Than S. Kehwar

Analytical approach to estimate normal tissue complication probability using best fit of normal tissue tolerance doses into the NTCP equation of the linear quadratic model.

AIMS AND OBJECTIVES Aims and objectives of this study are to get the best fit of the normal tissue tolerance doses to the NTCP model of the linear quadratic model. METHODS AND MATERIALS To compute the NTCP, the modified form of the Poisson cell kill model of NTCP, based on linear-quadratic model, is used. The model has been applied to compute the parameters of the NTCP model using clinical tolerance doses of various normal tissues / organs extracted from published reports of various authors. The normal tissue tolerance doses are calculated for partial volumes of the organs using the values of above-said parameters for published data on normal tissue tolerance doses. In this article, a graphical representation of the computed NTCP for bladder, brain, heart and rectum is presented. RESULTS AND CONCLUSION A fairly good correspondence is found between the curves of 2 sets of data for brain, heart and rectum. Hence the model may, therefore, be used to interpolate clinical data to provide an estimate of NTCP for these organs for any altered fractionated treatment schedule.

Concomitant Boost Radiotherapy Compared with Conventional Radiotherapy in Squamous Cell Carcinoma of the Head and Neck — a Phase III Trial from a Single Institution in India

AIMS To test the efficacy of an accelerated fractionation schedule (concomitant boost) against standard conventional fractionation in squamous cell carcinomas of the head and neck region in our patient population. MATERIALS AND METHODS Patients were randomised to receive either conventional radiotherapy with 2 Gy/fraction/day, to a dose of 66 Gy in 33 fractions over 6.5 weeks or accelerated radiotherapy in the form of concomitant boost to a dose of 67.5 Gy/40 fractions over 5 weeks (phase 1: 45 Gy/25 fractions/5 weeks and phase 2: 22.5 Gy/15 fractions/3 weeks as a second daily fraction after a 6h gap). The primary and secondary end points were disease-free survival and locoregional control respectively. RESULTS The compliance was 97.2% and 96.5% in the concomitant boost and conventional arms, respectively. Patients treated with concomitant boost had a better 2-year disease-free survival (71.7% vs 52.17%, P=0.0007) and locoregional control rates (73.6% vs 54.5%, P=0.0006) than with conventional fractionation. On exploratory subgroup analysis, the oropharynx (P<0.001), T4 lesions (P=0.017), N+ disease (P<0.001) and stage IV disease (P<0.001) were statistically significant prognostic variables in favour of the concomitant boost arm. Grade 3 mucositis was seen in 35% of patients in the concomitant boost arm, whereas in the conventional arm only 19% of patients had grade 3 mucositis (P=0.01). The median radiotherapy duration in the concomitant boost arm was 36 days (range 36-53 days), whereas in the conventional arm it was 46 days (range 46-64 days). The mean gap in radiation treatment in the concomitant boost arm was 1.68 days (range 0-14 days), whereas the mean gap in the conventional arm was 1.58 days (range 0-14 days). CONCLUSIONS Concomitant boost is a therapeutically superior and logistically feasible accelerated radiotherapy regimen in advanced head and neck cancers, especially in the setting of a developing country.

Evaluation of dosimetric effect of leaf position in a radiation field of an 80-leaf multileaf collimator fitted to the LINAC head as a tertiary collimator

This study evaluates changes in the dosimetric characteristics of a Varian Millennium 80‐leaf multileaf collimator (MLC) in a radiation field. In this study, dose rate, scatter factor, percentage depth dose, surface dose and dose in the buildup region, beam profile, flatness and symmetry, and penumbra width measurements were made for 6‐MV and 15‐MV photon beams. Analysis of widths between 50% dose levels of the beam profiles to reflect the field size at the level of profile measurement shows a significant difference between the fields defined by MLC and/or jaws and MLC (zero gap) and the fields defined by jaws only. The position of the MLC leaves in the radiation field also significantly affects scatter factors. A new relationship has, therefore, been established between the scatter factors and the position of the MLC, which will indeed be useful in the dose calculation for irregular fields. Penumbra widths increase with field size and were higher for fields defined by jaws and/or MLC than jaws and MLC (zero gap) by 1.5 mm to 4.2 mm and 3.8 mm to 5.0 mm, for 6‐MV, and 1.5 mm to 2.4 mm and 3.0 mm to 5.6 mm, for 15‐MV, at 20% to 80% and 10% to 90% levels, respectively. The surface dose and the dose in the buildup region were smaller for fields defined by jaws and MLC (zero gap) than the fields defined by jaws and/or MLC for both photon energies. No significant differences were found in percentage depth dose beyond dmax, beam profiles above 80% dose level, and flatness and symmetry for both energies. The results of this study suggest that while one collects linear accelerator beam data with a MLC, the effects of the positions of the MLC leaves play an important role in dosimetric characteristics of 3D conformal radiation therapy as well as intensity‐modulated radiotherapy. PACS number: 87.53.Dq

Effect of edema associated with 131Cs prostate permanent seed implants on dosimetric quality indices

This study was designed to investigate the effect of prostatic edema on various dosimetric quality indices following transperineal permanent C131s seed implant. Thirty-one patients with early prostate cancer, who received C131s permanent seed implant, were included in this study. Each patient received a prescribed dose of 115 Gy from the implant. Transrectal ultrasound (U.S.) was used to measure the preimplant prostate volume and pre- and postneedle implant volumes, and postimplant CT images were used to obtain postimplant prostate volumes at days 0, 14, and 28 for all patients. The magnitude of edema was determined by comparing the preneedle and postimplant prostate volumes, which was used to compute the half life of the edema using the least-squares method. Dose volume histograms were generated for each set of volumes to determine the percentage of the prostate volume that received a dose equal to or greater than the prescribed dose to compute the quality index (V100) and fractional D90 (FD90). There were no statistically significant differences between the postneedle and postimplant (day 0) volumes obtained by U.S. and CT scanned images (students t-test p=0.56). The mean half life of the edema was found to be (9.72±8.31) days (mean±1SD), ranging from 3.64 to 34.48 days. The mean values of V100 and FD90 from preimplant plan to postimplant plan at day 0 were decreased by 8.0% and 6.3%, respectively. On the other hand, the mean values of V100 and FD90 increased with increasing postimplant time and attained optimal values when postimplant volume reached the original volume of the prostate. The short half life C131s radioactive source delivered about 85% of the prescribed dose before the prostate reached its original volume. Therefore, improvement in V100 and FD90 due to edema decay does not improve the physical dose delivery to the prostate. It is important to note that at the time of C131s implant, the effect of edema must be accounted for when defining the seed positions. Implants performed based only on the guidance of a preimplant volume study would result in poor dosimetric results for C131s implants.

INFLUENCE OF PROSTATIC EDEMA ON 131Cs PERMANENT PROSTATE SEED IMPLANTS: A DOSIMETRIC AND RADIOBIOLOGICAL STUDY

PURPOSE To study the influence of prostatic edema on postimplant physical and radiobiological parameters using (131)Cs permanent prostate seed implants. METHODS AND MATERIALS Thirty-one patients with early prostate cancer who underwent (131)Cs permanent seed implantation were evaluated. Dose-volume histograms were generated for each set of prostate volumes obtained at preimplantation and postimplantion days 0, 14, and 28 to compute quality indices (QIs) and fractional doses at level x (FD(x)). A set of equations for QI, FD(x), and biologically effective doses at dose level D(x) (BED(x)) were defined to account for edema changes with time after implant. RESULTS There were statistically significant differences found between QIs of pre- and postimplant plans at day 0, except for the overdose index (ODI). QIs correlated with postimplant time, and FD(x) was found to increase with increasing postimplant time. With the effect of edema, BED at different dose levels showed less improvement due to the short half-life of (131)Cs, which delivers about 85% of the prescribed dose before the prostate reaches its original volume due to dissipation of edema. CONCLUSIONS Results of the study show that QIs, FD(x), and BEDs at the level of D(x) changed from preneedle plans to postimplant plans and have statistically significant differences (p < 0.05), except for the ODI (p = 0.106), which suggests that at the time of (131)C seed implantation, the effect of edema must be accounted for when defining the seed positions, to avoid the possibility of poor dosimetric and radiobiologic results for (131)Cs seed implants.

The nth root percent depth dose method for calculating monitor units for irregularly shaped electron fields

This study outlines an improved method for calculating dose per monitor unit values for irregularly shaped electron fields using the nth root percent depth dose method. This method calculates the percent depth dose and output factors for an irregularly shaped electron field directly from the measured electron beam percent depth dose curves and output factors for circular fields. The percent depth dose curves and output factors for circular fields are normalized and measured at a fixed depth of maximum dose for a reference field, respectively. When compared with the sector integral lateral buildup ratio method, the percent depth dose data calculated using the nth root method accounts more accurately for the change in lateral scatter with decreasing field size. Therefore, it provides more accurate values of dose per monitor unit at different depths for all type of field shapes and beam energies. For beam energies in the range of 6-21 MeV, the differences between measured and calculated dose per monitor unit values, at different depths, were found to be within +/- 1.0% when the nth root percent depth dose method was used for calculation and 12.6% when the sector integral lateral buildup ratio method was used. The nth root percent depth dose method was tested and compared with the sector integral lateral buildup ratio method for ten clinically used irregularly shaped inserts (cutouts). For small irregularly shaped fields, a maximum difference of 2% was found between calculated dose per monitor unit values and measurements when the nth root percent depth dose method was used; this difference changed to 7% when comparisons were made between measurements and calculations based on the sector integral lateral buildup ration method. For large irregular fields this difference was found to be within 1.5% and 3.5%, respectively.

Assessment of tumor control probability for high-dose-rate interstitial brachytherapy implants

Summary Aim The study was designed to propose a novel concept of biologically effective equivalent uniform dose to calculate tumor control probability for HDR implants. Materials and Methods The expression of biologically effective equivalent uniform dose was derived for non-uniform dose distribution in HDR implants using quality indices and voxel-based tumor control probability. Results The results of this study show that high dose regions of the implant have higher tumor control probability. But these regions may also have a large number of normal cells and consequently may lead to severe normal tissue complications. If tumor coverage was not proper then the overall tumor control probability would be low and might result in tumor recurrence. Higher values of external volume index, dose non-uniformity ratio and overdose volume index were related to higher normal tissue complication rates outside and inside the implants. Conclusion The present concept may provide an alternative approach to calculate tumor control probability for HDR implants.

Dosimetric and qualitative analysis of kinetic properties of millennium 80 multileaf collimator system for dynamic intensity modulated radiotherapy treatments

The aim of this paper is to analyze the positional accuracy, kinetic properties of the dynamic multileaf collimator (MLC) and dosimetric evaluation of fractional dose delivery for the intensity modulated radiotherapy (IMRT) for step and shoot and sliding window (dynamic) techniques of Varian multileaf collimator millennium 80. Various quality assurance tests such as accuracy in leaf positioning and speed, stability of dynamic MLC output, inter and intra leaf transmission, dosimetric leaf separation and multiple carriage field verification were performed. Evaluation of standard field patterns as pyramid, peaks, wedge, chair, garden fence test, picket fence test and sweeping gap output was done. Patient dose quality assurance procedure consists of an absolute dose measurement for all fields at 5 cm depth on solid water phantom using 0.6 cc water proof ion chamber and relative dose verification using Kodak EDR-2 films for all treatment fields along transverse and coronal direction using IMRT phantom. The relative dose verification was performed using Omni Pro IMRT film verification software. The tests performed showed acceptable results for commissioning the millennium 80 MLC and Clinac DHX for dynamic and step and shoot IMRT treatments.

Correlation between mean lethal dose and organ weight

The assessment of mean lethal dose (D o ) with organ weight in human organs yields a correlation coefficient of 0.94. Results indicate that as the organ weight increases, the D o decreases.

Variations in inter-observer contouring and its impact on dosimetric and radiobiological parameters for intensity-modulated radiotherapy planning in treatment of localised prostate cancer

Inter-observer variations in contouring and their impacts on dosimetric and radiobiological parameters in intensity-modulated radiotherapy (IMRT) treatment for localised prostate cancer patients were investigated. Four observers delineated the gross tumour volume (GTV) (prostate and seminal vesicles), bladder and rectum for nine patients. Contouring done by radiologist was considered as gold standard for comparison purposes and for IMRT plan optimisation. Maximum average variations in contoured prostate, bladder and rectum volumes were 3% (SD = 8.4), 2.5% (SD = 4.12) and 13.2% (SD = 6.77), respectively. The average conformity index for standard contouring set (observer A) was 0.85 (SD = 0.028) and statistically significant differences were observed for observers A–B ( p = 0.008), A–C ( p = 0.006) and A–D ( p = 0.011). Average values of normal tissue complication probability for bladder and rectum for observer A were 0.361% (SD = 0.036) and 1.59% (SD = 0.14). Maximum average tumour control probability was 99.94% (SD = 0.035) and statistically significant difference was observed for observers A–B ( p = 0.037) and observers A–C ( p = 0.01). Inter-observer contouring variations have significant impact on dosimetric and radiobiological outcome in IMRT treatment planning. So accurate contouring of tumour and normal organs is a fundamental prerequisite to make good correlation between calculated and clinical observed results.