar, but significantly different (5.3 mm for the normal mode and 7.1 mm for the coarse mode) in the anterio-posterior direction. The reference scheme based on the first three fractions (R3) was found to be the optimal one. CONCLUSIONS: The R3 reference scheme effectively reduced systematic and random errors. Larger margins in the anterio-posterior direction should be used during prostate treatment on the tomotherapy unit, as coarse imaging mode is chosen in order to reduce imaging time and dose.

A study of longitudinal tumor motion in helical tomotherapy using a cylindrical phantom.

Tumor motion during radiation treatment on a helical tomotherapy unit may create problems due to interplay with motion of the multileaf collimator, gantry rotation, and patient couch translation through the gantry. This study evaluated this interplay effect for typical clinical parameters using a cylindrical phantom consisting of 1386 diode detectors placed on a respiratory motion platform. All combinations of radiation field widths (1, 2.5, and 5 cm) and gantry rotation periods (16, 30, and 60 s) were considered for sinusoidal motions with a period of 4 s and amplitudes of 5, 6, 7, 8, 9, and 10 mm, as well as real patient breathing pattern. Gamma comparisons with 2% dose difference and 2 mm distance to agreement and dose profiles were used for evaluation. The required motion margins were determined for each set of parameters. The required margin size increased with decreasing field width and increasing tumor motion amplitude, but was not affected by rotation period. The plans with the smallest field width of 1 cm have required motion margins approximately equal to the amplitude of motion (± 25%), while those with the largest field width of 5 cm had required motion margins approximately equal to 20% of the motion amplitude (± 20%). For tumor motion amplitudes below 6 mm and field widths above 1 cm, the required additional motion margins were very small, at a maximum of 2.5 mm for sinusoidal breathing patterns and 1.2 mm for the real patient breathing pattern.

Dosimetric Comparison of Ultra-Hypofractionated and Conventionally Fractionated Radiation Therapy Boosts for Patients with High-Risk Prostate Cancer.

Recent comparison of an ultra-hypofractionated radiotherapy (UF-RT) boost to a conventionally fractionated (CF-RT) option showed similar toxicity and disease control outcomes. An analysis of the treatment plans for these patients is needed for evaluating calculated doses for different organs, treatment beam-on time, and requirements for human and financial resources. Eighty-six plans for UF-RT and 93 plans for CF-RT schemes were evaluated. The biologically equivalent dose, EQD2, summed for the first phase and the boost, was calculated for dose-volume parameters for organs at risk (OARs), as well as for the PTV1. ArcCHECK measurements for the boost plans were used for a comparison of planned and delivered doses. Monitor units and beam-on times were recorded by the Eclipse treatment planning system. Statistical analysis was performed with a significance level of 0.05. Dosimetric parameter values for OARs were well within tolerance for both groups. EQD2 for the PTV1 was on average 84 Gy for UF-RT patients and 76 Gy for CF-RT patients. Gamma passing rate for planned/delivered doses comparison was above 98% for both groups with 3 mm/3% distance to agreement/dose difference criteria. Total monitor units per fraction were 647 ± 94 and 2034 ± 570 for CF-RT and UF-RT, respectively. The total delivery time for boost radiation for the patients in the UF-RT arm was, on average, four times less than the total time for a conventional regimen with statistically equal clinical outcomes for the two arms in this study.

Surface dose measurement for helical tomotherapy.

PURPOSE: To compare the surface dose measurements made by different dosimeters for the helical tomotherapy (HT) plan in the case of the target close to the surface. METHODS: Surface dose measurements in different points for the HT plan to deliver 2 Gy to the planning target volume (PTV) at 5 mm below the surface of the cylindrical phantom were performed by radiochromic films, single use metal oxide semiconductor field-effect transistor (MOSFET) dosimeters, silicon IVD QED diode, and optically stimulated luminescence (OSL) dosimeters. RESULTS: The measured doses by all dosimeters were within 12 +/- 8% difference of each other. CONCLUSIONS: Radiochromic films, EBT, and EBT2, provide high spatial resolution, although it is difficult to get accurate measurements of dose. Both the OSL and QED measured similar dose to that of the MOSFET detectors. The QED dosimeter is promising as a reusable on-line wireless dosimeter, while the OSL dosimeters are easier to use, require minimum setup time and are very precise.

Evaluation of tomotherapy MVCT image enhancement program for tumor volume delineation.

The aims of this study were to investigate the variability between physicians in delineation of head and neck tumors on original tomotherapy megavoltage CT (MVCT) studies and corresponding software enhanced MVCT images, and to establish an optimal approach for evaluation of image improvement. Five physicians contoured the gross tumor volume (GTV) for three head and neck cancer patients on 34 original and enhanced MVCT studies. Variation between original and enhanced MVCT studies was quantified by DICE coefficient and the coefficient of variance. Based on volume of agreement between physicians, higher correlation in terms of average DICE coefficients was observed in GTV delineation for enhanced MVCT for patients 1, 2, and 3 by 15%, 3%, and 7%, respectively, while delineation variance among physicians was reduced using enhanced MVCT for 12 of 17 weekly image studies. Enhanced MVCT provides advantages in reduction of variance among physicians in delineation of the GTV. Agreement on contouring by the same physician on both original and enhanced MVCT was equally high.

KVCT, MVCT, and hybrid CT image studies--treatment planning and dose delivery equivalence on helical tomotherapy.

PURPOSE: To determine the equivalence of radiation therapy treatment planning and delivery for various imaging options on helical tomotherapy. METHODS: Seven treatment plans using identical anatomy and planning parameters were created based on the following CT studies: Standard kilovoltage CT (kVCT); 2, 4, and 6 mm spacing megavoltage CT (MVCT); and 2, 4, and 6 mm hybrid MVCT/kVCT studies. In addition, two kVCT based plans were created to explore the effect of the choice of dose calculation grid for optimization. Calculated and measured dose distributions were compared via volumetric and dosimetric analysis at the planning stage, point dose measurements with ion chamber, along with EDR2 film data for gamma function analysis for distance to agreement of 3 mm and dose differences of 3%, 5%, and 7% using both the commercially available "cheese" phantom and the new QUASAR Verification (QVer) phantom. RESULTS: Plans created for each imaging option showed residual error increasing as image slice spacing increased and critical structure size decreased. With the exception of the low dose area in hybrid studies, point dose measurements were within the calculated/measured dose acceptance criteria of 5% on both the QVer and cheese phantoms. Gamma analysis for the original kVCT plan delivery showed an average of 98.5% +/- 0.5% and 98.8% +/- 0.3% of dose pixels passing kVCT study treatment and delivery quality assurance procedures, respectively. The QVer phantom allows for delivery quality assurance with simultaneous use of two films and more convenient gamma function assessment but shows some measurement discrepancy up to 10% compared to the cheese phantom. CONCLUSIONS: The kVCT, MVCT, and kVCT/MVCT hybrid studies showed considerable agreement at both planning and delivery stages. The choice of calculation grid is more important when dealing with smaller anatomical structures.

Application of helical tomotherapy in genitourinary malignancies.

INTRODUCTION: Helical tomotherapy (HT) is an innovative approach to the delivery of intensity-modulated radiation therapy which combines the imaging elements of helical computed tomography (CT) with megavoltage linear accelerator treatment. The purpose of this report is to describe our experience with the clinical implementation of HT for genitourinary malignancies. MATERIALS AND METHODS: All patients treated with a primary genitourinary malignancy were included in this study cohort. Descriptive statistics for various demographic and treatment-related parameters such as patient age, primary site of disease, site of radiotherapy, goal of treatment, dose/fractionation, immobilization and clinical trial enrolment were calculated. RESULTS: A total of 57 patients diagnosed with a primary genitourinary malignancy were treated on the helical tomotherapy unit during the study period. Median age was 69 years (range 45 to 83 years) and 56 (98.2%) patients were male. Prostate cancer was the most frequently treated genitourinary cancer in this cohort of 57 (94.7%) cases. Ten patients (17.5%) were treated with palliative intent, 46 (80.7%) with radical intent (including full dose prostate bed adjuvant/salvage RT), and one (1.8%) patient was treated in a purely adjuvant manner (high risk postop bladder). CONCLUSIONS: HT is a technology that can be utilized in both radical and palliative genitourinary treatment situations in order to deliver precise conformal IMRT therapy with unique localization and critical structure avoidance properties.

Dosimetric comparison of intensity-modulated radiosurgery and helical tomotherapy for the treatment of multiple intracranial metastases.

The purpose of this study was to evaluate the dosimetry of single fraction, single-isocenter intensity-modulated radiosurgery (IMRS) plans for multiple intracranial metastases and to compare Helical Tomotherapy (HT). Ten treatment plans with 3-6 brain metastases treated with IMRS were re-planned with HT. The mean number of lesions was 5 and mean PTV 22 cm(3). The prescribed dose was 16-20 Gy. The mean V100% was similar for IMRS and HT, and the mean conformity index was 1.4, mean Paddick confirmity index was 0.7, and mean MDPD was 1.1 for both. The mean gradient index was similar for both. The mean 50% _isodose volume was 179.2 cm(3) for IMRS and 277.0 cm(3) for HT (p=0.01). The mean maximum doses to organs at risk were lower for IMRS except brainstem and right optic nerve. For brain, the integral dose was 5.1 and 6.8 Gy-kg (p<0.001) and mean dose 4.0 and 5.4 Gy (p<0.001) for IMRS and HT, respectively. The mean treatment times were 23 (IMRS) and 41 (HT) minutes. Conformity and homogeneity indices were equivalent and sparing of the organs at risk was clinically acceptable for both IMRS and HT. Though the gradient index was similar for IMRS and HT, the mean 50% isodose volume and integral dose to normal brain were lower for IMRS as was treatment time.

Successful treatment of primary renal lymphoma using image guided helical tomotherapy.

PURPOSE: To describe a clinical pilot case of renal lymphoma successfully treated using helical tomotherapy, and to evaluate alternative hypofractionated treatment schedules and their potential applicability to future cases of renal cell carcinoma (RCC). PATIENTS AND METHODS: An 82-year-old female patient with a large right perinephric mass encircling the lower pole of the right kidney was treated on the Hi-ART unit (TomoTherapy Inc. Madison, WI, USA) with daily pretreatment megavoltage CT imaging. Gross tumor volumes (GTVs) were outlined on every MVCT study. The Planned Adaptive software was used for calculation of dosimetric parameters for both the target and organs at risk (OARs). In response to observed GTV regression, a hypothetical anatomy changes adjusted plan was generated and analyzed. Six alternative treatment schedules were investigated: 48 Gy in 4 and 3 fractions, and 60 Gy in 30, 5, 4 and 3 fractions, as possible clinical scenarios for RCC. Normal tissue complication probability (NTCP) and tumor control probability (TCP) values were estimated for each scenario in the study. RESULTS: During 30 days, the GTV was reduced by 50.6%. The smaller GTV and the reduced planning target volume (PTV) margins from 15 mm to 10 mm after 12 fractions would allow for a decrease of the planned mean liver and spinal cord dose by 3.8 Gy and 4 Gy, respectively. Improvements to portions of the colon include a 3.3 Gy and 9.2 Gy reduction in planned mean dose to the descending and ascending colons, respectively. NTCP and TCP estimates have shown that hypofractionated treatment schedules provide a much higher probability of local control, but the risk of tissue complication rises simultaneously. For this particular case, hypofractionation would not be suitable due to the potential adverse affects brought on to the liver. CONCLUSIONS: Caution should be observed in high dose hypofractionated radiotherapy in right sided, whole kidney carcinoma due to increased risk of liver complication. The accelerated treatment may however be justified by the significantly higher TCP rates for left sided kidney cases. Further investigation of small renal tumors is needed to evaluate control rates, vasculopathy, and residual normal function.

Proliferation saturation index in an adaptive Bayesian approach to predict patient-specific radiotherapy responses.

Purpose: Radiotherapy prescription dose and dose fractionation protocols vary little between individual patients having the same tumor grade and stage. To personalize radiotherapy a predictive model is needed to simulate radiation response. Previous modeling attempts with multiple variables and parameters have been shown to yield excellent data fits at the cost of non-identifiability and clinically unrealistic results. Materials and methods: We develop a mathematical model based on a proliferation saturation index (PSI) that is a measurement of pre-treatment tumor volume-to-carrying capacity ratio that modulates intrinsic tumor growth and radiation response rates. In an adaptive Bayesian approach, we utilize an increasing number of data points for individual patients to predict patient-specific responses to subsequent radiation doses. Results: Model analysis shows that using PSI as the only patient-specific parameter, model simulations can fit longitudinal clinical data with high accuracy (R2=0.84). By analyzing tumor response to radiation using daily CT scans early in the treatment, response to the remaining treatment fractions can be predicted after two weeks with high accuracy (c-index = 0.89). Conclusion: The PSI model may be suited to forecast treatment response for individual patients and offers actionable decision points for mid-treatment protocol adaptation. The presented work provides an actionable image-derived biomarker prior to and during therapy to personalize and adapt radiotherapy.

A Phase II Multi-institutional Clinical Trial Assessing Fractionated Simultaneous In-Field Boost Radiotherapy for Brain Oligometastases.

Purpose/Objective Published preclinical and phase I clinical trial data suggest that fractionated lesional radiotherapy with 60 Gy in 10 fractions can serve as an alternative approach to single fraction radiosurgical boost for brain oligometastases.  Methods and Materials A phase II clinical trial (NCT01543542) of a total of 60 Gy in 10 fractions of lesional (one to three) radiotherapy (given simultaneously with whole-brain helical tomotherapy with 30 Gy in 10 fractions) was conducted at five institutions. We hypothesized that fractionated radiotherapy would be considered unsuitable if the median overall survival (OS) was degraded by two months or if six-month intracranial control (ICC) and intracranial lesion (ILC) were inferior by 10% compared with the published RTOG 9508 results. Results A total of 87 patients were enrolled over a 4.5-year accrual period. Radiological lesion and extralesional central nervous system progression were documented in 15/87 (17%) and 11/87 (13%) patients, respectively. Median OS for all patients was 5.4 months. Six-month actuarial estimates of ICC and ILC were 78% and 89%, respectively. However, only the ILC estimate achieved statistical significance (p=0.02), demonstrating non-inferiority to the a priori historical controls (OS: p=0.09, ICC=0.31). Two patients developed suspected asymptomatic radionecrosis. Conclusions The phase II estimates of ILC were demonstrated to be non-inferior to the results of the RTOG 9508.

Image registration of a moving target phantom with helical tomotherapy: effect of the CT acquisition technique and action level proposal.

This study aims to quantify the effects of target motion and resultant motion artifacts in planning and megavoltage CT (MVCT) studies on the automatic registration processes of helical tomotherapy. Clinical and experimental data were used to derive an action level for patient repositioning on helical tomotherapy. Planning CT studies of a respiratory motion phantom were acquired using conventional and four-dimensional CT (4D CT) techniques. MVCT studies were acquired on helical tomotherapy in the presence and absence of target motion and were registered with different planning CT studies. The residual errors of the registration process were calculated from the registration values to quantify the ability of the process to detect 5 or 10 mm translations of the phantom in two directions. Twenty-seven registration combinations of MVCT inter-slice spacing, technique and resolution were investigated. The residual errors were used as an estimate of the localization error of the registration process, and the accuracy of couch repositioning was determined from couch position measurements during 866 treatment fractions. These two parameters were used to calculate the action level for patient repositioning on helical tomotherapy. Automatic registration of an MVCT study with 0% breathing phase, average intensity and maximum intensity 4D CT projections did not differ from that of an MVCT study with a conventional planning CT. Motion artifacts in the MVCT or planning CT studies changed the accuracy of the automatic registration process by less than 2.0%. The action level for patient repositioning using MVCT studies of 6 mm inter-slice spacing was determined to be 0.7, 1.1 and 0.6 mm in the x-, y- and z-directions, respectively. These action levels have the greatest effect on treatments for disease sites in the brain.

Prognostic Significance of Tumor Location for Liver Cancer Radiotherapy.

Introduction According to the Surveillance, Epidemiology and End Results (SEER) data, cancerous involvement of the liver is on an increase over the last three decades. It occurs worldwide in all races and carries a poor prognosis. Currently, considerable progress has been made in patient selection, staging, surgery, chemotherapy agents, and stereotactic radiotherapy in both primary and metastatic liver cancers with improved outcomes. While there is evidence of the prognostic factors of liver function, the involvement of the portal vein, inferior vena cava thrombosis, lesion size, radiation dose, number of fractions, and SBRT techniques, there is no study evaluating outcomes with the location of the lesion. Our aim in this retrospective study was to explore the correlation of tumor location from the portal vein bifurcation (vascular wall) and the radiotherapy outcome (survival) in hepatocellular cancer. Methods Contrast-enhanced computed tomography (CT) studies in 86 patients with liver cancer were retrospectively reviewed in an institutional review board (IRB)-approved database to determine the distance to the bifurcation point of the portal vein from tumor's centre of mass (distance tumor bifurcation: DTB) and from the edge point of the planning target volume closest to the bifurcation (distance edge bifurcation: DEB). The mean dose to the sphere of 1 cm diameter around the bifurcation point (mean dose at bifurcation: MDB) was calculated. These parameters were tested as predictors of patient outcomes using univariate and multivariate analysis as two groups of patients. Results Only the DEB correlation with survival for hepatocellular carcinoma (HCC) was found to be significant (P = 0.028). A larger MDB is caused by a smaller DTB and a smaller DEB. The hazard ratio for DTB, DEB, and MDB were 0.48, 0.41, and 1.05, respectively. The DEB was found to be a better predictor of outcomes (overall survival) compared to the DTB and MDB parameters. The close proximity of the tumor to the blood supply vessels was a decisive factor. The DTB parameter is also dependent on the size of the tumor and this factor weakens the correlation of this parameter on survival data. The inclusion of the dosimetric and geometric location, as well as distance parameters in predictive models for liver cancer patients, was shown to benefit the pre-selection of treatment options for liver cancer patients treated with radiotherapy. Conclusion For hepatocellular cancer patients, the distance between the edge point of the planning treatment volume (PTV) to the portal vein bifurcation (DEB) of more than 2 cm was found to be a predictor of survival.

Adaptive radiotherapy planning on decreasing gross tumor volumes as seen on megavoltage computed tomography images.

PURPOSE: To evaluate gross tumor volume (GTV) changes for patients with non-small-cell lung cancer by using daily megavoltage (MV) computed tomography (CT) studies acquired before each treatment fraction on helical tomotherapy and to relate the potential benefit of adaptive image-guided radiotherapy to changes in GTV. METHODS AND MATERIALS: Seventeen patients were prescribed 30 fractions of radiotherapy on helical tomotherapy for non-small-cell lung cancer at London Regional Cancer Program from Dec 2005 to March 2007. The GTV was contoured on the daily MVCT studies of each patient. Adapted plans were created using merged MVCT-kilovoltage CT image sets to investigate the advantages of replanning for patients with differing GTV regression characteristics. RESULTS: Average GTV change observed over 30 fractions was -38%, ranging from -12 to -87%. No significant correlation was observed between GTV change and patient's physical or tumor features. Patterns of GTV changes in the 17 patients could be divided broadly into three groups with distinctive potential for benefit from adaptive planning. CONCLUSIONS: Changes in GTV are difficult to predict quantitatively based on patient or tumor characteristics. If changes occur, there are points in time during the treatment course when it may be appropriate to adapt the plan to improve sparing of normal tissues. If GTV decreases by greater than 30% at any point in the first 20 fractions of treatment, adaptive planning is appropriate to further improve the therapeutic ratio.