R Zwicker

Partial breast brachytherapy after lumpectomy: Low-dose-rate and high-dose-rate experience

PURPOSE The use of partial breast brachytherapy (PBB) after lumpectomy for selected patients with early-stage breast cancer reduces the adjuvant radiotherapy treatment time to <1 week. Despite the advantages of accelerated treatment, maintaining an acceptable cosmetic outcome is important. In a cohort of patients who received low-dose-rate (LDR) or high-dose-rate (HDR) PBB after lumpectomy, the clinical characteristics and treatment parameters were analyzed to identify predictors for an unfavorable cosmetic outcome. METHODS AND MATERIALS Early-stage breast cancer patients with clear resection margins and 0-3 positive lymph nodes were eligible for PBB. Uniform guidelines for target definition and brachytherapy catheter placement were applied. The HDR PBB dose was 34 Gy in 10 fractions within 5 days, and the LDR dose was 45 Gy given at a rate of 50 cGy/h. The end points included incidence of radiation recall reaction, telangiectasias, and cosmetic-altering fibrosis. RESULTS Between 1995 and 2000, 44 patients with early-stage breast cancer received PBB without adjuvant external beam radiotherapy after lumpectomy (31 HDR PBB, 13 LDR PBB). After a median follow-up of 42 months (range 18-86), all patients remained locally controlled. The overall rate of good/excellent cosmetic outcome was 79.6% overall and 90% with HDR PBB. Radiation recall reactions occurred in 43% of patients (6 of 14) who received adriamycin. LDR PBB and adriamycin were significant predictors for late unfavorable cosmetic changes in univariate analysis (p = 0.003 and p = 0.005, respectively). CONCLUSION Although a high rate of local control and good/excellent cosmetic outcome is provided with HDR PBB, the risk of unfavorable cosmetic changes when treated with both LDR PBB and adriamycin is noteworthy. This suggests that HDR PBB is preferred in patients for whom adriamycin is indicated.

SU-F-T-607: Study of Dose Falloff Slope in RapidArc Planning of Lung SBRT.

PURPOSE SBRT of lung tumor results in steep dose fall-off outside the PTV to limit the normal tissue dose. RTOG report 0915 recommended metrics for plan quality measure, CI, D2cm, R50% and HDloc were evaluated. This study was aimed at ascertaining moderate to low dose spillage using 5 mm ring structures outside the PTV. METHODS In this retrospective study, 41 clinical RapidArc lung SBRT plans (Eclipse TPS ver 8.9) previously treated at our institution in a TrueBeam STx linac were analyzed. The delivered plans were re-planed using either 2 or 3 complete or partial arcs in coplanar or non-coplanar arrangement with upto ±15 degree couch angle. Seven concentric 5 mm wide rings were created outside the PTV for evaluation of dose fall-off based on mean, maximum doses as a function of distance. Mean and maximum percent dose fall-off in the rings (% dose/mm) were compared between the delivered and research plans. The differences in the mean and maximum percent doses between coplanar and non-coplanar arc plans were studied for any significant differences. RESULTS Mean dose falloff in the rings outside the PTV followed an exponential curve. For all the patients, interestingly the slope of maximum dose falloff increased at the intersection of 1st and 2nd ring and monotonically fell at outer rings. Altering coplanar to non-coplanar arc arrangements had a minimal impact on the CI as well as D2cm, R50% values (p-value > 0.05). CONCLUSION Slope of dose falloff from the PTV surface could serve as an additional metric. Coplanar arc plan had slightly steep dose falloff outside the PTV than non-coplanar arcs, but the differences are statistically insignificant.

SU‐GG‐T‐199: Feasibility Study of Parallel‐Opposed GRID Therapy Using a Multileaf Collimator

Purpose: Parallel‐opposed approach of megavoltage spatially fractionated (GRID) radiation therapy for deep‐seated tumors was demonstrated using a multileaf collimator(MLC GRID). Its therapeutic advantage was assessed based on dosimetric measurements using a linear quadratic model. Method and Materials: A Varian Clinac 2100EX linear accelerator equipped with an MLC was used for the MLC GRID therapy test. Two MLC GRID blocks denoted by MLC5 and MLC10 representing 5×5 and 10×10 mm openings projected at isocenter, respectively, were assessed in both single and parallel‐opposed (POP) setups. A linear‐quadratic (LQ) model was used to calculate the survival fraction (SF) of tumor and normal tissues. Therapeutic gain was obtained by the SF ratios of normal tissues under an MLC GRID to that under equivalent open field for both single and POP setup. Results: Beam profiles from MLC5 and MLC10 GRIDs created using 6 MV and 18 MV x rays were found to show a GRID field feature of spatially periodic intensity modulation for both single and POP setups. Their dose distributions measured at different depths using a film dosimetry were used to determine therapeutic gain. Therapeutic ratios varied from 1 to 45 for a wide range of tumor sensitivities at single fraction doses of up to 30 Gy. MLC GRID therapy with POP setup showed a higher therapeutic gain than one with single MLC GRID. Conclusion:Dosimetric properties of MLC GRIDs allowed for therapeutic evaluation using a modified LQ model. With high, single‐fraction doses, POP MLC GRID radiotherapy exhibited a significant therapeutic advantage over the single field radiotherapy when the tumor cells were more radioresistant.

Comments on shielding for dual energy accelerators

Determination of shielding requirements for medical linear accelerators has been greatly facilitated by the publication of the National Council on Radiation Protection and Measurements (NCRP) latest guidelines on this subject in NCRP Report No. 151. In the present report the authors review their own recent experience with patient treatments on conventional dual energy linear accelerators to examine the various input parameters needed to follow the NCRP guidelines. Some discussion is included of workloads, occupancy, use factors, and field size, with the effects of intensity modulated radiotherapy (IMRT) treatments included. Studies of collimator settings showed average values of 13.1 x 16.2 cm2 for 6 MV and 14.1 x 16.8 cm2 for 18 MV conventional ports, and corresponding average unblocked areas of 228 and 254 cm2, respectively. With an average of 77% of the field area unblocked, this gives a mean irradiated area of 196 cm2 for the 18 MV beam, which dominates shielding considerations for most dual energy machines. Assuming conservatively small room dimensions, a gantry bin angle of 18 degrees was found to represent a reasonable unit for tabulation of use factors. For conventional 18 MV treatments it was found that the usual treatment angles of 0, 90, 180, and 270 degrees were still favored, and use factors of 0.25 represent reasonable estimates for these beams. As expected, the IMRT fields (all at 6 MV) showed a high degree of gantry angle randomization, with no bin having a use factor in excess of 0.10. It is concluded that unless a significant number of patients are treated with high energy IMRT, the traditional use factors of 0.25 are appropriate for the dominant high energy beam.

SU‐FF‐T‐418: Therapeutic Advantage of GRID Therapy Using a Multileaf Collimator

Purpose:Dosimetry of megavoltage spatially fractionated (GRID) radiation therapy using a multileaf collimator(MLC GRID) was carried out using films. Its therapeutic advantage was assessed based on dosimetric measurements by a linear quadratic model. Therapeutic advantages obtained from MLC GRIDs were compared with a Cerrobend GRID. Materials and Methods: A Varian Clinac 2100EX linear accelerator equipped with an MLC was used for the MLC GRID therapy. The two MLC GRID blocks were denoted by MLC5 and MLC10, representing 5×5 and 10×10 mm openings projected at isocenter, respectively. Cerrobend GRID blocks were used for comparison with MLC GRIDs. A linear‐quadratic (LQ) model was used to calculate the survival fraction (SF) of tumor and normal tissues. Therapeutic gain was obtained by the SF ratios of normal tissues under an MLC GRID to that under equivalent open field. Results: 5×5 and 10×10 mm GRIDS (i.e., MLC5 and MLC10 GRIDs) were created using a MLC using 6 MV and 18 MV x rays. Their dose distributions were measured at different depths using a film dosimetry. The peak‐to‐valley dose ratios at the depth of maximum dose at 100cm SSD were found to be 19% for MLC5 and 17% for MLC10. Therapeutic ratios varied from 0.9 to 45 for a wide range of tumor sensitivities at single fraction doses of up to 30 Gy. MLC10 GRID therapy showed a higher therapeutic gain than MLC5 and an 8mm Cerrobend GRID block. Conclusion: With high, single‐fraction doses,MLC GRID radiotherapy exhibited a significant therapeutic advantage over the open field radiotherapy when the tumor cells were more radioresistant. Dosimetric properties of MLC GRIDs allowed for therapeutic evaluation using a modified LQ model. One of the GRIDs investigated, MLC10 showed a great increase in therapeutic benefit compared to MLC5 and a Cerrobend GRID block with 8mm apertures.

Clinical Applications of Low Dose Rate and Medium Dose Rate Brachytherapy

C. A. Perez, MD Professor, Department of Radiation Oncology, Washington University Medical Center, 4511 Forest Park Boulevard, Suite 200, St. Louis, MO 63108, USA R. D. Zwicker, PhD Department of Radiation Oncology, University of Kentucky Medical Center, 800 Rose Street, Lexington, KY 40536, USA Z. Li, DSc Associate Professor, Division of Radiation Physics, Washington University Medical Center, 4921 Parkview Place, Campus Box 8224, Saint Louis, MO 63110, USA This chapter is an update of a similar chapter published in Principles and Practice of Radiation Oncology, 4th edn., Lippincott Williams Wilkins (Perez et al. 2004). CONTENTS

SU‐FF‐T‐285: Dose Property of Grid Therapy

The possibility of a therapeutic advantage in the use of a megavoltage grid to deliver large single dose fractions has been previously demonstrated. Assessment of dose response characteristics is essential to the understanding and use of grid therapy in the clinic. A Wellhofer scanning system, an EDR2 film dose measurement device, and Monte Carlo calculations were used in evaluating the dosimetric properties of a megavoltage grid. A range of grid hole diameters was simulated by the Monte Carlo technique, and a 0.8 cm diameter grid was singled out to carry out a comprehensive comparison between the measurements and Monte Carlo calculations. The maximum and minimum doses, and dose profiles at the depth of maximum dose dmax as well as the percentage depth dose were obtained. With the dose normalized at 100 cGy at dmax in a 10×10 cm2 open field, the maximum dose for the grid was found to range from 11.9 to 94.5 cGy when the diameter of grid was varied from 0.2 to 1.0 cm, while the minimum dose between holes increased only from 6.8 to 16.1 cGy. A fairly good agreement between the Monte Carlo simulated and measured data was demonstrated for the 0.8 cm diameter grid. With our calculated results, the cell survival rates for grid therapy were further derived using a linear‐quadratic survival model, and a therapeutic advantage for the grid was confirmed for large single fractions.