Parallel-opposed Lateral Fields Research Articles

Biological impact of dosimetric perturbations of a fiducial marker and the daily number of fields in proton therapy for prostate cancer

The purpose of this study was to estimate the biological impact of dosimetric perturbations of a fiducial marker and the daily number of fields in proton therapy for prostate cancer. Using a linear-quadratic model, normalized total doses (NTDs) of points where deposited dose was reduced from the prescribed dose by dosimetric perturbation of a fiducial marker were calculated in two hypothetical prostate cancer treatment schedules: a) irradiation of both parallel-opposed lateral fields and b) irradiation of alternate field in each daily treatment. The impact of hypofractionation and sublethal damage repair between irradiation on NTD was also estimated. The NTD of two fields/day schedule becomes lower than that of one field/day schedule. The difference becomes larger as dose reduction from one of two fields becomes more enhanced. The NTD reduction from the total dose in the two fields/day schedule is largest (30% of total dose) where the dose from one beam is completely lost by a fiducial marker. In contrast, the NTD reduction from the total dose in the one field/day schedule is largest (9% of total dose) where the half dose from one beam is decreased by a fiducial marker. In addition, the NTD reduction becomes larger as the fractional dose increases in a hypofractionated regimen, and when the effect of sublethal damage repair was incorporated. These influences become significant in prostate cancer since the radiobiological sensitivity α/β of prostate cancer is lower than other cancer types and normal tissues late complication. Treating with one alternate field in a daily treatment can improve a deteriorating treatment effect by dosimetric distortion of a fiducial marker in prostate cancer treatment. However, the choice of the number of beams in a fraction must also be determined by considering the sparing of normal tissues and patient-specific status.

Proton Therapy vs. VMAT for Prostate Cancer: A Treatment Planning Study

Abstract Purpose: The main objective of this study was to compare the dosimetric quality of volumetric modulated arc therapy (VMAT) with that of proton therapy for high-risk prostate cancer. Patients and Materials: Twelve patients with high-risk prostate cancer previously treated with uniform scanning proton therapy (USPT) were included in this study. Proton planning was done using the XiO treatment planning system (TPS) with two 1800 parallel-opposed lateral fields. The VMAT planning was done using the RapidArc technique with two arcs in the Eclipse TPS. The VMAT and proton plans were calculated using the anisotropic analytical algorithm and pencil-beam algorithm, respectively. The calculated VMAT and proton plans were then normalized so that at least 95% of the planning target volume (PTV) received the prescription dose. The dosimetric evaluation was performed by comparing the physical dose-volume parameters, which were obtained from the VMAT and proton plans. Results: The average difference in the PTV ...

3DCRT for posterior fossa: Sparing of surrounding organs at risk

IntroductionRadiotherapy of the posterior fossa is routinely delivered using 3DCRT parallel-opposed lateral fields. However high incidence of sensorineural hearing loss, hypothalamic–pituitary dysfunction, thyroid and gonadal dysfunction during radiotherapy makes the need for treatment plan which provides adequate coverage of the target volume while sparing of the cochlea and other surrounding organs at risk (OARs) at same time inevitable. Aim of the workTo compare the coverage of posterior fossa and the dose to surrounding OARs including non-posterior fossa brain, pituitary, cochlea, eyes, optic nerves, optic chiasm, cervical spinal cord, thyroid gland, pharynx, parotid glands and mandible using three different 3DCRT plans. MethodsTen patients underwent CT simulation for treatment planning of posterior fossa boost. The CT data were transferred to Precise Elekta treatment planning system where posterior fossa, non-posterior fossa brain, pituitary, cochlea, parotid glands, cervical spinal cord, thyroid gland, pharynx, mandible, eyes, lenses, optic nerves and optic chiasm were all contoured.For each patient, three plans were carried out; two parallel opposed open lateral photon fields, a pair of wedged posterior oblique fields, and a pair of wedged posterior oblique fields and an open vertex field.For all plans, the dose distributions and dose volume histogram parameters (DVPs) for the PTV and OARs were compared and analyzed statistically using excel sheet 2003 and SPSS spreadsheet (SPSS base 18). ResultsPosterior fossa dose coverage and its homogeneity were adequate and comparable for the three plans. A part from high mean dose received by cochlea, plan 1 shows the best sparing for other OARs. Conclusion3DCRT using parallel opposed fields is recommended for posterior fossa irradiation boost as it minimizes the exit dose to all structures other than the cochlea, however its mean dose was within the tolerance.

Radiotherapy Treatment of Early-Stage Prostate Cancer with IMRT and Protons: A Treatment Planning Comparison

To compare intensity-modulated photon radiotherapy (IMRT) with three-dimensional conformal proton therapy (3D-CPT) for early-stage prostate cancer, and explore the potential utility of intensity-modulated proton therapy (IMPT). Ten patients were planned with both 3D-CPT (two parallel-opposed lateral fields) and IMRT (seven equally spaced coplanar fields). Prescribed dose was 79.2 Gy (or cobalt Gray-equivalent, [CGE] for protons) to the prostate gland. Dose-volume histograms, dose conformity, and equivalent uniform dose (EUD) were compared. Additionally, plans were optimized for 3D-CPT with nonstandard beam configuration, and for IMPT assuming delivery with beam scanning. At least 98% of the planning target volume received the prescription dose. IMRT plans yielded better dose conformity to the target, whereas proton plans achieved higher dose homogeneity and better sparing of rectum and bladder in the range below 30 Gy/CGE. Bladder volumes receiving more than 70 Gy/CGE (V70) were reduced, on average, by 34% with IMRT vs. 3D-CPT, whereas rectal V70 were equivalent. EUD from 3D-CPT and IMRT plans were indistinguishable within uncertainties for both bladder and rectum. With the use of small-angle lateral-oblique fields in 3D-CPT and IMPT, the rectal V70 was reduced by up to 35% compared with the standard lateral configuration, whereas the bladder V70 increased by less than 10%. In the range higher than 60 Gy/CGE, IMRT achieved significantly better sparing of the bladder, whereas rectal sparing was similar with 3D-CPT and IMRT. Dose to healthy tissues in the range lower than 50% of the target prescription was substantially lower with proton therapy.

Head and neck field matching verification using three dimensional PAGAT polymer gel dosimetry

Independent jaw collimation on modern linear accelerators allows matching of non-diverging field edges (laterals and anterior) when using head and neck treatment techniques. It is a common practice to split a head and neck cancer radiotherapy treatment volume into two separate abutting mono-isocenter fields because the treatment volume lies above and below the line of the shoulders. In a standard three field technique parallel-opposed lateral fields are used to treat the primary tumour site, upper and mid-cervical nodes and a single anterior field is used to treat the lower neck and superior mediastinal lymph nodes. If these fields are non-diverging fields, there is a possibility of creating an under dose or over dose region at the junction of the abutting fields. The purpose of this study was to evaluate the suitability of the PAGAT polymer gel dosimeter as a tool for verification of the dosimetric match of non-diverging field junctions of a mono-isocenter setup.

Ependymal tumors in childhood

Ependymal tumors are classified as ependymoma (benign or low grade) versus anaplastic ependymoma (malignant or high grade). Ependymomas represent 5-10% of intracranial neoplasm in children. In this study, demographic data and the treatment results of pediatric patients with ependymal tumors, treated in a single institute, is reported. Between 1989 and 2001, 40 (22 M/18 F) previously untreated patients with a median age of 5.5 years (3 months-15 years), of histologically proven ependymal tumors (except ependymoblastomas) were referred to the Institute of Oncology, University of Istanbul. The localization was supratentorial in 18, infratentorial in 20, both supra and infratentorial in two patients. Histologic subgroups were 18 ependymomas (43.6%), and 22 anaplastic ependymomas (56.4%). Total tumor resection was performed in 20 patients (50%), subtotal in 18 patients (45%), and biopsy only in 2 patients (5%). Postoperative treatment consisted of regional (8 patients) or craniospinal (CSI) (9 patients) radiotherapy (RT) in patients with ependymoma; regional (7 patients) or CSI RT (14 patients) with chemotherapy (ChT) in patients with anaplastic ependymoma; ChT only (1 patient) in patients less than 3 years of age. The standard technique for posterior fossa irradiation was parallel-opposed lateral fields and total dose was 45-54 Gy. Between September 1989 and May 1991 patients received regimen A, which consisted of RT followed by eight-in-one ChT, given every 4 weeks for eight courses. Patients who were treated between June 1991 and July 1994, received regimen B, which included two courses of postoperative "VEC" (vincristine, etoposide, cisplatin) ChT, administered every 3 weeks, followed by RT applied with low dose concomitant cisplatin used as a radiosensitizer. Patients with objective response to postoperative "VEC" continued to have "VEC" after completion of RT for six more courses. From August 1994 on, patients received regimen C, consisting of RT and concomitant infusion of cisplatin followed by "VCPCU" (vincristine, cyclophosphamide, procarbazine, lomustine) administered every 4 weeks for eight courses. A total of 40 patients were included in the outcome and survival data. The 5-year overall survival (OS) rate was 64.9%, and the 5-year progression-free survival rate was 50.8% for the whole series. Median time for progression or relapse was 24.3 months and there were 19 patients (43.6%) with relapse or progression. Non-metastatic patients (P = 0.0008, 5-year OS rate was 82% vs. 29%), and totally resected patients (P = 0.01, 5-year OS rate was 80% vs. 55%), and > or =3 years of age (P = 0.04, 5-year OS rate was 75% vs. 38%) had significantly better outcome. The majority of complete responders were patients who had total tumor removal. Treatment failure occurred mainly within the first 2 years, and outcome was dismal for patients who relapsed or had progressive disease. The median age at diagnosis is 6 years in our patient group; younger children (less than 3 years old) have less favorable outcome. There was no significant difference in survival or progression-free survival between the two histologic subtypes.

Posterior fossa boost in medulloblastoma: an analysis of dose to surrounding structures using 3-dimensional (conformal) radiotherapy

Purpose: The purpose of this study was to compare three different techniques of delivering the posterior fossa boost in patients with medulloblastoma. Methods and Materials: Five patients underwent CT simulation for treatment planning of the posterior fossa boost. For each slice, the posterior fossa was contoured in addition to the cochlea, non–posterior fossa brain, pituitary gland, mandible, parotid glands, thyroid gland, pharynx, and cervical spinal cord. For each patient, plans for three techniques of delivering the posterior fossa boost were compared. Technique A utilized parallel-opposed lateral fields using bony landmarks (2-dimensional radiotherapy); the other two techniques were planned using 3-dimensional radiotherapy. Technique B utilized a pair of coplanar wedged posterior oblique beams, whereas Technique C utilized a pair of posterior oblique fields and a vertex field. Dose–volume histograms (DVH) were obtained for each of the organs contoured and for each technique and patient. The maximum, minimum, and mean dose to each organ were determined using the DVH program in our treatment planning system. Results: In 3 of 5 patients, the planning target volume (PTV) was not included in the treatment field with Technique A. The cochlea received 100%, 50%, and 42% of the prescribed posterior fossa dose using Techniques A, B, and C respectively. The mean dose to the non–posterior fossa brain was highest with Technique C, intermediate with Technique A, and lowest for Technique B. The mean dose to the non–posterior fossa brain with Technique B was comparable to the mean non–posterior fossa brain dose delivered using parallel-opposed lateral fields based on the CT definition of the PTV. Likewise, mean dose to the pituitary gland was lowest for Technique B. Both Techniques B and C were associated with a higher mean dose to the thyroid gland, mandible, parotid glands, and pharynx. Conclusions: The use of Technique B minimized the radiotherapy dose to the cochlea, pituitary gland, and non–posterior fossa brain. Contrary to what one may expect, conformal radiotherapy using Technique B did not deliver a higher dose to the non–posterior fossa brain over standard parallel-opposed lateral fields. Other advantages of conformal techniques B and C over 2-dimensional radiotherapy are the inclusion of the PTV in all patients and a lower mean dose to the pituitary gland. The main disadvantage of conformal Techniques B and C employed in our patients is a higher mean dose to the thyroid gland and other tissues in the neck.

A dynamic match-line wedge

Purpose : To implement match-line wedges at the abutting edges of x-ray fields using dynamic collimation. Methods and Materials : Experiments were made using a computer-controlled linear accelerator equipped with developmental software that allows for collimator jaw motion while the beam is on. The jaws defining the abutting field edges were programmed to move from 1.5 cm inside to 1.5 cm outside the prescribed field during irradiation. Films were taken in plastic phantoms to assess the resulting edge gradient and to evaluate the sensitivity of this technique to setup errors. Results : The measured edge gradient for a single field was 30% per cm. Parallel-opposed lateral fields produced a gradient of 28% per cm along their midline. A simulated central nervous system irradiation with cranial and spinal fields kept dose variations in the field-match region to less than 10% with setup errors of 3 mm. Conclusion : The use of collimator motion during irradiation is an effective and simple means of reducing the dose variation in a field-match region due to setup errors and system tolerances. Treatment time is not increased and labor savings can be achieved when compared to feathering techniques commonly used.

Some factors influencing salivary function when treating with radiotherapy

Salivary flow rate (FR) was studied in 29 patients treated with external irradiation to bead and neck areas. “Resting” saliva samples were collected before, during the radiotherapy course and follow-up. Several parameters were investigated: field arrangement, amount of salivary glands irradiated, dose to these glands, initial FR, its recovery during and after irradiation, and influence of therapy interruption (split course) in FR. We found that the level of the upper border of the field is a critical factor when using parallel-opposed lateral fields to the upper neck area and lateral face. More than 50% of the parotids have to be outside the fields to prevent severe dryness. Neck fields which do not encompass salivary glands do not decrease salivary secretion. There is some relation between the initial FR and the dose necessary to produce dryness: patients with high initial salivary FR require higher doses. FR recovery occurs during weekend interruptions before xerostomia develops. Interruptions of therapy for more than two weeks during the radiotherapy course prior to development of dryness might decrease late xerostomia, at least in patients with high initial FR. Objective recovery of the FR has not been observed after treatment in spite of the subjective improvement in the sensation of dryness of some patients.