Ravindra Yaparpalvi

Random Positional Variation Among the Skull, Mandible, and Cervical Spine With Treatment Progression During Head-and-Neck Radiotherapy

PURPOSE With 54 degrees of freedom from the skull to mandible to C7, ensuring adequate immobilization for head-and-neck radiotherapy (RT) is complex. We quantify variations in skull, mandible, and cervical spine movement between RT sessions. METHODS AND MATERIALS Twenty-three sequential head-and-neck RT patients underwent serial computed tomography. Patients underwent planned rescanning at 11, 22, and 33 fractions for a total of 93 scans. Coordinates of multiple bony elements of the skull, mandible, and cervical spine were used to calculate rotational and translational changes of bony anatomy compared with the original planning scan. RESULTS Mean translational and rotational variations on rescanning were negligible, but showed a wide range. Changes in scoliosis and lordosis of the cervical spine between fractions showed similar variability. There was no correlation between positional variation and fraction number and no strong correlation with weight loss or skin separation. Semi-independent rotational and translation movement of the skull in relation to the lower cervical spine was shown. Positioning variability measured by means of vector displacement was largest in the mandible and lower cervical spine. CONCLUSIONS Although only small overall variations in position between head-and-neck RT sessions exist on average, there is significant random variation in patient positioning of the skull, mandible, and cervical spine elements. Such variation is accentuated in the mandible and lower cervical spine. These random semirigid variations in positioning of the skull and spine point to a need for improved immobilization and/or confirmation of patient positioning in RT of the head and neck.

Point vs. volumetric bladder and rectal doses in combined intracavitary-interstitial high-dose-rate brachytherapy: Correlation and comparison with published Vienna applicator data

PURPOSE We correlated rectal and bladder point and volumetric dose data in patients treated for advanced cervix cancers with combined intracavitary-interstitial high-dose-rate (HDR) brachytherapy (BT). The results are compared with published Vienna applicator data. METHODS AND MATERIALS We retrospectively analyzed 30 individual combined intracavitary plus interstitial implants from 10 patients treated with external beam radiation therapy (EBRT) followed by HDR BT for locally advanced cervix carcinoma. EBRT consisted of 45 Gy to the pelvis followed by 9-14.4 Gy boost to involved parametria. BT consisted of a total dose of 21 Gy delivered in 7 Gy fraction. For each implant, CT-image-based simulation and image-guided BT treatment planning was performed. Bladder and rectal doses were evaluated and analyzed using both International commission on Radiation Units and Measurements (ICRU) reference points and dose-volume histograms. The cumulative doses to the rectum and bladder were calculated by combining contributions from external beam therapy and BT. To facilitate comparison with published literature, the total doses were normalized to equivalent dose in 2-Gy fractions (EQD2) using the equation EQD2total = EQD2EBRT + EQD2BT. RESULTS For the patient population considered, the mean ICRU bladder dose was 75 (+/-4) Gy3 compared to bladder D0.1 cc and D2 cc doses of 84 (+/-4) and 78 (+/-3) Gy3, respectively. The mean ICRU rectal dose was 73 (+/-4) Gy3 compared to rectal D0.1 cc and D2 cc doses of 79 (+/-5) and 74 (+/-4) Gy3, respectively. For rectum, the mean dose ratios (D0.1 cc/D(ICRU)) and (D2 cc/D(ICRU)) were 1.08 and 1.01, respectively, compared to Vienna applicator study mean dose ratios of 1.08 and 0.93, respectively. ICRU rectal dose correlated with volumetric rectal doses and best with volumetric D2 cc dose (rS = 0.91, p = 0.0003); however, ICRU bladder dose did not correlate with volumetric bladder dose. CONCLUSIONS Our study findings reveal a strong correlation between ICRU rectal reference dose and volumetric rectal D2 cc dose in combined intracavitary-interstitial HDR brachytherapy. This surrogate rectal-dose relationship is valuable in establishing rectal tolerance dose levels in transitioning from traditional two-dimensional to image-based three-dimensional dose planning.

The use of diode dosimetry in quality improvement of patient care in radiation therapy

The purpose of this work is to improve the quality of patient care in radiation therapy by implementing a comprehensive quality assurance (QA) program aiming to enhance patient in vivo dosimetry on a routine basis. The characteristics of two commercially available semi-conductor diode dosimetry systems were evaluated. The diodes were calibrated relative to an ionization chamber-electrometer system with calibrations traceable to the National Institute of Standards and Technology (NIST). Correction factors of clinical relevance were quantified to convert the diode readings into patient dose. The results of dose measurements on 6 patients undergoing external beam radiation therapy for carcinoma of the prostate on three different therapy units are presented. Field shaping during treatments was accomplished either by multileaf collimation or by cerrobend blocking. A deviation of less than +/-4% between the measured and prescribed patient doses was observed. The results indicate that the diodes exhibit excellent linearity, dose reproducibility, minimal anisotropy, and can be used with confidence for patient dose verification. Furthermore, diodes render real time verification of dose delivered to patients.

Technical Aspects of Positron Emission Tomography/Computed Tomography in Radiotherapy Treatment Planning

The usage of functional data in radiation therapy (RT) treatment planning (RTP) process is currently the focus of significant technical, scientific, and clinical development. Positron emission tomography (PET) using ((18)F) fluorodeoxyglucose is being increasingly used in RT planning in recent years. Fluorodeoxyglucose is the most commonly used radiotracer for diagnosis, staging, recurrent disease detection, and monitoring of tumor response to therapy (Lung Cancer 2012;76:344-349; Lung Cancer 2009;64:301-307; J Nucl Med 2008;49:532-540; J Nucl Med 2007;48:58S-67S). All the efforts to improve both PET and computed tomography (CT) image quality and, consequently, lesion detectability have a common objective to increase the accuracy in functional imaging and thus of coregistration into RT planning systems. In radiotherapy, improvement in target localization permits reduction of tumor margins, consequently reducing volume of normal tissue irradiated. Furthermore, smaller treated target volumes create the possibility of dose escalation, leading to increased chances of tumor cure and control. This article focuses on the technical aspects of PET/CT image acquisition, fusion, usage, and impact on the physics of RTP. The authors review the basic elements of RTP, modern radiation delivery, and the technical parameters of coregistration of PET/CT into RT computerized planning systems.

Clinical experiences with onboard imager KV images for linear accelerator-based stereotactic radiosurgery and radiotherapy setup.

PURPOSE To report our clinical experiences with on-board imager (OBI) kV image verification for cranial stereotactic radiosurgery (SRS) and radiotherapy (SRT) treatments. METHODS AND MATERIALS Between January 2007 and May 2008, 42 patients (57 lesions) were treated with SRS with head frame immobilization and 13 patients (14 lesions) were treated with SRT with face mask immobilization at our institution. No margin was added to the gross tumor for SRS patients, and a 3-mm three-dimensional margin was added to the gross tumor to create the planning target volume for SRT patients. After localizing the patient with stereotactic target positioner (TaPo), orthogonal kV images using OBI were taken and fused to planning digital reconstructed radiographs. Suggested couch shifts in vertical, longitudinal, and lateral directions were recorded. kV images were also taken immediately after treatment for 21 SRS patients and on a weekly basis for 6 SRT patients to assess any intrafraction changes. RESULTS For SRS patients, 57 pretreatment kV images were evaluated and the suggested shifts were all within 1 mm in any direction (i.e., within the accuracy of image fusion). For SRT patients, the suggested shifts were out of the 3-mm tolerance for 31 of 309 setups. Intrafraction motions were detected in 3 SRT patients. CONCLUSIONS kV imaging provided a useful tool for SRS or SRT setups. For SRS setup with head frame, it provides radiographic confirmation of localization using the stereotactic target positioner. For SRT with mask, a 3-mm margin is adequate and feasible for routine setup when TaPo is combined with kV imaging.

ICRU reference dose in an era of intensity-modulated radiation therapy clinical trials: correlation with planning target volume mean dose and suitability for intensity-modulated radiation therapy dose prescription.

BACKGROUND AND PURPOSE IMRT clinical trials lack dose prescription and specification standards similar to ICRU standards for two- and three-dimensional external beam planning. In this study, we analyzed dose distributions for patients whose treatment plans incorporated IMRT, and compared the dose determined at the ICRU reference point to the PTV doses determined from dose-volume histograms. Additionally, we evaluated if ICRU reference type single-point dose prescriptions are suitable for IMRT dose prescriptions. MATERIALS AND METHODS For this study, IMRT plans of 117 patients treated at our institution were randomly selected and analyzed. The treatment plans were clinically applied to the following disease sites: abdominal (11), anal (10), brain (11), gynecological (15), head and neck (25), lung (15), male pelvis (10) and prostate (20). The ICRU reference point was located in each treatment plan following ICRU Report 50 guidelines. The reference point was placed in the central part of the PTV and at or near the isocenter. In each case, the dose was calculated and recorded to this point. For each patient--volume and dose (PTV, PTV mean, median and modal) information was extracted from the planned dose-volume histogram. RESULTS The ICRU reference dose vs PTV mean dose relationship in IMRT exhibited a weak positive association (Pearson correlation coefficient 0.63). In approximately 65% of the cases studied, dose at the ICRU reference point was greater than the corresponding PTV mean dose. The dose difference between ICRU reference and PTV mean doses was 2% in approximately 79% of the cases studied (average 1.21% (+/-1.55), range -4% to +4%). Paired t-test analyses showed that the ICRU reference doses and PTV median doses were statistically similar (p=0.42). The magnitude of PTV did not influence the difference between ICRU reference and PTV mean doses. CONCLUSIONS The general relationship between ICRU reference and PTV mean doses in IMRT is similar to that in 3D CRT distributions. Point doses in IMRT are influenced by the degree of intensity modulation as well as calculation grid size utilized. Although the ICRU reference point type prescriptions conceptually may be extended for IMRT dose prescriptions and used as a representative of tumor dose, new universally acceptable dose prescription and specification standards for IMRT based on RTOG IMRT prescription model incorporating dose-volume specification would likely lead to greater consistency among treatment centers.

Radiation therapy of breast carcinoma: confirmation of prescription dose using diodes

PURPOSE To quantitate the dose delivered during tangential breast radiation therapy and measure the scatter dose to the contralateral breast for three different breast setup techniques. METHODS AND MATERIALS A commercial semiconductor diode system is used for dose measurements. The diode characteristics were studied by comparing the diode response against a standard ionization chamber response in a reference configuration. In vivo dose measurements on 11 patients undergoing tangential breast radiation therapy with 6 MV photons were performed. Medial and lateral field entrance and exit doses were measured and compared with the expected values from the treatment planning system. Scatter doses to the contralateral breast for three breast setup techniques were measured and documented as a function of distance from the field edge and various beam modifiers commonly used in breast radiation therapy. RESULTS The diodes used in this study exhibited excellent linearity, dose reproducibility, and minimal anisotropy. The in-phantom measurements resulted in dose accuracy within +/- 1.5%. Dose measurements on patients resulted in standard deviations of 1.2 and 2.3% for the medial entrance and exit doses and 1.7 and 2.2% for the lateral entrance and exit doses, respectively. In patients, the scatter doses to the opposite breast at a 5 cm perpendicular distance from the medial field edge resulted in cumulative scatter doses of 2.47 to 5.30 Gy from the tangential fields and an additional 0.50 Gy from the supraclavicular or axillary field, if included. CONCLUSION Quantitative verification of the prescribed daily dose is important in breast radiation therapy to ensure precision in patient setup and accuracy in dose delivery. Diodes provide a convenient way of real-time patient dose verification and are easy to use by the therapists.

An efficient Volumetric Arc Therapy treatment planning approach for hippocampal-avoidance whole-brain radiation therapy (HA-WBRT).

An efficient and simple class solution is proposed for hippocampal-avoidance whole-brain radiation therapy (HA-WBRT) planning using the Volumetric Arc Therapy (VMAT) delivery technique following the NRG Oncology protocol NRG-CC001 treatment planning guidelines. The whole-brain planning target volume (PTV) was subdivided into subplanning volumes that lie in plane and out of plane with the hippocampal-avoidance volume. To further improve VMAT treatment plans, a partial-field dual-arc technique was developed. Both the arcs were allowed to overlap on the in-plane subtarget volume, and in addition, one arc covered the superior out-of-plane sub-PTV, while the other covered the inferior out-of-plane subtarget volume. For all plans (n = 20), the NRG-CC001 protocol dose-volume criteria were met. Mean values of volumes for the hippocampus and the hippocampal-avoidance volume were 4.1 cm(3) ± 1.0 cm(3) and 28.52 cm(3) ± 3.22 cm(3), respectively. For the PTV, the average values of D(2%) and D(98%) were 36.1 Gy ± 0.8 Gy and 26.2 Gy ± 0.6 Gy, respectively. The hippocampus D(100%) mean value was 8.5 Gy ± 0.2 Gy and the maximum dose was 15.7 Gy ± 0.3 Gy. The corresponding plan quality indices were 0.30 ± 0.01 (homogeneity index), 0.94 ± 0.01 (target conformality), and 0.75 ± 0.02 (confirmation number). The median total monitor unit (MU) per fraction was 806 MU (interquartile range [IQR]: 792 to 818 MU) and the average beam total delivery time was 121.2 seconds (IQR: 120.6 to 121.35 seconds). All plans passed the gamma evaluation using the 5-mm, 4% criteria, with γ > 1 of not more than 9.1% data points for all fields. An efficient and simple planning class solution for HA-WBRT using VMAT has been developed that allows all protocol constraints of NRG-CC001 to be met.

Feasibility study and optimum loading pattern of a multi-ring inflatable intravaginal applicator

Purpose A cylinder applicator is the standard treatment device for intravaginal brachytherapy. However, they are limited in their ability to simultaneously spare the organs at risk (OAR), and reduce the hot spot in the vaginal mucosa, while achieving adequate dose conformality. This study aims to compare the dosimetric characteristics of single and multi-channel cylinders, and utilizes volume point dose optimizations to investigate the feasibility and optimum loading method for a multi-ring inflatable intravaginal applicator. Material and methods Studies were designed to: (1) test the feasibility of multi-ring applicators, (2) compare dose distributions between different multi-channel applicators and loading patterns, (3) test non-uniform prescription depths around the multi-ring cylinder. Results Compared to a cylinder with a single central channel, a cylinder with 6 lumina arranged around the periphery, providing the lumina had adequate distance to the cylinder surface, could reduce dose beyond the prescription depth. However, when the number of outer lumina increased from 6 to 12, no further dose reduction could be achieved and the high dose volume close to the surface of the cylinder increased. Moreover, an additional ring, with lumina further away from the surface, provided increased dose shaping capabilities, allowing for individualized dose distributions. Conclusions Dose could be reduced to normal tissue and the inner mucosa, and better conformity was seen to unique anatomical shapes. A modified peripheral loading pattern provided the optimum dose distribution, yielding good conformity, dose sparing at adjacent organs, and dose reduction in the high dose region of the vaginal mucosa.

Day to day treatment variations of accelerated partial breast brachytherapy using a multi-lumen balloon

Purpose To evaluate the variations of multi-lumen balloon (MLB)-based brachytherapy from simulation day to treatment day and their dosimetric impacts during accelerated partial breast irradiation (APBI). Material and methods A total of 42 CT images scanned from seven patients were evaluated with regards to daily variation due to of: 1) internal uncertainty: size and shape of balloon, seroma volume; 2) geometrical uncertainty-random: length of each catheter was measured for each fraction (total 70); 3) geometrical uncertainty-systematic: virtual systematic errors were tested by offsetting dwell positions. The original plans (as group A) had a mean value of 96.8% on V95 of the PTV_Eval. Plans were rerun (as group B) such that the mean value of the V95 was relaxed to 90.4%. By applying the reference plan to each daily CT image, variations of target coverage under different sources of error were evaluated. Results Shape and size of the balloon had means of < 1 mm decreased in diameter and < 0.4 cm3 decreased in volume; the mean seroma volume increased by 0.2 cm3. This internal variation has a mean of < 1% difference for both V90 and V95. The geometrical uncertainty made a mean deviation of 2.7 mm per root of sum of square. It caused the degradations of V90 and V95 by mean values of 1.0% and 1.2%, respectively. A systematic error of 3 mm and 4 mm would degrade both of V90 and V95 by 4% and 6%, respectively. The degradations on target coverage of the plans in group A were statistically the same as those in group B. Conclusions Overall, APBI treatments with MLB based brachytherapy are precise from day to day. However, minor variation due to daily treatment uncertainties can still degrade tumor bed coverage to an unacceptable coverage when V95 of the original plan is close to 90%.