Commissioning of Bilateral (Parallel Opposed) Extended Source-to-surface Distance Total Body Irradiation Technique and Its Long-term Stability

Abstract

Aim :

This article describes the commissioning of a total body irradiation (TBI) technique using bilateral-parallel-opposed fields at extended source-to-surface distance (SSD).

Material:

The measurements are based on the actual patient treatment geometry, which requires the patient to be placed inside a Perspex box. The gaps between the patient and the walls of the Perspex box were filled with rice bags to achieve full scattering conditions. The TBI box’s lateral separation can be adjusted to 42 cm, 52 cm, or 62 cm by inserting the removable side walls to either of the three pairs of slots. A Farmer chamber, inserted inside a water-equivalent plastic slab phantom placed under full scattering conditions, was used for depth dose and profile measurements. The extended SSD was 333.5 cm, and the available field size was 132 cm × 132 cm. For output measurement, dose-to-water calibration factors for 6 MV and 15 MV energies were derived for the extended SSD. Bilateral-opposed fields were measured at three different separations to calculate lateral tissue effects.

Result:

For the 6 MV and 15 MV beams at a 42 cm separation, the midline-to-surface dose ratios were 1:1.17 and 1:1.08, respectively. As the separation increased, this ratio increased faster for the 6 MV beam and slower for the 15 MV beam. For the end-to-end quality assurance test (monitor unit to dose verification), the noted deviations were − 2.16% for the 6 MV beam and − 1.27% for the 15 MV beam.

Conclusion:

This article presents the detailed commissioning and long-term stability of the extended SSD TBI technique.

INTRODUCTION

Total body irradiation (TBI) has been an essential constituent of conditioning regimen to allogeneic hematopoietic stem-cell transplantation for both myeloid leukemia and acute lymphoblastic leukemia acute.[1] The primary aim of TBI is to eliminate the malignant cells from the bone marrow, circulating blood, and lymph nodes. Unlike chemotherapy kinematics, radiation delivery to leukemic sites is neither dependent on metabolism, blood supply, and biodistribution nor on the inter-patient variability of drug absorption or clearance kinetics; radiation therapy also affects sites like brain or testes, which are often inaccessible for chemotherapeutic drugs.[2] TBI can induce effective immunosuppression to avoid the rejection of donor hematopoietic cells.[3] The TBI technique, originally developed by Edward Donnall Thomas in 1975, still remains the same, unable to irradiate the target without exposing healthy structures to the planned dose.[4,5] Use of TBI in clinical practice increased over the last few years for its better effectivity in sterilizing leukemic cells. Two useable methods used in the clinical practice are the TBI and total marrow and lymphoid irradiation (TMLI). TBI can be performed in open field extended source-to-surface distance (SSD) technique, or in 100 cm source-to-axis distance in C-Arm or Ring Gantry linear accelerators using intensity modulated technique.[6,7,8,9] Similarly, TMLI can be performed in C-Arm linear accelerator, ring gantry linear accelerator, and Tomotherapy.[10,11,12] Both techniques have their own merits and demerits. The open-field, extended SSD TBI technique without any intensity modulation still remains as the most popular technique.[9]

Commissioning TBI is a challenging task in clinical medical physics practice, as it is a labor-intensive and time-consuming process. Numerous publications address TBI installation, but a few, if any, present the full commissioning process with all the intricate details. In addition, the International Commission on Radiation Units and Measurements (ICRU Report numbers 50 and 62) recommends an overall accuracy in dose delivery of ± 5%, which must be maintained within the prescribed dose – a challenging requirement in extended SSD conditions.[13,14] This study aims to describe the extended-SSD TBI technique and its periodic maintenance in comprehensive detail, enabling potential users to perform complete commissioning even if they have not previously been involved in the process.

MATERIALS AND METHODS

Before starting the dosimetric measurements, it is essential to understand the treatment scheme for the actual patient, and the measurements need to be arranged accordingly. In the actual treatment, the patient will be placed inside a top-open Perspex box. All the empty spaces between the patient and the walls of the box will be tightly filled with rice bags of different sizes. Figure 1a shows a schematic diagram of the patient treatment; Figure 1b shows the patient placed in the specially designed Perspex box, while Figure 1c shows the final setup where the Perspex box is tightly filled with the rice bags. The Perspex walls are 1 cm thick. The lateral separation of the Perspex box be adjusted as 42 cm, 52 cm, or 62 cm, using three symmetrical pairs of slots in which the side walls are inserted. For clinical treatment, the minimum separation in which the patient can be comfortably accommodated is selected, thus ensuring minimal amount of rice packing and better dose distribution. The output measurements were also carried out for these three slots separately. During the actual treatment, regardless of which slot was used for the patient, the Perspex wall facing the linear accelerator was at 333.5 cm.

Figure 1.

(a) Patient setup geometry. Diagram illustration of the total body irradiation (TBI) technique using bilateral parallel opposed fields. Geometric diagonal field of size 223 cm × 223 cm is incident on the patient. Collimator rotation 45° and Gantry rotation 270°. (b) Actual patient placed in the TBI box prepared for anesthesia. Three different channels in left side can be seen, right side a Perspex guard wall is used. Three slots at a separation of 42 cm, 52 cm and 62 cm, used as per the patient width. (c) Patient after completion of the treatment preparation, filled with rice bags. Most inner slot (42 cm) is used for the chield patient. Blow TBI box thermocal sheets are visible, this is to make the TBI table horizontal to gantry when parked at 270° for tretement. SSD: Source-to-surface distance

Before measurements were performed at the extended SSD for both photon energies (6 MV and 15 MV), the percentage depth dose (PDD), beam profiles, output at 100 cm SSD were checked in the water phantom for a 10 cm × 10 cm field size and compared with our baseline values. The tissue phantom ratio (TPR20/10) was also verified.

Extended SSD TBI was installed on a Synergy platform (Elekta AB, Stockholm, Sweden), medical accelerator machine with a maximum field size of 40 cm × 40 cm at 100 cm SSD. The linear accelerator operates at two clinical photon energies, 6 MV and 15 MV, with a minimum dose rate of 260 monitor unit (MU)/min. TBI needs to be delivered at the lowest possible dose rate to avoid any central nervous system-related toxicity due to full body irradiation.[15] All the beams were delivered with a gantry rotation of 270° and a 45° collimator rotation, which offered the maximum possible field size in the diagonal direction. The extended SSD used was 333.5 cm. At this SSD, the field size according to divergence was 188 cm, but due to the circular primary collimator, it was only 165 cm due to clipping at the corners. The useful field length, excluding the penumbra, was only the central 80% of the total length and was equal to 132 cm.

PDD and beam profile measurements at extended SSD were done in the horizontal direction instead of the standard vertical measurement, forcing the use of water equivalent (density = 1.030 g/cc) solid water phantom (RW3) for these measurements. All the measurements were carried out at 333.5 cm SSD with the same field setting as described above.

For both absolute and relative dose measurements, a PTW (PTW-Freiburg, GmBH) Farmer type cylindrical ionization (0.6 cc, model 30013,) in phantom with full scattering condition, and a UNIDOS-E electrometer (PTW-Freiburg, GmBH) were used. Figure 2 shows a typical full scattering condition measurement. At the time of dose verification, and at the time of patient treatment, a suitable acrylic buildup cap having a density of 1.185 g/cm³ was used with the Farmer chamber for dose measurements. The buildup caps’ wall thicknesses (radius) were 12.7 mm for 6 MV and 17.8 mm for 15 MV, respectively. During measurements, the chamber was placed in a vertical position pointing toward floor, perpendicular to the central axis and the chamber cable inside the field was covered with a 5-mm bolus to reduce the stem and cable effects. During all the measurements, rice-filled bags were kept on either side of the phantom to provide full-scatter conditions.

Figure 2.

Full scattering condition measurement. Dosimetric measurements for total body irradiation technique bilateral parallel opposed fields at 333.5 cm source to surface distance

Dosimetry measurements

Both PDD and profile measurements were done using step- and-shoot method, i.e., chamber was kept at a known location, delivered 200 MU and the meter reading (nC) was noted and the whole process was repeated for all the required locations to generate the PDD and the profile curves with acceptable resolution. Further full curve was normalized to the depth of maximum dose (d_max) and the central axis for the PDD and the crossbeam profile, respectively.

Central axis depth dose (percentage depth dose)

The central axis depth dose was measured at the extended SSD of 333.5 cm. In the build-up region from 0 to 2 cm, an increment of 1 mm was used for the chamber movement, and beyond 2 cm depth a positional increment of 1 cm was used. Figure 2 shows a typical measurement condition with phantom and rice filled bags. The PDD was measured until 29 cm and further extended to 60 cm using an extrapolation method.

Beam profile

Two crossbeam profiles: one at the depth of d_max and another at the depth of 20 cm along the long axis of the field were measured with a spatial resolution of 1 cm in the region of penumbra and up to 2 cm beyond field edge while in the central region of the field a resolution of 5 cm was employed. All the profiles were normalized to central axis.

Measurement of cross calibration factor valid at maximum dose

A set of different calibration factors for both energies were calculated for extended SSD, which is used for regular day to day measurement for reference dosimetry using reference chamber (0.6 cc) at the beam quality Q. The chamber was initially placed at 10 cm depth in a water phantom and reading was measured. Alternatively, the same reference chamber (0.6 cc) was placed again depth of d_max and charge was collected. The calibration factor in terms of absorbed dose to water for the reference chamber valid at d_max for general methodology is given by

where M_{Q at 10cm} and M_{Q at dmax} are the meter readings per 100 MU for the same field chamber 10 cm and d_max depth, corrected for the external influence quantities of temperature and pressure, polarity correction as well as ion recombination correction factor. is the calibration factor in terms of absorbed dose to water for the reference chamber provided by the Secondary Standard Dosimetry Laboratory valid for calibration energy Q₀. is the calibration factor absorbed dose to in-water valid at d_max valid at user energy.

The cross calibration factors measured in a full scattering condition with rice bags at extended SSD accounts for the dose buildup and scattering in this medium which is slightly different from the in- water measurements.

Output measurement

The Tissue Phantom Ratio (TPR_20/10) was verified at the nominal SAD of 100 cm. Following that, the output measurement for the 10 cm × 10 cm field size in a water phantom at 100 cm SSD as per TRS-398 protocol was performed and the dose to water at d_max was established. Subsequently, with the chamber placed at the depth of d_max in the water phantom, the average meter reading was obtained for a series of measurements. Since the dose in water at this depth is known from the reference dosimetry, the cross-calibration coefficient N_D,w valid only for measurements in water phantom at the depth of d_max was established. Dose to water was calculated for 10 × 10 field size in a solid water phantom using TRS-398 protocol.

For the extended SSD output measurement, the Farmer chamber (0.6 cc) was positioned at the depth of dose maximum in solid water phantom (RW3 plates) under full-scatter conditions using rice bags. The linear accelerator was operated at a dose rate of .

Dose per MU was calculated using the N_D,w and depth dose values and verified with the measured values. Long-term stability of the commissioning was evaluated by periodic dose per MU measurements over 5 years, at an interval of 6 months, for a total of 10 times.

RESULTS

Depth dose and profile measurement result

Table 1 shows the PDD along the central axis of a single direct field for 6 and 15 MV Photon energies for the extended SSD of 333.5 cm 165 cm × 165 cm field size or clipped field size of 165 cm × 165 cm. The depths of dose maximum for 6 and 15 MV were found to occur at 1.5 cm and 2.8 cm, respectively, matching with the values at the nominal SSD. Figure 2 shows the measurement setup in RW3 plastic phantom under full-scattering conditions. Figure 3a and b shows the depth dose curves measured at the extended SSD for the field size of 40 cm × 40 cm at 100 cm and 165 cm × 165 cm at 333.5 cm distance. The measurement was carried out up to a depth of 29 cm and further extrapolated to deeper depths using a curve fitting technique.[16] Figure 4a-d show the crossbeam profiles at the depths of d_max and 20 cm for 6 MV and 15 MV at 333.5 cm SSD. The variation in dose along the central axis for the bilateral parallel-opposed beams setup was studied for the three separations, viz. 42 cm, 52 cm and 62 cm by adding up the CADD values from each beam at every depth. Figure 5a-c show the variation in dose along the central axis for such bilateral parallel-opposed setup for 42 cm, 52 cm, and 62 cm lateral separation, respectively. The dose at the midplane was normalized to 100% in each curve. Similar curves are given in Figure 6a-c for the 15 MV energy. The central axis D_max near the surface is higher relative to the midpoint dose due to tissue lateral effect. The TBI Box has three pairs of slots; therefore, it was essential to establish the parallel-opposed beams were producing homogeneous dose at the midplane. Figure 5a-c shows the midplane dose measurement of 6 MV beam for 42 cm, 52 cm, and 62 cm, respectively. Similarly, Figure 6a-c shows the midplane dose measurement for 15 MV beam. All doses were normalized to respective midplane. For the 42 cm separation, the dose at d_max was 117% in 6 MV and was 109% in 15 MV. For the separation of 52 cm, the corresponding values were 132% and 118% for the 6 MV and 15 MV beam. For the 62 cm thickness, the values were 148% and 127% for the 6 MV and 15 MV, respectively. The peripheral dose was higher for larger separations compared to the midplane dose and this effect was more pronounced for the 6 MV beam. Figure 7 shows the ratio of maximum peripheral dose to the midplane dose along the central axis plotted as a function of patient thickness for the two photon energies. The dose heterogeneity was more for lower beam energy at higher separation compared to higher energy beam. Therefore, depending on the lateral separation of the patient, the beam energy should be decided to reduce the dose heterogeneity.

Table 1.

Percentage depth dose along the central axis of a single direct field for 6 and 15 MV photon energies at extended source-to-surface distance of 333.5 cm 165 cm² × 165 cm² field size

6 MV photon		15 MV photon
Depth z (cm)	Percentage depth dose (%)	Depth z (cm)	Percentage depth dose (%)
1	98.7	1	91.0
-	-	1.5	96.7
1.5	100.0	2.8	100.0
5.0	92.4	5.0	96.4
10.0	79.7	10.0	85.4
15.0	67.4	15.0	74.8
20.0	56.6	20.0	61.5
25.0	47.0	25.0	56.4
30.0	39.4	30.0	49.0
35.0	33.6	35.0	43.0
40.0	28.2	40.0	37.5
45.0	24.0	45.0	32.6
50.0	20.1	50.0	28.5
55.0	16.9	55.0	24.8
60.0	14.2	60.0	21.6

Figure 3.

(a) 6 MV percentage depth dose (PDD), field size 165 cm × 165 cm at extended source-to-surface distance (SSD) of 333.5 cm, collimator angle 45°. Measurement done up to 29 cm and extrapolated to 62 cm. (b) 15 MV PDD, field size 165 cm × 165 cm at extended SSD of 333.5 cm, collimator angle 45°. Measurement done up to 29 cm and extrapolated to 62 cm. SSD: Source-to-surface distance

Figure 4.

(a) 6 MV profile at depth of maximum dose, field size 165 cm × 165 cm at extended source-to-surface distance (SSD) of 333.5 cm, collimator angle 45°, gantry angle 270°. (b) 6 MV profile at depth at 20 cm, field size 165 cm × 165 cm at extended SSD of 333.5 cm, collimator angle 45°, gantry angle 270°. (c) 15 MV profile at depth of maximum dose, field size 165 cm × 165 cm at extended SSD of 333.5 cm, collimator angle 45°, gantry angle 270°. (d) 15 MV profile at depth at 20 cm, field size 165 cm × 165 cm at extended SSD of 333.5 cm, collimator angle 45°, gantry angle 270°

Figure 5.

(a) 6 MV parallel opposed (lateral) profile at a depth of 21 cm for total separation of 42 cm, dose normalized to 100% at mid-plane. (b) 6 MV parallel opposed (lateral) profile at a depth of 26 cm for total separation of 52 cm, dose normalized to 100% at mid-plane. (c) 6 MV parallel opposed (lateral) profile at a depth of 31 cm for 62 cm separation, dose normalized to 100% at mid-plane

Figure 6.

(a) 15 MV parallel opposed (lateral) profile at a depth of 21 cm for total separation of 42 cm, dose normalized to 100% at mid-plane. (b) 15 MV parallel opposed (lateral) profile at a depth of 26 cm for total separation of 52 cm, dose normalized to 100% at mid-plane. (c) 15 MV parallel opposed (lateral) profile at a depth of 31 cm for 62 cm separation, dose normalized to 100% at mid-plane

Figure 7.

Ratio of the maximum peripheral dose to the midpoint dose, plotted as a function of patient thickness for photon energies. Bilateral parallel opposed fields, field size 40 cm × 40 cm at source to surface distance of 333.5 cm

Detail of output measurement for both the energies is presented.[17] Output measurement for TBI is a two-step procedure: first the output at the nominal SSD was measured in a water phantom with the chamber placed at the reference depth of 10 cm and then at the depth of dose maximum to determine the cross-calibration coefficient valid at d_max; followed by dose measurement in solid water phantom using this coefficient at the extended SSD. The TPR_20/10 value for 6 MV and 15 MV values for nominal SSD ware 0.684 and 0.762 respectively which match well with our baseline values.

Reference dosimetry for 6 MV at nominal SSD for 10 cm × 10 cm field size, 100 MU:

The chamber was then placed at the depth of d_max in water and measurements were obtained for same field setup and 100 MU.

Meter reading = 18.67 (corrected only for pressure and temperature).

N_D,w,dmax,6MV = 5.369 cGy/nC for 6 MV.

Reference dosimetry for 15 MV at nominal SSD for 10 cm × 10 cm field size, 100 MU:

The chamber was then placed at the depth of d_max in water and measurements were obtained for same field setup and 100 MU.

Meter reading = 18.83 (corrected only for pressure and temperature).

N_{D,w,dmax,15MV} = 5.306 cGy/nC for 15 MV.

The interpolated PDDs at 21 cm, 26 cm, and 31 cm were 54.5%, 45.34%, and 38.2%, respectively. The dose rates at these midplane depths of 21 cm, 26 cm, and 31 cm were 53.58 cGy, 44.57 cGy, and 37.55 cGy/1000 MUs, respectively.

The cross calibration factor for 6 MV (using equation-i)

And 15 MV

Supplementary Documents 1 ^{(3.6MB, tif)} and 2 ^{(3.6MB, tif)} shows further details of the dosimetry and MU calculation for 6 MV and 15 MV beams respectively. Supplementary Documents 3 ^{(7MB, tif)} and 4 ^{(3.5MB, tif)} provide the output factor and depth dose values measured in TBI measurement.

End-to-end quality assurance test verification

An end-to-end dosimetric verification was done by placing a pelvic Rando phantom inside the TBI box, packing it tightly with rice bags like in real situation, and placing the Farmer chamber in between the thighs of the phantom. The lateral separation of the TBI box was 42 cm and chamber was placed exactly at 21 cm with rice bags placed all around. The Rando phantom was irradiated to a midplane dose of 200 cGy at the umbilicus level by bilateral parallel-opposed fields. A dose of 100 cGy was delivered by the first field from one side and the box was rotated before another 100 cGy was delivered from the other side.

A typical dose calculation is shown below. The dosimetric error for MV and for after the MV beam for measurement dose difference observed between measured and compared dose for 6 MV and 15 MV beam at 333.5 cm SSD was found to be − 2.16% and − 1.27%, respectively.

MU calculation and dose verification for 6 MV beam

With the Farmer chamber placed at the depth of d_max in the solid phantom, the meter reading for 100 MU at 333.5 cm SSD was 1.852 nC

Dose rate at 333.5 cm SSD at d_max =

Dose rate at 333.5 cm SSD at d_max =

Calculated PDD at 21 cm depth was 54.51%

Dose rate at 333.5 cm SSD at 21 cm =

To deliver 100 cGy, MU required =

To deliver 200 cGy at mid-plane, MUs = 1868.1 MU by each lateral field. The Farmer chamber with appropriate buildup cap was placed in between the thighs of the Rando phantom and readings were taken by delivering the calculated MUs.

Meter reading measured for 1^st lateral field = 19.105 nC

Meter reading measured for 2^nd lateral field = 17.784 nC

Dose from both lateral fields = (19.105 + 17.784)

For the 6 MV beam, the interpolated PDDs at 21 cm, 26 cm, and 31 cm were 54.5%, 45.34%, and 38.2%, respectively. The dose rates at these mid-plane depths of 21 cm, 26 cm, and 31 cm were 53.58 cGy, 44.57 cGy, and 37.55 cGy/1000 MUs, respectively.

MU calculation and dose verification for 15 MV beam

With the Farmer chamber placed at the depth of d_max in the solid phantom, meter reading for 100 MU at 333.5 cm SSD was 1.871 nC

Dose rate at 333.5 cm SSD at d_max = M.R./100 MU x k_T,P x N_{D,w,Q,dmax,15 MV} =

1.871 nC/100 MU x 0.988 x 5.306 cGy/nC

Dose rate at 333.5 cm SSD at d_max = 9.808 cGy/100 MU

Calculated PDD at 21 cm depth was 63.26%

Dose rate at 333.5 cm SSD at 21 cm depth = 9.808 cGy/100 MU x 0.6326 = 6.204 cGy/100 MU

To deliver 100 cGy, MU required =

100 MU/6.204 cGy x 100 cGy = 1611 MU

To deliver 200 cGy at mid-plane, MUs = 1611 MU by each lateral field. The Farmer chamber with appropriate buildup cap was placed in between the thighs of the Rando phantom and readings were taken by delivering the calculated MUs.

Meter reading measured for 1st lateral field = 19.37 nC

Meter reading measured for 2nd lateral field = 18.30 nC

Dose from both lateral fields = (19.37 + 18.30)

= 37.67 nC x 5.306 cGy/nC = 197.47 cGy

For the 15 MV beam, the interpolated PDDs at 21 cm, 26 cm, and 31 cm were 63.26%, 54.92%, and 47.85%, respectively. The dose rates at these mid-plane depths of 21 cm, 26 cm, and 31 cm were 61.99 cGy, 53.82 cGy, and 46.89 cGy/1000 MUs, respectively.

The difference between calculated and measured MU for delivering 200 cGy at mid plane was −2.16% and −1.27%, respectively, for 6 MV and 15 MV beams, respectively. Detailed calculations are provided in Supplementary Documents 5 ^{(3.5MB, tif)} and 6 ^{(7MB, tif)} for the 6 MV and 15 MV beams, respectively.

The long-term stability results indicate that the Dose-to-MU verification deviated by −2.1 ± 1.9% for the 6 MV beam and 1.7 ± 2.3% for the 15 MV beam, measured at d_max depth.

DISCUSSION

The commissioning experience presented here was carried out in 2015–2016. A total of 67 patients were treated with extended SSD TBI between 2015 and 2020, including 19 females and 48 males. Among them, 25 patients received a single fraction treatment, while 42 received treatment in six fractions.[18,19] Specifically, 31 patients were treated with a 6 MV beam and 36 with a 15 MV beam. A separate article is being written to present the long-term clinical result. We presented the TBI commissioning experience lately after establishing that the commissioned technique served well over long years.[18,19] Meticulous periodic maintenance was also carried out to keep the process intact over these years.

There is a perception among the practitioners that modern techniques like volumetric modulated arc therapy (VMAT) and Tomotherapy are more suitable for performing TBI. In 2020, the Children’s Oncology Group (COG) surveyed 152 COG institutions regarding their practice patterns in pediatric TBI.[9] The survey revealed that all the respondents (doctors) were keen to refining the conventional TBI technique to reduce lung dose, and three fourth of the respondents were interested in implementing VMAT-or Tomotherapy-based TBI. However, the survey revealed that only 14% of COG institutions had adopted more advanced TBI techniques. In a vast country like India where more than 400 radiotherapy centers are equipped with one or more advanced linear accelerators, only three are using VMAT-based technique for TBI/TMLI delivery.[11] Hence, the presented technique, though a decade old, still has relevance for the needy centers who either do not have the advanced treatment machines or prefer to practice it the traditional way.

The literature proposes several advanced TBI and TMLI/total marrow irradiation (TMI) techniques, including helical tomotherapy, volumetric arc therapy-based technique in C-arm or O-ring linear accelerators.[9,20,21,22] TMI/TMLI techniques are possible with C-arm, O-ring and Tomotherapy machines.[10,11,12] Of these, Tomotherapy is the easiest to implement due to the helical treatment capability of the machine.[12] In contrast, the intensity-modulated TBI treatments on C-arm linear accelerators face significant challenges in widespread implementation because of their complexity in treatment planning and delivery.[11,12] Springer et al. reported that the treatment planning for the multi-isocentric axial VMAT technique could take up to 4.5 days, including contouring, planning, optimization, and dose calculation.[23] This is only possible if the planning system has a customized computer with high computation capability or in present time using a graphics processing unit-based system. Otherwise, in a planning system operating on a standard Zen-4 processor, a single cycle of optimization and dose calculation will take average 48 h of time for a full length adult for TBI/TMLI. Although TBI contouring does not take much time, contouring a TMLI takes around 2–4 days if two clinicians were engaged.[11] Therefore, most challenging part of the VMAT based technique is the computational time. In addition, uncertainty in the completing the optimization and dose calculation within a stipulated time is sometime uncertain in terms of crashing of the operating system due to excessive memory use during the optimization or dose calculation. The challenge in terms of VMAT-based TBI/TMLI delivery is the positional uncertainty. This is most prominent for TMLI delivery in the lower limb, especially for the pediatric group of patients both with or without anesthesia. Due to long treatment delivery time, patients get fatigued and it became difficult to reproduce the position even with proper immobilization. Although the VMAT-based TBI/TMLI is governed by a low-gradient junction technique which was originally devolved for multi-isocentric craniospinal irradiation technique,[24,25] this technique is very robust in dealing with the longitudinal positional errors, but with lateral positional error it fails.[12,24,25] It is observed that often the end user prefers to do three-dimensional conformal radiotherapy open field technique for lower limb which saves the computation time as well as reduce the possibility of geometrical mis.

Although the commissioning of extended SSD TBI technique is a challenging task in terms of labor and time, once it is implemented, it is free of any challenges like computation time and uncertainties and geometrical miss. Probably, that is the reason only a small fraction of modern radiotherapy centers are keen to implement the VMAT-based techniques replacing open field extended SSD techniques, although former having a significant dosimetric advantage like lung and kidney sparing.[9]

We have implemented safe and reproducible bilateral parallel opposed irradiation technique for TBI and established the long-term stability of the commissioned technique. Supplementary Document 7 ^{(13.7MB, tif)} summaries all the details of TBI technique in our institution. In addition, we have also presented the long term in vivo dosimetry result for bilateral oppose extended SSD TBI in an recent article.[26] The further detail of the TBI installation and measurements can be found in AAPM Task Group Report-17 and earlier publications like Ganapathy et al. where they have done point dose analysis for different anatomical region using Si-diode.[27,28] We fabricated special Perspex box for treatment filling space with rice bags and delivered uniform dose to the entire box. Our technique does not require any shielding material to OARs (Organ at Risk). The side walls of the box are adjustable accordingly to the thickness of either thin or thick patients. Before implementation, we did measurements, both absolute and relative dosimetry, at normal treatment distance and as well as extended SSD. At extended measurements, the d_max depth was same as compared to 100 cm SSD. Additionally, the cross beam profiles for both the energies were flat at both depths. Our end-to-end quality assurance test for second MU check as part of dose verification results measured for 6 MV were − 2.16% and 15 MV were − 1.27% variations from the prescription dose.

We recommend the lying position for TBI over the standing (vertical) position based on our clinical experience over the last decade with both open-field and VMAT-based TBI.[11,18,19,22,26] Patients with both myeloablative and nonmyeloablative benign diseases are generally in poor general condition. Therefore, it is challenging for this group of patients to stand for an hour or more to complete the treatment. Additionally, patients requiring anesthesia cannot be treated in the standing position for TBI. In our setup, approximately 30% of patients generally require anesthesia. It may be argued that in the lying position, with lateral beams, lung shielding is not possible; however, this is partially compensated by the arms. Based on the above considerations, our recommendation is to use the lying (horizontal) position for TBI. The time required in commissioning was little more than 4 months, since in a clinically functional department, measurements could be started only after the completion of the treatment for the day.

CONCLUSION

With all advancement in the linear accelerators, computational processes, we present the commissioning of the extended SSD TBI technique which performed well over a long period of time. This article summarizes all the details of the dosimetric measurements, its values, and other requirements like Perspex box and scattering materials. We believe this article will be helpful to a new medical physicist willing to implement the extended SSD TBI technique in clinical routine.

Author’s note

This article should be read in conjunction with our previously published work on TBI in vivo dosimetry: Sarkar B, Pradhan A. Long-term in vivo dosimetry results for bilateral opposed extended source-to-surface distance (SSD) total body irradiation. Radiation Physics and Chemistry. 2025 Jan 18:112541.

Conflicts of interest

There are no conflicts of interest.

Supplementary Document 1

6 MV Synergy Platform cross-calibration

Supplementary Document 2

6 MV - Total body irradiation - MU calculation and experimental verification

Supplementary Document 3

6 MV - Total body irradiation - Dosimetry MU calculation chart

Supplementary Document 4

15 MV Synergy Platform cross calibration

Supplementary Document 5

15 MV - Total body irradiation - MU calculation and experimental verification

Supplementary Document 6

15 MV - Total body irradiation - Dosimetry and MU chart

Supplementary Document 7

TBI - Protocol - FMRI

Funding Statement

Nil.